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Change branch handling in ADCE to fix errors (#4596)

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Consider the new test case.  The conditional branch in the continue
block is never marked as live.  However, `IsDead` will say it is not
dead, so it does not get deleted.  Because it was never marked as live,
`%false` was not mark as live either, but it gets deleted.  This results
in invalid code.

To fix this properly, we had to reconsider how branches are handle.  We
make the following changes:

1) Terminator instructions that are not branch or OpUnreachable must be
kept, so they are marked as live when initializing the worklist.

2) Branches and OpUnreachable instructions are marked as live if
  a) the block does not have a merge instruction and another instruction
     in the block is marked as live, or
  b) the merge instruction in the same block is marked as live.

3) Any instruction that is not marked as live is removed.

4) If a terminator is to be removed, an OpUnreachable is added.  This
happens when the entire block is dead, and the block will be removed.
The OpUnreachable is generated to make sure the block still has a
terminator, and is valid.

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/4509.
This commit is contained in:
Steven Perron 2021-10-29 10:46:43 -04:00 committed by GitHub
parent bd5bf754b1
commit 6c7885dbde
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GPG Key ID: 4AEE18F83AFDEB23
3 changed files with 113 additions and 59 deletions
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125
source/opt/aggressive_dead_code_elim_pass.cpp
View File

