mirror of
https://github.com/KhronosGroup/SPIRV-Tools
synced 2024-11-23 12:10:06 +00:00
c9190a54da
When a phi candidate is marked as trivial, we are suppose to update all of its uses to the reference the value that it is being folded to. However, the code updates the uses misses `defs_at_block_`. So at a later time, the id for the trivial phi can reemerge. Fixes #2744
603 lines
21 KiB
C++
603 lines
21 KiB
C++
// Copyright (c) 2018 Google LLC.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// This file implements the SSA rewriting algorithm proposed in
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//
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// Simple and Efficient Construction of Static Single Assignment Form.
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// Braun M., Buchwald S., Hack S., Leißa R., Mallon C., Zwinkau A. (2013)
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// In: Jhala R., De Bosschere K. (eds)
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// Compiler Construction. CC 2013.
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// Lecture Notes in Computer Science, vol 7791.
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// Springer, Berlin, Heidelberg
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//
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// https://link.springer.com/chapter/10.1007/978-3-642-37051-9_6
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//
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// In contrast to common eager algorithms based on dominance and dominance
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// frontier information, this algorithm works backwards from load operations.
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//
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// When a target variable is loaded, it queries the variable's reaching
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// definition. If the reaching definition is unknown at the current location,
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// it searches backwards in the CFG, inserting Phi instructions at join points
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// in the CFG along the way until it finds the desired store instruction.
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//
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// The algorithm avoids repeated lookups using memoization.
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//
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// For reducible CFGs, which are a superset of the structured CFGs in SPIRV,
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// this algorithm is proven to produce minimal SSA. That is, it inserts the
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// minimal number of Phi instructions required to ensure the SSA property, but
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// some Phi instructions may be dead
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// (https://en.wikipedia.org/wiki/Static_single_assignment_form).
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#include "source/opt/ssa_rewrite_pass.h"
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#include <memory>
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#include <sstream>
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#include "source/opcode.h"
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#include "source/opt/cfg.h"
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#include "source/opt/mem_pass.h"
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#include "source/util/make_unique.h"
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// Debug logging (0: Off, 1-N: Verbosity level). Replace this with the
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// implementation done for
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// https://github.com/KhronosGroup/SPIRV-Tools/issues/1351
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// #define SSA_REWRITE_DEBUGGING_LEVEL 3
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#ifdef SSA_REWRITE_DEBUGGING_LEVEL
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#include <ostream>
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#else
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#define SSA_REWRITE_DEBUGGING_LEVEL 0
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#endif
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namespace spvtools {
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namespace opt {
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namespace {
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const uint32_t kStoreValIdInIdx = 1;
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const uint32_t kVariableInitIdInIdx = 1;
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} // namespace
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std::string SSARewriter::PhiCandidate::PrettyPrint(const CFG* cfg) const {
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std::ostringstream str;
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str << "%" << result_id_ << " = Phi[%" << var_id_ << ", BB %" << bb_->id()
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<< "](";
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if (phi_args_.size() > 0) {
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uint32_t arg_ix = 0;
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for (uint32_t pred_label : cfg->preds(bb_->id())) {
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uint32_t arg_id = phi_args_[arg_ix++];
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str << "[%" << arg_id << ", bb(%" << pred_label << ")] ";
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}
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}
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str << ")";
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if (copy_of_ != 0) {
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str << " [COPY OF " << copy_of_ << "]";
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}
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str << ((is_complete_) ? " [COMPLETE]" : " [INCOMPLETE]");
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return str.str();
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}
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SSARewriter::PhiCandidate& SSARewriter::CreatePhiCandidate(uint32_t var_id,
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BasicBlock* bb) {
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// TODO(1841): Handle id overflow.
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uint32_t phi_result_id = pass_->context()->TakeNextId();
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auto result = phi_candidates_.emplace(
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phi_result_id, PhiCandidate(var_id, phi_result_id, bb));
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PhiCandidate& phi_candidate = result.first->second;
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return phi_candidate;
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}
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void SSARewriter::ReplacePhiUsersWith(const PhiCandidate& phi_to_remove,
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uint32_t repl_id) {
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for (uint32_t user_id : phi_to_remove.users()) {
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PhiCandidate* user_phi = GetPhiCandidate(user_id);
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BasicBlock* bb = pass_->context()->get_instr_block(user_id);
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if (user_phi) {
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// If the user is a Phi candidate, replace all arguments that refer to
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// |phi_to_remove.result_id()| with |repl_id|.
