[DirectX] Legalize Freeze instruction

[farzonl]

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.

[llvmbot]

@llvm/pr-subscribers-backend-directx

Author: Farzon Lotfi (farzonl)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/136043.diff

2 Files Affected:

• (modified) llvm/lib/Target/DirectX/DXILLegalizePass.cpp (+14)
• (added) llvm/test/CodeGen/DirectX/legalize-freeze.ll (+17)

diff --git a/llvm/lib/Target/DirectX/DXILLegalizePass.cpp b/llvm/lib/Target/DirectX/DXILLegalizePass.cpp
index 395311e430fbb..c133e59d2e3c6 100644
--- a/llvm/lib/Target/DirectX/DXILLegalizePass.cpp
+++ b/llvm/lib/Target/DirectX/DXILLegalizePass.cpp
@@ -8,6 +8,7 @@
 
 #include "DXILLegalizePass.h"
 #include "DirectX.h"
+#include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstIterator.h"
@@ -20,6 +21,18 @@
 
 using namespace llvm;
 
+static void legalizeFreeze(Instruction &I,
+                           SmallVectorImpl<Instruction *> &ToRemove,
+                           DenseMap<Value *, Value *>) {
+  auto *FI = dyn_cast<FreezeInst>(&I);
+  if (!FI)
+    return;
+
+  Value *Input = FI->getOperand(0);
+  FI->replaceAllUsesWith(Input);
+  ToRemove.push_back(FI);
+}
+
 static void fixI8TruncUseChain(Instruction &I,
                                SmallVectorImpl<Instruction *> &ToRemove,
                                DenseMap<Value *, Value *> &ReplacedValues) {
@@ -169,6 +182,7 @@ class DXILLegalizationPipeline {
   void initializeLegalizationPipeline() {
     LegalizationPipeline.push_back(fixI8TruncUseChain);
     LegalizationPipeline.push_back(downcastI64toI32InsertExtractElements);
+    LegalizationPipeline.push_back(legalizeFreeze);
   }
 };
 
diff --git a/llvm/test/CodeGen/DirectX/legalize-freeze.ll b/llvm/test/CodeGen/DirectX/legalize-freeze.ll
new file mode 100644
index 0000000000000..29446adeef215
--- /dev/null
+++ b/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
+}