[LLVM][InstCombine] Preserve vector types when shrinking FP constants. #163598

paulwalker-arm · 2025-10-15T17:19:12Z

While my objective is to make the shrinkfp path safe for ConstantFP based splats I discovered the following issues also affect ConstantVector based splats:

PreferBFloat is not set for bfloat vectors.
getMinimumFPType() returns a scalar type for vector constants where getSplatValue() is successful.

Please let me know if you rather I upstream those fixes separate from the use-constant-fp-for-fixed-length-splat support, which is mainly just refactoring shrinkFPConstant.

While my original objective was to make the shrinkfp path safe for ConstantFP based splats I discovered the following issues for ConstantVector based splats: 1. PreferBFloat is not set for bfloat vectors. 2. getMinimumFPType() returns a scalar type for vector constants where getSplatValue() is successful. Please let me know if you rather me upstream those fixes seperate from the use-constant-fp-for-fixed-length-splat support.

llvmbot · 2025-10-15T17:20:01Z

@llvm/pr-subscribers-llvm-transforms

Author: Paul Walker (paulwalker-arm)

Changes

While my objective is to make the shrinkfp path safe for ConstantFP based splats I discovered the following issues also affect ConstantVector based splats:

PreferBFloat is not set for bfloat vectors.
getMinimumFPType() returns a scalar type for vector constants where getSplatValue() is successful.

Please let me know if you rather I upstream those fixes separate from the use-constant-fp-for-fixed-length-splat support, which is mainly just refactoring shrinkFPConstant.

Full diff: https://github.com/llvm/llvm-project/pull/163598.diff

2 Files Affected:

(modified) llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp (+31-22)
(modified) llvm/test/Transforms/InstCombine/fpextend.ll (+23)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 4c9b10a094981..46857b7f66bfe 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1643,33 +1643,43 @@ Instruction *InstCombinerImpl::visitSExt(SExtInst &Sext) {
 
 /// Return a Constant* for the specified floating-point constant if it fits
 /// in the specified FP type without changing its value.
-static bool fitsInFPType(ConstantFP *CFP, const fltSemantics &Sem) {
+static bool fitsInFPType(APFloat F, const fltSemantics &Sem) {
   bool losesInfo;
-  APFloat F = CFP->getValueAPF();
   (void)F.convert(Sem, APFloat::rmNearestTiesToEven, &losesInfo);
   return !losesInfo;
 }
 
-static Type *shrinkFPConstant(ConstantFP *CFP, bool PreferBFloat) {
-  if (CFP->getType() == Type::getPPC_FP128Ty(CFP->getContext()))
-    return nullptr;  // No constant folding of this.
+static Type *shrinkFPConstant(LLVMContext &Ctx, APFloat F, bool PreferBFloat) {
   // See if the value can be truncated to bfloat and then reextended.
-  if (PreferBFloat && fitsInFPType(CFP, APFloat::BFloat()))
-    return Type::getBFloatTy(CFP->getContext());
+  if (PreferBFloat && fitsInFPType(F, APFloat::BFloat()))
+    return Type::getBFloatTy(Ctx);
   // See if the value can be truncated to half and then reextended.
-  if (!PreferBFloat && fitsInFPType(CFP, APFloat::IEEEhalf()))
-    return Type::getHalfTy(CFP->getContext());
+  if (!PreferBFloat && fitsInFPType(F, APFloat::IEEEhalf()))
+    return Type::getHalfTy(Ctx);
   // See if the value can be truncated to float and then reextended.
-  if (fitsInFPType(CFP, APFloat::IEEEsingle()))
-    return Type::getFloatTy(CFP->getContext());
-  if (CFP->getType()->isDoubleTy())
-    return nullptr;  // Won't shrink.
-  if (fitsInFPType(CFP, APFloat::IEEEdouble()))
-    return Type::getDoubleTy(CFP->getContext());
-  // Don't try to shrink to various long double types.
+  if (fitsInFPType(F, APFloat::IEEEsingle()))
+    return Type::getFloatTy(Ctx);
+  // See if the value can be truncated to double and then reextended.
+  if (fitsInFPType(F, APFloat::IEEEdouble()))
+    return Type::getDoubleTy(Ctx);
+  // Does not shrink.
   return nullptr;
 }
 