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for (uint32_t& arg : user_phi->phi_args()) {
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if (arg == phi_to_remove.result_id()) {
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arg = repl_id;
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}
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}
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} else if (bb->id() == user_id) {
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// The phi candidate is the definition of the variable at basic block
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// |bb|. We must change this to the replacement.
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WriteVariable(phi_to_remove.var_id(), bb, repl_id);
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} else {
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// For regular loads, traverse the |load_replacement_| table looking for
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// instances of |phi_to_remove|.
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for (auto& it : load_replacement_) {
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if (it.second == phi_to_remove.result_id()) {
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it.second = repl_id;
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}
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}
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}
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}
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}
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uint32_t SSARewriter::TryRemoveTrivialPhi(PhiCandidate* phi_candidate) {
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uint32_t same_id = 0;
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for (uint32_t arg_id : phi_candidate->phi_args()) {
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if (arg_id == same_id || arg_id == phi_candidate->result_id()) {
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// This is a self-reference operand or a reference to the same value ID.
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continue;
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}
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if (same_id != 0) {
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// This Phi candidate merges at least two values. Therefore, it is not
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// trivial.
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assert(phi_candidate->copy_of() == 0 &&
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"Phi candidate transitioning from copy to non-copy.");
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return phi_candidate->result_id();
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}
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same_id = arg_id;
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}
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// The previous logic has determined that this Phi candidate |phi_candidate|
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// is trivial. It is essentially the copy operation phi_candidate->phi_result
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// = Phi(same, same, same, ...). Since it is not necessary, we can re-route
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// all the users of |phi_candidate->phi_result| to all its users, and remove
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// |phi_candidate|.
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// Mark the Phi candidate as a trivial copy of |same_id|, so it won't be
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// generated.
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phi_candidate->MarkCopyOf(same_id);
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assert(same_id != 0 && "Completed Phis cannot have %0 in their arguments");
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// Since |phi_candidate| always produces |same_id|, replace all the users of
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// |phi_candidate| with |same_id|.
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ReplacePhiUsersWith(*phi_candidate, same_id);
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return same_id;
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}
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uint32_t SSARewriter::AddPhiOperands(PhiCandidate* phi_candidate) {
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assert(phi_candidate->phi_args().size() == 0 &&
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"Phi candidate already has arguments");
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bool found_0_arg = false;
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for (uint32_t pred : pass_->cfg()->preds(phi_candidate->bb()->id())) {
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BasicBlock* pred_bb = pass_->cfg()->block(pred);
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// If |pred_bb| is not sealed, use %0 to indicate that
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// |phi_candidate| needs to be completed after the whole CFG has
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// been processed.
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//
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// Note that we cannot call GetReachingDef() in these cases
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// because this would generate an empty Phi candidate in
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// |pred_bb|. When |pred_bb| is later processed, a new definition
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// for |phi_candidate->var_id_| will be lost because
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// |phi_candidate| will still be reached by the empty Phi.
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//
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// Consider:
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//
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// BB %23:
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// %38 = Phi[%i](%int_0[%1], %39[%25])
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//
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// ...
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//
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// BB %25: [Starts unsealed]
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// %39 = Phi[%i]()
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// %34 = ...
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// OpStore %i %34 -> Currdef(%i) at %25 is %34
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// OpBranch %23
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//
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// When we first create the Phi in %38, we add an operandless Phi in
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// %39 to hold the unknown reaching def for %i.
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//
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// But then, when we go to complete %39 at the end. The reaching def
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// for %i in %25's predecessor is %38 itself. So we miss the fact
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// that %25 has a def for %i that should be used.
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//
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// By making the argument %0, we make |phi_candidate| incomplete,
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// which will cause it to be completed after the whole CFG has
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// been scanned.
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uint32_t arg_id = IsBlockSealed(pred_bb)
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? GetReachingDef(phi_candidate->var_id(), pred_bb)
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: 0;
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phi_candidate->phi_args().push_back(arg_id);
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if (arg_id == 0) {
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found_0_arg = true;
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} else {
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// If this argument is another Phi candidate, add |phi_candidate| to the
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// list of users for the defining Phi.
