[InstCombine] Fix frexp(frexp(x)) -> frexp(x) fold

[el-ev]

Fixes #138819

When frexp is applied twice, the second result should be zero.

[llvmbot]

@llvm/pr-subscribers-backend-amdgpu
@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: Iris Shi (el-ev)

Changes

Fixes #138819

When frexp is applied twice, the second result should be zero.

---

Full diff: https://github.com/llvm/llvm-project/pull/138837.diff

3 Files Affected:

• (modified) llvm/lib/Analysis/InstructionSimplify.cpp (-9)
• (modified) llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp (+12)
• (renamed) llvm/test/Transforms/InstCombine/frexp.ll (+7-4)

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 5a2943de9066e..85e3be9cc45c3 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -6377,15 +6377,6 @@ static Value *simplifyUnaryIntrinsic(Function *F, Value *Op0,
     if (isSplatValue(Op0))
       return Op0;
     break;
-  case Intrinsic::frexp: {
-    // Frexp is idempotent with the added complication of the struct return.
-    if (match(Op0, m_ExtractValue<0>(m_Value(X)))) {
-      if (match(X, m_Intrinsic<Intrinsic::frexp>(m_Value())))
-        return X;
-    }
-
-    break;
-  }
   default:
     break;
   }
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 3e78b20e41f0d..624c97cf39de3 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -3799,6 +3799,18 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     }
     break;
   }
+  case Intrinsic::frexp: {
+    Value *X;
+    // Frexp is idempotent with the added complication of the struct return.
+    if (match(II->getArgOperand(0), m_ExtractValue<0>(m_Value(X)))) {
+      if (match(X, m_Intrinsic<Intrinsic::frexp>(m_Value()))) {
+        X = Builder.CreateInsertValue(
+            X, ConstantInt::get(II->getType()->getStructElementType(1), 0), 1);
+        return replaceInstUsesWith(*II, X);
+      }
+    }
+    break;
+  }
   default: {
     // Handle target specific intrinsics
     std::optional<Instruction *> V = targetInstCombineIntrinsic(*II);
diff --git a/llvm/test/Transforms/InstSimplify/frexp.ll b/llvm/test/Transforms/InstCombine/frexp.ll
similarity index 96%
rename from llvm/test/Transforms/InstSimplify/frexp.ll
rename to llvm/test/Transforms/InstCombine/frexp.ll
index 34cfce92bac43..6541f0d77a093 100644
--- a/llvm/test/Transforms/InstSimplify/frexp.ll
+++ b/llvm/test/Transforms/InstCombine/frexp.ll
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 3
-; RUN: opt -S -passes=instsimplify %s | FileCheck %s
+; RUN: opt -S -passes=instcombine %s | FileCheck %s
 
 declare { float, i32 } @llvm.frexp.f32.i32(float)
 declare { <2 x float>, <2 x i32> } @llvm.frexp.v2f32.v2i32(<2 x float>)
@@ -12,7 +12,8 @@ define { float, i32 } @frexp_frexp(float %x) {
 ; CHECK-LABEL: define { float, i32 } @frexp_frexp(
 ; CHECK-SAME: float [[X:%.*]]) {
 ; CHECK-NEXT:    [[FREXP0:%.*]] = call { float, i32 } @llvm.frexp.f32.i32(float [[X]])
-; CHECK-NEXT:    ret { float, i32 } [[FREXP0]]
+; CHECK-NEXT:    [[FREXP1:%.*]] = insertvalue { float, i32 } [[FREXP0]], i32 0, 1
+; CHECK-NEXT:    ret { float, i32 } [[FREXP1]]
 ;
   %frexp0 = call { float, i32 } @llvm.frexp.f32.i32(float %x)
   %frexp0.0 = extractvalue { float, i32 } %frexp0, 0
@@ -24,7 +25,8 @@ define { <2 x float>, <2 x i32> } @frexp_frexp_vector(<2 x float> %x) {
 ; CHECK-LABEL: define { <2 x float>, <2 x i32> } @frexp_frexp_vector(
 ; CHECK-SAME: <2 x float> [[X:%.*]]) {
 ; CHECK-NEXT:    [[FREXP0:%.*]] = call { <2 x float>, <2 x i32> } @llvm.frexp.v2f32.v2i32(<2 x float> [[X]])
-; CHECK-NEXT:    ret { <2 x float>, <2 x i32> } [[FREXP0]]
+; CHECK-NEXT:    [[FREXP1:%.*]] = insertvalue { <2 x float>, <2 x i32> } [[FREXP0]], <2 x i32> zeroinitializer, 1
+; CHECK-NEXT:    ret { <2 x float>, <2 x i32> } [[FREXP1]]
 ;
   %frexp0 = call { <2 x float>, <2 x i32> } @llvm.frexp.v2f32.v2i32(<2 x float> %x)
   %frexp0.0 = extractvalue { <2 x float>, <2 x i32> } %frexp0, 0
@@ -47,7 +49,8 @@ define { <vscale x 2 x float>, <vscale x 2 x i32> } @frexp_frexp_scalable_vector
 ; CHECK-LABEL: define { <vscale x 2 x float>, <vscale x 2 x i32> } @frexp_frexp_scalable_vector(
 ; CHECK-SAME: <vscale x 2 x float> [[X:%.*]]) {
 ; CHECK-NEXT:    [[FREXP0:%.*]] = call { <vscale x 2 x float>, <vscale x 2 x i32> } @llvm.frexp.nxv2f32.nxv2i32(<vscale x 2 x float> [[X]])
-; CHECK-NEXT:    ret { <vscale x 2 x float>, <vscale x 2 x i32> } [[FREXP0]]
+; CHECK-NEXT:    [[FREXP1:%.*]] = insertvalue { <vscale x 2 x float>, <vscale x 2 x i32> } [[FREXP0]], <vscale x 2 x i32> zeroinitializer, 1
+; CHECK-NEXT:    ret { <vscale x 2 x float>, <vscale x 2 x i32> } [[FREXP1]]
 ;
   %frexp0 = call { <vscale x 2 x float>, <vscale x 2 x i32> } @llvm.frexp.nxv2f32.nxv2i32(<vscale x 2 x float> %x)
   %frexp0.0 = extractvalue { <vscale x 2 x float>, <vscale x 2 x i32> } %frexp0, 0

[github-actions]

⚠️ undef deprecator found issues in your code. ⚠️

You can test this locally with the following command:

git diff -U0 --pickaxe-regex -S '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)' 'HEAD~1' HEAD llvm/lib/Analysis/InstructionSimplify.cpp llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp llvm/test/CodeGen/AMDGPU/frexp-constant-fold.ll llvm/test/Transforms/InstCombine/frexp.ll

The following files introduce new uses of undef:

• llvm/test/Transforms/InstCombine/frexp.ll

Undef is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields undef. You should use poison values for placeholders instead.

In tests, avoid using undef and having tests that trigger undefined behavior. If you need an operand with some unimportant value, you can add a new argument to the function and use that instead.

For example, this is considered a bad practice:

define void @fn() {
  ...
  br i1 undef, ...
}

Please use the following instead:

define void @fn(i1 %cond) {
  ...
  br i1 %cond, ...
}

Please refer to the Undefined Behavior Manual for more information.