Add unrolling to the legalization passes (#1903)

Adds unrolling to the legalization passes.

After enabling unrolling I found a bug when there is a self-referencing
phi node.  That has been fixed.

The test that checks for that the order of optimizations is correct also
needed to be updated.

source/opt/loop_unroller.cpp

@@ -244,6 +244,10 @@ class LoopUnrollerUtilsImpl {
   // ect).
   void AssignNewResultIds(BasicBlock* basic_block);
 
+  // Using the map built by AssignNewResultIds, replace the uses in |inst|
+  // by the id that the use maps to.
+  void RemapOperands(Instruction* inst);
+
   // Using the map built by AssignNewResultIds, for each instruction in
   // |basic_block| use
   // that map to substitute the IDs used by instructions (in the operands) with
@@ -757,6 +761,11 @@ void LoopUnrollerUtilsImpl::CloseUnrolledLoop(Loop* loop) {
   for (BasicBlock* block : loop_blocks_inorder_) {
     RemapOperands(block);
   }
+
+  // Rewrite the last phis, since they may still reference the original phi.
+  for (Instruction* last_phi : state_.previous_phis_) {
+    RemapOperands(last_phi);
+  }
 }
 
 // Uses the first loop to create a copy of the loop with new IDs.
@@ -842,10 +851,7 @@ void LoopUnrollerUtilsImpl::AssignNewResultIds(BasicBlock* basic_block) {
   }
 }
 
-// For all instructions in |basic_block| check if the operands used are from a
-// copied instruction and if so swap out the operand for the copy of it.
-void LoopUnrollerUtilsImpl::RemapOperands(BasicBlock* basic_block) {
-  for (Instruction& inst : *basic_block) {
+void LoopUnrollerUtilsImpl::RemapOperands(Instruction* inst) {
   auto remap_operands_to_new_ids = [this](uint32_t* id) {
     auto itr = state_.new_inst.find(*id);
 
@@ -854,7 +860,12 @@ void LoopUnrollerUtilsImpl::RemapOperands(BasicBlock* basic_block) {
     }
   };
 
-    inst.ForEachInId(remap_operands_to_new_ids);
+  inst->ForEachInId(remap_operands_to_new_ids);
+}
+
+void LoopUnrollerUtilsImpl::RemapOperands(BasicBlock* basic_block) {
+  for (Instruction& inst : *basic_block) {
+    RemapOperands(&inst);
   }
 }
 

source/opt/optimizer.cpp

@@ -131,6 +131,7 @@ Optimizer& Optimizer::RegisterLegalizationPasses() {
           // Propagate constants to get as many constant conditions on branches
           // as possible.
           .RegisterPass(CreateCCPPass())
+          .RegisterPass(CreateLoopUnrollPass(true))
           .RegisterPass(CreateDeadBranchElimPass())
           // Copy propagate members.  Cleans up code sequences generated by
           // scalar replacement.  Also important for removing OpPhi nodes.

test/opt/loop_optimizations/unroll_simple.cpp

@@ -2952,6 +2952,52 @@ OpFunctionEnd
   EXPECT_NE(loop_2.GetLatchBlock(), loop_2.GetContinueBlock());
 }
 
+// Test that a loop with a self-referencing OpPhi instruction is handled
+// correctly.
+TEST_F(PassClassTest, OpPhiSelfReference) {
+  const std::string text = R"(
+  ; Find the two adds from the unrolled loop
+  ; CHECK: OpIAdd
+  ; CHECK: OpIAdd
+  ; CHECK: OpIAdd %uint %uint_0 %uint_1
+  ; CHECK-NEXT: OpReturn
+          OpCapability Shader
+     %1 = OpExtInstImport "GLSL.std.450"
+          OpMemoryModel Logical GLSL450
+          OpEntryPoint GLCompute %2 "main"
+          OpExecutionMode %2 LocalSize 8 8 1
+          OpSource HLSL 600
+  %uint = OpTypeInt 32 0
+  %void = OpTypeVoid
+     %5 = OpTypeFunction %void
+%uint_0 = OpConstant %uint 0
+%uint_1 = OpConstant %uint 1
+  %bool = OpTypeBool
+  %true = OpConstantTrue %bool
+     %2 = OpFunction %void None %5
+    %10 = OpLabel
+          OpBranch %19
+    %19 = OpLabel
+    %20 = OpPhi %uint %uint_0 %10 %20 %21
+    %22 = OpPhi %uint %uint_0 %10 %23 %21
+    %24 = OpULessThanEqual %bool %22 %uint_1
+          OpLoopMerge %25 %21 Unroll
+          OpBranchConditional %24 %21 %25
+    %21 = OpLabel
+    %23 = OpIAdd %uint %22 %uint_1
+          OpBranch %19
+    %25 = OpLabel
+    %14 = OpIAdd %uint %20 %uint_1
+          OpReturn
+          OpFunctionEnd
+  )";
+
+  const bool kFullyUnroll = true;
+  const uint32_t kUnrollFactor = 0;
+  SinglePassRunAndMatch<opt::LoopUnroller>(text, true, kFullyUnroll,
+                                           kUnrollFactor);
+}
+
 }  // namespace
 }  // namespace opt
 }  // namespace spvtools

test/tools/opt/flags.py

@@ -235,6 +235,7 @@ class TestLegalizationPasses(expect.ValidObjectFile1_3,
       'eliminate-local-multi-store',
       'eliminate-dead-code-aggressive',
       'ccp',
+      'loop-unroll',
       'eliminate-dead-branches',
       'simplify-instructions',
       'eliminate-dead-code-aggressive',