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PhiCandidate* defining_phi = GetPhiCandidate(arg_id);
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if (defining_phi && defining_phi != phi_candidate) {
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defining_phi->AddUser(phi_candidate->result_id());
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}
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}
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}
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// If we could not fill-in all the arguments of this Phi, mark it incomplete
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// so it gets completed after the whole CFG has been processed.
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if (found_0_arg) {
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phi_candidate->MarkIncomplete();
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incomplete_phis_.push(phi_candidate);
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return phi_candidate->result_id();
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}
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// Try to remove |phi_candidate|, if it's trivial.
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uint32_t repl_id = TryRemoveTrivialPhi(phi_candidate);
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if (repl_id == phi_candidate->result_id()) {
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// |phi_candidate| is complete and not trivial. Add it to the
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// list of Phi candidates to generate.
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phi_candidate->MarkComplete();
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phis_to_generate_.push_back(phi_candidate);
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}
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return repl_id;
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}
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uint32_t SSARewriter::GetReachingDef(uint32_t var_id, BasicBlock* bb) {
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// If |var_id| has a definition in |bb|, return it.
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const auto& bb_it = defs_at_block_.find(bb);
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if (bb_it != defs_at_block_.end()) {
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const auto& current_defs = bb_it->second;
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const auto& var_it = current_defs.find(var_id);
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if (var_it != current_defs.end()) {
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return var_it->second;
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}
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}
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// Otherwise, look up the value for |var_id| in |bb|'s predecessors.
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uint32_t val_id = 0;
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auto& predecessors = pass_->cfg()->preds(bb->id());
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if (predecessors.size() == 1) {
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// If |bb| has exactly one predecessor, we look for |var_id|'s definition
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// there.
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val_id = GetReachingDef(var_id, pass_->cfg()->block(predecessors[0]));
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} else if (predecessors.size() > 1) {
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// If there is more than one predecessor, this is a join block which may
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// require a Phi instruction. This will act as |var_id|'s current
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// definition to break potential cycles.
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PhiCandidate& phi_candidate = CreatePhiCandidate(var_id, bb);
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// Set the value for |bb| to avoid an infinite recursion.
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WriteVariable(var_id, bb, phi_candidate.result_id());
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val_id = AddPhiOperands(&phi_candidate);
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}
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// If we could not find a store for this variable in the path from the root
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// of the CFG, the variable is not defined, so we use undef.
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if (val_id == 0) {
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val_id = pass_->GetUndefVal(var_id);
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}
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WriteVariable(var_id, bb, val_id);
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return val_id;
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}
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void SSARewriter::SealBlock(BasicBlock* bb) {
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auto result = sealed_blocks_.insert(bb);
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(void)result;
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assert(result.second == true &&
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"Tried to seal the same basic block more than once.");
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}
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void SSARewriter::ProcessStore(Instruction* inst, BasicBlock* bb) {
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auto opcode = inst->opcode();
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assert((opcode == SpvOpStore || opcode == SpvOpVariable) &&
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"Expecting a store or a variable definition instruction.");
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uint32_t var_id = 0;
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uint32_t val_id = 0;
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if (opcode == SpvOpStore) {
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(void)pass_->GetPtr(inst, &var_id);
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val_id = inst->GetSingleWordInOperand(kStoreValIdInIdx);
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} else if (inst->NumInOperands() >= 2) {
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var_id = inst->result_id();
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val_id = inst->GetSingleWordInOperand(kVariableInitIdInIdx);
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}
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if (pass_->IsTargetVar(var_id)) {
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WriteVariable(var_id, bb, val_id);
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#if SSA_REWRITE_DEBUGGING_LEVEL > 1
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std::cerr << "\tFound store '%" << var_id << " = %" << val_id << "': "
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<< inst->PrettyPrint(SPV_BINARY_TO_TEXT_OPTION_FRIENDLY_NAMES)
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<< "\n";
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#endif
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}
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}
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void SSARewriter::ProcessLoad(Instruction* inst, BasicBlock* bb) {
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uint32_t var_id = 0;
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(void)pass_->GetPtr(inst, &var_id);
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if (pass_->IsTargetVar(var_id)) {
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// Get the immediate reaching definition for |var_id|.