@ -1,7 +1,7 @@
// Copyright (c) 2017 The Khronos Group Inc.
// Copyright (c) 2017 Valve Corporation
// Copyright (c) 2017 LunarG Inc.
// Copyright (c) 2018 Google LLC
// Copyright (c) 2018-2021 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
@ -23,6 +23,7 @@
#include "source/cfa.h"
#include "source/latest_version_glsl_std_450_header.h"
#include "source/opt/eliminate_dead_functions_util.h"
#include "source/opt/ir_builder.h"
#include "source/opt/iterator.h"
#include "source/opt/reflect.h"
#include "source/spirv_constant.h"
@ -166,14 +167,6 @@ bool AggressiveDCEPass::AllExtensionsSupported() const {
return true;
}
bool AggressiveDCEPass::IsDead(Instruction* inst) {
if (IsLive(inst)) return false;
if ((inst->IsBranch() || inst->opcode() == SpvOpUnreachable) &&
context()->get_instr_block(inst)->GetMergeInst() == nullptr)
return false;
return true;
}
bool AggressiveDCEPass::IsTargetDead(Instruction* inst) {
const uint32_t tId = inst->GetSingleWordInOperand(0);
Instruction* tInst = get_def_use_mgr()->GetDef(tId);
@ -190,7 +183,7 @@ bool AggressiveDCEPass::IsTargetDead(Instruction* inst) {
});
return dead;
}
return IsDead(tInst);
return !IsLive(tInst);
}
void AggressiveDCEPass::ProcessLoad(Function* func, uint32_t varId) {
@ -285,7 +278,7 @@ bool AggressiveDCEPass::KillDeadInstructions(
for (auto bi = structured_order.begin(); bi != structured_order.end();) {
uint32_t merge_block_id = 0;
(*bi)->ForEachInst([this, &modified, &merge_block_id](Instruction* inst) {
if (!IsDead(inst)) return;
if (IsLive(inst)) return;
if (inst->opcode() == SpvOpLabel) return;
// If dead instruction is selection merge, remember merge block
// for new branch at end of block
@ -323,6 +316,12 @@ bool AggressiveDCEPass::KillDeadInstructions(
live_insts_.Set(merge_terminator->unique_id());
}
} else {
Instruction* inst = (*bi)->terminator();
if (!IsLive(inst)) {
// If the terminator is not live, this block has no live instructions,
// and it will be unreachable.
AddUnreachable(*bi);
}
++bi;
}
}
@ -457,26 +456,38 @@ uint32_t AggressiveDCEPass::GetVariableId(uint32_t ptr_id) {
void AggressiveDCEPass::MarkBlockAsLive(Instruction* inst) {
BasicBlock* basic_block = context()->get_instr_block(inst);
if (basic_block != nullptr) {
AddToWorklist(basic_block->GetLabelInst());
if (basic_block == nullptr) {
return;
}
// If in a structured if or loop construct, add the controlling
// conditional branch and its merge.
BasicBlock* blk = context()->get_instr_block(inst);
Instruction* branchInst = GetHeaderBranch(blk);
if (branchInst != nullptr) {
AddToWorklist(branchInst);
Instruction* mergeInst = GetMergeInstruction(branchInst);
AddToWorklist(mergeInst);
// If we intend to keep this instruction, we need the block label and
// block terminator to have a valid block for the instruction.
AddToWorklist(basic_block->GetLabelInst());
// We need to mark the successors blocks that follow as live. If this is
// header of the merge construct, the construct may be folded, but we will
// definitely need the merge label. If it is not a construct, the terminator
// must be live, and the successor blocks will be marked as live when
// processing the terminator.
uint32_t merge_id = basic_block->MergeBlockIdIfAny();
if (merge_id == 0) {
AddToWorklist(basic_block->terminator());
} else {
AddToWorklist(context()->get_def_use_mgr()->GetDef(merge_id));
}
// If the block is a header, add the next outermost controlling
// conditional branch and its merge.
Instruction* nextBranchInst = GetBranchForNextHeader(blk);
if (nextBranchInst != nullptr) {
AddToWorklist(nextBranchInst);
Instruction* mergeInst = GetMergeInstruction(nextBranchInst);
// Mark the structured control flow constructs that contains this block as
// live. If |inst| is an instruction in the loop header, then it is part of
// the loop, so the loop construct must be live. We exclude the label because
// it does not matter how many times it is executed. This could be extended
// to more instructions, but we will need it for now.
if (inst->opcode() != SpvOpLabel)
MarkLoopConstructAsLiveIfLoopHeader(basic_block);
Instruction* next_branch_inst = GetBranchForNextHeader(basic_block);
if (next_branch_inst != nullptr) {
AddToWorklist(next_branch_inst);
Instruction* mergeInst = GetMergeInstruction(next_branch_inst);
AddToWorklist(mergeInst);
}
@ -485,14 +496,20 @@ void AggressiveDCEPass::MarkBlockAsLive(Instruction* inst) {
AddBreaksAndContinuesToWorklist(inst);
}
}
void AggressiveDCEPass::MarkLoopConstructAsLiveIfLoopHeader(
BasicBlock* basic_block) {
// If this is the header for a loop, then loop structure needs to keep as well
// because the loop header is also part of the loop.
Instruction* merge_inst = basic_block->GetLoopMergeInst();
if (merge_inst != nullptr) {
AddToWorklist(basic_block->terminator());
AddToWorklist(merge_inst);
}
}
void AggressiveDCEPass::AddOperandsToWorkList(const Instruction* inst) {
