[DirectX] Legalize Freeze instruction (#136043)

fixes #135719

LLVM 3.7 did not have a freeze instruction
Further this instruction is really only used as syntactic sugar
in LLVM's optimizer passes to not aggressively optimize things that
could be undef or poison ie x*2 to x+x.
Most backends treat it as a no-op so we will do the same
by removing it and replacing its uses with its input.

Author: farzonl

llvm/lib/Target/DirectX/DXILLegalizePass.cpp

@@ -20,6 +20,17 @@
 
 using namespace llvm;
 
+static void legalizeFreeze(Instruction &I,
+                           SmallVectorImpl<Instruction *> &ToRemove,
+                           DenseMap<Value *, Value *>) {
+  auto *FI = dyn_cast<FreezeInst>(&I);
+  if (!FI)
+    return;
+
+  FI->replaceAllUsesWith(FI->getOperand(0));
+  ToRemove.push_back(FI);
+}
+
 static void fixI8TruncUseChain(Instruction &I,
                                SmallVectorImpl<Instruction *> &ToRemove,
                                DenseMap<Value *, Value *> &ReplacedValues) {
@@ -169,6 +180,7 @@ class DXILLegalizationPipeline {
   void initializeLegalizationPipeline() {
     LegalizationPipeline.push_back(fixI8TruncUseChain);
     LegalizationPipeline.push_back(downcastI64toI32InsertExtractElements);
+    LegalizationPipeline.push_back(legalizeFreeze);
   }
 };
 

llvm/test/CodeGen/DirectX/legalize-freeze.ll

@@ -0,0 +1,17 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+
+; RUN: opt -S -passes='dxil-legalize' -mtriple=dxil-pc-shadermodel6.3-library %s | FileCheck %s
+
+
+define i32 @test_remove_freeze(i32 %x) {
+; CHECK-LABEL: define i32 @test_remove_freeze(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    [[Y:%.*]] = add i32 [[X]], 1
+; CHECK-NEXT:    ret i32 [[Y]]
+;
+entry:
+  %f = freeze i32 %x
+  %y = add i32 %f, 1
+  ret i32 %y
+}