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uint32_t val_id = GetReachingDef(var_id, bb);
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// Schedule a replacement for the result of this load instruction with
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// |val_id|. After all the rewriting decisions are made, every use of
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// this load will be replaced with |val_id|.
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const uint32_t load_id = inst->result_id();
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assert(load_replacement_.count(load_id) == 0);
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load_replacement_[load_id] = val_id;
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PhiCandidate* defining_phi = GetPhiCandidate(val_id);
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if (defining_phi) {
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defining_phi->AddUser(load_id);
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}
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#if SSA_REWRITE_DEBUGGING_LEVEL > 1
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std::cerr << "\tFound load: "
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<< inst->PrettyPrint(SPV_BINARY_TO_TEXT_OPTION_FRIENDLY_NAMES)
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<< " (replacement for %" << load_id << " is %" << val_id << ")\n";
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#endif
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}
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}
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void SSARewriter::PrintPhiCandidates() const {
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std::cerr << "\nPhi candidates:\n";
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for (const auto& phi_it : phi_candidates_) {
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std::cerr << "\tBB %" << phi_it.second.bb()->id() << ": "
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<< phi_it.second.PrettyPrint(pass_->cfg()) << "\n";
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}
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std::cerr << "\n";
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}
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void SSARewriter::PrintReplacementTable() const {
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std::cerr << "\nLoad replacement table\n";
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for (const auto& it : load_replacement_) {
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std::cerr << "\t%" << it.first << " -> %" << it.second << "\n";
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}
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std::cerr << "\n";
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}
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void SSARewriter::GenerateSSAReplacements(BasicBlock* bb) {
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#if SSA_REWRITE_DEBUGGING_LEVEL > 1
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std::cerr << "Generating SSA replacements for block: " << bb->id() << "\n";
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std::cerr << bb->PrettyPrint(SPV_BINARY_TO_TEXT_OPTION_FRIENDLY_NAMES)
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<< "\n";
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#endif
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for (auto& inst : *bb) {
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auto opcode = inst.opcode();
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if (opcode == SpvOpStore || opcode == SpvOpVariable) {
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ProcessStore(&inst, bb);
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} else if (inst.opcode() == SpvOpLoad) {
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ProcessLoad(&inst, bb);
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}
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}
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// Seal |bb|. This means that all the stores in it have been scanned and it's
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// ready to feed them into its successors.
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SealBlock(bb);
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#if SSA_REWRITE_DEBUGGING_LEVEL > 1
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PrintPhiCandidates();
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PrintReplacementTable();
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std::cerr << "\n\n";
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#endif
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}
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uint32_t SSARewriter::GetReplacement(std::pair<uint32_t, uint32_t> repl) {
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uint32_t val_id = repl.second;
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auto it = load_replacement_.find(val_id);
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while (it != load_replacement_.end()) {
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val_id = it->second;
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it = load_replacement_.find(val_id);
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}
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return val_id;
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}
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uint32_t SSARewriter::GetPhiArgument(const PhiCandidate* phi_candidate,
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uint32_t ix) {
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assert(phi_candidate->IsReady() &&
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"Tried to get the final argument from an incomplete/trivial Phi");
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uint32_t arg_id = phi_candidate->phi_args()[ix];
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while (arg_id != 0) {
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PhiCandidate* phi_user = GetPhiCandidate(arg_id);
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if (phi_user == nullptr || phi_user->IsReady()) {
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// If the argument is not a Phi or it's a Phi candidate ready to be
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// emitted, return it.
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return arg_id;
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}
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arg_id = phi_user->copy_of();
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}
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assert(false &&
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"No Phi candidates in the copy-of chain are ready to be generated");
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return 0;
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}
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bool SSARewriter::ApplyReplacements() {
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bool modified = false;
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#if SSA_REWRITE_DEBUGGING_LEVEL > 2
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std::cerr << "\n\nApplying replacement decisions to IR\n\n";
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PrintPhiCandidates();
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PrintReplacementTable();
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std::cerr << "\n\n";
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#endif
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// Add Phi instructions from completed Phi candidates.