inst->ForEachInId([&inst, this](const uint32_t* iid) {
inst->ForEachInId([this](const uint32_t* iid) {
Instruction* inInst = get_def_use_mgr()->GetDef(*iid);
// Do not add label if an operand of a branch. This is not needed
// as part of live code discovery and can create false live code,
// for example, the branch to a header of a loop.
if (inInst->opcode() == SpvOpLabel && inst->IsBranch()) return;
AddToWorklist(inInst);
});
if (inst->type_id() != 0) {
@ -504,6 +521,7 @@ void AggressiveDCEPass::InitializeWorkList(
Function* func, std::list<BasicBlock*>& structured_order) {
AddToWorklist(&func->DefInst());
MarkFunctionParameterAsLive(func);
MarkFirstBlockAsLive(func);
// Add instructions with external side effects to the worklist. Also add
// branches that are not attached to a structured construct.
@ -512,6 +530,9 @@ void AggressiveDCEPass::InitializeWorkList(
for (auto& bi : structured_order) {
for (auto ii = bi->begin(); ii != bi->end(); ++ii) {
SpvOp op = ii->opcode();
if (ii->IsBranch()) {
continue;
}
switch (op) {
case SpvOpStore: {
uint32_t var_id = 0;
@ -526,19 +547,9 @@ void AggressiveDCEPass::InitializeWorkList(
(void)GetPtr(target_addr_id, &var_id);
if (!IsLocalVar(var_id, func)) AddToWorklist(&*ii);
} break;
case SpvOpSwitch:
case SpvOpBranch:
case SpvOpBranchConditional:
case SpvOpUnreachable: {
bool branch_related_to_construct =
(GetMergeInstruction(&*ii) == nullptr &&
GetHeaderBlock(context()->get_instr_block(&*ii)) == nullptr);
if (branch_related_to_construct) {
AddToWorklist(&*ii);
}
} break;
case SpvOpLoopMerge:
case SpvOpSelectionMerge:
case SpvOpUnreachable:
break;
default: {
// Function calls, atomics, function params, function returns, etc.
@ -762,7 +773,7 @@ bool AggressiveDCEPass::ProcessGlobalValues() {
uint32_t counter_buffer_id = annotation->GetSingleWordInOperand(2);
Instruction* counter_buffer_inst =
get_def_use_mgr()->GetDef(counter_buffer_id);
if (IsDead(counter_buffer_inst)) {
if (!IsLive(counter_buffer_inst)) {
context()->KillInst(annotation);
modified = true;
}
@ -777,7 +788,7 @@ bool AggressiveDCEPass::ProcessGlobalValues() {
for (uint32_t i = 1; i < annotation->NumOperands();) {
Instruction* opInst =
get_def_use_mgr()->GetDef(annotation->GetSingleWordOperand(i));
if (IsDead(opInst)) {
if (!IsLive(opInst)) {
// Don't increment |i|.
annotation->RemoveOperand(i);
modified = true;
@ -804,7 +815,7 @@ bool AggressiveDCEPass::ProcessGlobalValues() {
for (uint32_t i = 1; i < annotation->NumOperands();) {
Instruction* opInst =
get_def_use_mgr()->GetDef(annotation->GetSingleWordOperand(i));
if (IsDead(opInst)) {
if (!IsLive(opInst)) {
// Don't increment |i|.
annotation->RemoveOperand(i + 1);
annotation->RemoveOperand(i);
@ -838,13 +849,13 @@ bool AggressiveDCEPass::ProcessGlobalValues() {
}
for (auto& dbg : get_module()->ext_inst_debuginfo()) {
if (!IsDead(&dbg)) continue;
if (IsLive(&dbg)) continue;
// Save GlobalVariable if its variable is live, otherwise null out variable
// index
if (dbg.GetCommonDebugOpcode() == CommonDebugInfoDebugGlobalVariable) {
auto var_id = dbg.GetSingleWordOperand(kGlobalVariableVariableIndex);
Instruction* var_inst = get_def_use_mgr()->GetDef(var_id);
if (!IsDead(var_inst)) continue;
if (IsLive(var_inst)) continue;
context()->ForgetUses(&dbg);
dbg.SetOperand(
kGlobalVariableVariableIndex,
@ -859,7 +870,7 @@ bool AggressiveDCEPass::ProcessGlobalValues() {
// Since ADCE is disabled for non-shaders, we don't check for export linkage
// attributes here.
for (auto& val : get_module()->types_values()) {
if (IsDead(&val)) {
if (!IsLive(&val)) {
// Save forwarded pointer if pointer is live since closure does not mark
// this live as it does not have a result id. This is a little too
// conservative since it is not known if the structure type that needed
@ -867,7 +878,7 @@ bool AggressiveDCEPass::ProcessGlobalValues() {
if (val.opcode() == SpvOpTypeForwardPointer) {
uint32_t ptr_ty_id = val.GetSingleWordInOperand(0);
Instruction* ptr_ty_inst = get_def_use_mgr()->GetDef(ptr_ty_id);
if (!IsDead(ptr_ty_inst)) continue;
if (IsLive(ptr_ty_inst)) continue;
}
to_kill_.push_back(&val);
modified = true;
@ -886,7 +897,7 @@ bool AggressiveDCEPass::ProcessGlobalValues() {
} else {
auto* var =
get_def_use_mgr()->GetDef(entry.GetSingleWordInOperand(i));
if (!IsDead(var)) {
if (IsLive(var)) {
new_operands.push_back(entry.GetInOperand(i));
}
}
@ -1063,5 +1074,17 @@ bool AggressiveDCEPass::HasCall(Function* func) {
[](Instruction* inst) { return inst->opcode() != SpvOpFunctionCall; });
}
void AggressiveDCEPass::MarkFirstBlockAsLive(Function* func) {
BasicBlock* first_block = &*func->begin();
MarkBlockAsLive(first_block->GetLabelInst());
}
void AggressiveDCEPass::AddUnreachable(BasicBlock*& block) {
InstructionBuilder builder(
context(), block,
IRContext::kAnalysisInstrToBlockMapping | IRContext::kAnalysisDefUse);
builder.AddUnreachable();
}
} // namespace opt
} // namespace spvtools