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std::vector<Instruction*> generated_phis;
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for (const PhiCandidate* phi_candidate : phis_to_generate_) {
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#if SSA_REWRITE_DEBUGGING_LEVEL > 2
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std::cerr << "Phi candidate: " << phi_candidate->PrettyPrint(pass_->cfg())
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<< "\n";
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#endif
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assert(phi_candidate->is_complete() &&
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"Tried to instantiate a Phi instruction from an incomplete Phi "
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"candidate");
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// Build the vector of operands for the new OpPhi instruction.
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uint32_t type_id = pass_->GetPointeeTypeId(
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pass_->get_def_use_mgr()->GetDef(phi_candidate->var_id()));
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std::vector<Operand> phi_operands;
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uint32_t arg_ix = 0;
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std::unordered_map<uint32_t, uint32_t> already_seen;
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for (uint32_t pred_label : pass_->cfg()->preds(phi_candidate->bb()->id())) {
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uint32_t op_val_id = GetPhiArgument(phi_candidate, arg_ix++);
|
|
if (already_seen.count(pred_label) == 0) {
|
|
phi_operands.push_back(
|
|
{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {op_val_id}});
|
|
phi_operands.push_back(
|
|
{spv_operand_type_t::SPV_OPERAND_TYPE_ID, {pred_label}});
|
|
already_seen[pred_label] = op_val_id;
|
|
} else {
|
|
// It is possible that there are two edges from the same parent block.
|
|
// Since the OpPhi can have only one entry for each parent, we have to
|
|
// make sure the two edges are consistent with each other.
|
|
assert(already_seen[pred_label] == op_val_id &&
|
|
"Inconsistent value for duplicate edges.");
|
|
}
|
|
}
|
|
|
|
// Generate a new OpPhi instruction and insert it in its basic
|
|
// block.
|
|
std::unique_ptr<Instruction> phi_inst(
|
|
new Instruction(pass_->context(), SpvOpPhi, type_id,
|
|
phi_candidate->result_id(), phi_operands));
|
|
generated_phis.push_back(phi_inst.get());
|
|
pass_->get_def_use_mgr()->AnalyzeInstDef(&*phi_inst);
|
|
pass_->context()->set_instr_block(&*phi_inst, phi_candidate->bb());
|
|
auto insert_it = phi_candidate->bb()->begin();
|
|
insert_it.InsertBefore(std::move(phi_inst));
|
|
pass_->context()->get_decoration_mgr()->CloneDecorations(
|
|
phi_candidate->var_id(), phi_candidate->result_id(),
|
|
{SpvDecorationRelaxedPrecision});
|
|
|
|
modified = true;
|
|
}
|
|
|
|
// Scan uses for all inserted Phi instructions. Do this separately from the
|
|
// registration of the Phi instruction itself to avoid trying to analyze uses
|
|
// of Phi instructions that have not been registered yet.
|
|
for (Instruction* phi_inst : generated_phis) {
|
|
pass_->get_def_use_mgr()->AnalyzeInstUse(&*phi_inst);
|
|
}
|
|
|
|
#if SSA_REWRITE_DEBUGGING_LEVEL > 1
|
|
std::cerr << "\n\nReplacing the result of load instructions with the "
|
|
"corresponding SSA id\n\n";
|
|
#endif
|
|
|
|
// Apply replacements from the load replacement table.
|
|
for (auto& repl : load_replacement_) {
|
|
uint32_t load_id = repl.first;
|
|
uint32_t val_id = GetReplacement(repl);
|
|
Instruction* load_inst =
|
|
pass_->context()->get_def_use_mgr()->GetDef(load_id);
|
|
|
|
#if SSA_REWRITE_DEBUGGING_LEVEL > 2
|
|
std::cerr << "\t"
|
|
<< load_inst->PrettyPrint(
|
|
SPV_BINARY_TO_TEXT_OPTION_FRIENDLY_NAMES)
|
|
<< " (%" << load_id << " -> %" << val_id << ")\n";
|
|
#endif
|
|
|
|
// Remove the load instruction and replace all the uses of this load's
|
|
// result with |val_id|. Kill any names or decorates using the load's
|
|
// result before replacing to prevent incorrect replacement in those
|
|
// instructions.