13
source/opt/aggressive_dead_code_elim_pass.h
View File

@ -77,9 +77,6 @@ class AggressiveDCEPass : public MemPass {
return live_insts_.Get(inst->unique_id());
}
// Returns true if |inst| is dead.
bool IsDead(Instruction* inst);
// Adds entry points, execution modes and workgroup size decorations to the
// worklist for processing with the first function.
void InitializeModuleScopeLiveInstructions();
@ -217,6 +214,16 @@ class AggressiveDCEPass : public MemPass {
// Returns true if |func| contains a function call.
bool HasCall(Function* func);
// Marks the first block, which is the entry block, in |func| as live.
void MarkFirstBlockAsLive(Function* func);
// Adds an OpUnreachable instruction at the end of |block|.
void AddUnreachable(BasicBlock*& block);
// Marks the OpLoopMerge and the terminator in |basic_block| as live if
// |basic_block| is a loop header.
void MarkLoopConstructAsLiveIfLoopHeader(BasicBlock* basic_block);
// The cached results for |IsEntryPointWithNoCalls|. It maps the function's
// result id to the return value.
std::unordered_map<uint32_t, bool> entry_point_with_no_calls_cache_;

34
test/opt/aggressive_dead_code_elim_test.cpp
View File

@ -8008,11 +8008,35 @@ OpFunctionEnd
EXPECT_EQ(text, std::get<0>(result));
}
// TODO(greg-lunarg): Add tests to verify handling of these cases:
//
// Check that logical addressing required
// Check that function calls inhibit optimization
// Others?
TEST_F(AggressiveDCETest, EmptyContinueWithConditionalBranch) {
const std::string text = R"(OpCapability Shader
%1 = OpExtInstImport "GLSL.std.450"
OpMemoryModel Logical GLSL450
OpEntryPoint Fragment %2 "main"
OpExecutionMode %2 OriginUpperLeft
%void = OpTypeVoid
%4 = OpTypeFunction %void
%bool = OpTypeBool
%false = OpConstantFalse %bool
%2 = OpFunction %void None %4
%9 = OpLabel
OpBranch %10
%10 = OpLabel
OpLoopMerge %11 %12 None
OpBranch %13
%13 = OpLabel
OpKill
%12 = OpLabel
OpBranchConditional %false %10 %10
%11 = OpLabel
OpUnreachable
OpFunctionEnd
)";
SetTargetEnv(SPV_ENV_VULKAN_1_2);
SetAssembleOptions(SPV_TEXT_TO_BINARY_OPTION_PRESERVE_NUMERIC_IDS);
SinglePassRunAndCheck<AggressiveDCEPass>(text, text, false);
}
} // namespace
} // namespace opt