|
|
pass_->context()->KillNamesAndDecorates(load_id);
|
|
pass_->context()->ReplaceAllUsesWith(load_id, val_id);
|
|
pass_->context()->KillInst(load_inst);
|
|
modified = true;
|
|
}
|
|
|
|
return modified;
|
|
}
|
|
|
|
void SSARewriter::FinalizePhiCandidate(PhiCandidate* phi_candidate) {
|
|
assert(phi_candidate->phi_args().size() > 0 &&
|
|
"Phi candidate should have arguments");
|
|
|
|
uint32_t ix = 0;
|
|
for (uint32_t pred : pass_->cfg()->preds(phi_candidate->bb()->id())) {
|
|
BasicBlock* pred_bb = pass_->cfg()->block(pred);
|
|
uint32_t& arg_id = phi_candidate->phi_args()[ix++];
|
|
if (arg_id == 0) {
|
|
// If |pred_bb| is still not sealed, it means it's unreachable. In this
|
|
// case, we just use Undef as an argument.
|
|
arg_id = IsBlockSealed(pred_bb)
|
|
? GetReachingDef(phi_candidate->var_id(), pred_bb)
|
|
: pass_->GetUndefVal(phi_candidate->var_id());
|
|
}
|
|
}
|
|
|
|
// This candidate is now completed.
|
|
phi_candidate->MarkComplete();
|
|
|
|
// If |phi_candidate| is not trivial, add it to the list of Phis to generate.
|
|
if (TryRemoveTrivialPhi(phi_candidate) == phi_candidate->result_id()) {
|
|
// If we could not remove |phi_candidate|, it means that it is complete
|
|
// and not trivial. Add it to the list of Phis to generate.
|
|
assert(!phi_candidate->copy_of() && "A completed Phi cannot be trivial.");
|
|
phis_to_generate_.push_back(phi_candidate);
|
|
}
|
|
}
|
|
|
|
void SSARewriter::FinalizePhiCandidates() {
|
|
#if SSA_REWRITE_DEBUGGING_LEVEL > 1
|
|
std::cerr << "Finalizing Phi candidates:\n\n";
|
|
PrintPhiCandidates();
|
|
std::cerr << "\n";
|
|
#endif
|
|
|
|
// Now, complete the collected candidates.
|
|
while (incomplete_phis_.size() > 0) {
|
|
PhiCandidate* phi_candidate = incomplete_phis_.front();
|
|
incomplete_phis_.pop();
|
|
FinalizePhiCandidate(phi_candidate);
|
|
}
|
|
}
|
|
|
|
bool SSARewriter::RewriteFunctionIntoSSA(Function* fp) {
|
|
#if SSA_REWRITE_DEBUGGING_LEVEL > 0
|
|
std::cerr << "Function before SSA rewrite:\n"
|
|
<< fp->PrettyPrint(0) << "\n\n\n";
|
|
#endif
|
|
|
|
// Collect variables that can be converted into SSA IDs.
|
|
pass_->CollectTargetVars(fp);
|
|
|
|
// Generate all the SSA replacements and Phi candidates. This will
|
|
// generate incomplete and trivial Phis.
|
|
pass_->cfg()->ForEachBlockInReversePostOrder(
|
|
fp->entry().get(),
|
|
[this](BasicBlock* bb) { GenerateSSAReplacements(bb); });
|
|
|
|
// Remove trivial Phis and add arguments to incomplete Phis.
|
|
FinalizePhiCandidates();
|
|
|
|
// Finally, apply all the replacements in the IR.
|
|
bool modified = ApplyReplacements();
|
|
|
|
#if SSA_REWRITE_DEBUGGING_LEVEL > 0
|
|
std::cerr << "\n\n\nFunction after SSA rewrite:\n"
|
|
<< fp->PrettyPrint(0) << "\n";
|
|
#endif
|
|
|
|
return modified;
|
|
}
|
|
|
|
Pass::Status SSARewritePass::Process() {
|
|
bool modified = false;
|
|
for (auto& fn : *get_module()) {
|
|
modified |= SSARewriter(this).RewriteFunctionIntoSSA(&fn);
|
|
}
|
|
return modified ? Pass::Status::SuccessWithChange
|
|
: Pass::Status::SuccessWithoutChange;
|
|
}
|
|
|
|
} // namespace opt
|
|
} // namespace spvtools
|