+static Type *shrinkFPConstant(ConstantFP *CFP, bool PreferBFloat) {
+  Type *Ty = CFP->getType();
+  if (Ty->getScalarType() == Type::getPPC_FP128Ty(CFP->getContext()))
+    return nullptr; // No constant folding of this.
+
+  Type *ShrinkTy =
+      shrinkFPConstant(CFP->getContext(), CFP->getValueAPF(), PreferBFloat);
+  if (auto *VecTy = dyn_cast<VectorType>(Ty))
+    ShrinkTy = VectorType::get(ShrinkTy, VecTy);
+
+  // Does it shrink?
+  return ShrinkTy != Ty ? ShrinkTy : nullptr;
+}
+
 // Determine if this is a vector of ConstantFPs and if so, return the minimal
 // type we can safely truncate all elements to.
 static Type *shrinkFPConstantVector(Value *V, bool PreferBFloat) {
@@ -1720,10 +1730,10 @@ static Type *getMinimumFPType(Value *V, bool PreferBFloat) {
 
   // Try to shrink scalable and fixed splat vectors.
   if (auto *FPC = dyn_cast<Constant>(V))
-    if (isa<VectorType>(V->getType()))
+    if (auto *VTy = dyn_cast<VectorType>(V->getType()))
       if (auto *Splat = dyn_cast_or_null<ConstantFP>(FPC->getSplatValue()))
         if (Type *T = shrinkFPConstant(Splat, PreferBFloat))
-          return T;
+          return VectorType::get(T, VTy);
 
   // Try to shrink a vector of FP constants. This returns nullptr on scalable
   // vectors
@@ -1796,10 +1806,9 @@ Instruction *InstCombinerImpl::visitFPTrunc(FPTruncInst &FPT) {
   Type *Ty = FPT.getType();
   auto *BO = dyn_cast<BinaryOperator>(FPT.getOperand(0));
   if (BO && BO->hasOneUse()) {
-    Type *LHSMinType =
-        getMinimumFPType(BO->getOperand(0), /*PreferBFloat=*/Ty->isBFloatTy());
-    Type *RHSMinType =
-        getMinimumFPType(BO->getOperand(1), /*PreferBFloat=*/Ty->isBFloatTy());
+    bool PreferBFloat = Ty->getScalarType()->isBFloatTy();
+    Type *LHSMinType = getMinimumFPType(BO->getOperand(0), PreferBFloat);
+    Type *RHSMinType = getMinimumFPType(BO->getOperand(1), PreferBFloat);
     unsigned OpWidth = BO->getType()->getFPMantissaWidth();
     unsigned LHSWidth = LHSMinType->getFPMantissaWidth();
     unsigned RHSWidth = RHSMinType->getFPMantissaWidth();
diff --git a/llvm/test/Transforms/InstCombine/fpextend.ll b/llvm/test/Transforms/InstCombine/fpextend.ll
index 9125339c00ecf..076a5280eaf63 100644
--- a/llvm/test/Transforms/InstCombine/fpextend.ll
+++ b/llvm/test/Transforms/InstCombine/fpextend.ll
@@ -1,5 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+; RUN: opt < %s -passes=instcombine --use-constant-fp-for-fixed-length-splat -S | FileCheck %s
 
 define float @test(float %x) nounwind  {
 ; CHECK-LABEL: @test(
@@ -449,6 +450,28 @@ define bfloat @bf16_frem(bfloat %x) {
   ret bfloat %t3
 }
 
+define <4 x bfloat> @v4bf16_frem_x_const(<4 x bfloat> %x) {
+; CHECK-LABEL: @v4bf16_frem_x_const(
+; CHECK-NEXT:    [[TMP1:%.*]] = frem <4 x bfloat> [[X:%.*]], splat (bfloat 0xR40C9)
+; CHECK-NEXT:    ret <4 x bfloat> [[TMP1]]
+;
+  %t1 = fpext <4 x bfloat> %x to <4 x float>
+  %t2 = frem <4 x float> %t1, splat(float 6.281250e+00)
+  %t3 = fptrunc <4 x float> %t2 to <4 x bfloat>
+  ret <4 x bfloat> %t3
+}
+
+define <4 x bfloat> @v4bf16_frem_const_x(<4 x bfloat> %x) {
+; CHECK-LABEL: @v4bf16_frem_const_x(
+; CHECK-NEXT:    [[TMP1:%.*]] = frem <4 x bfloat> splat (bfloat 0xR40C9), [[X:%.*]]
+; CHECK-NEXT:    ret <4 x bfloat> [[TMP1]]
+;
+  %t1 = fpext <4 x bfloat> %x to <4 x float>
+  %t2 = frem <4 x float> splat(float 6.281250e+00), %t1
+  %t3 = fptrunc <4 x float> %t2 to <4 x bfloat>
+  ret <4 x bfloat> %t3
+}
+
 define <4 x float> @v4f32_fadd(<4 x float> %a) {
 ; CHECK-LABEL: @v4f32_fadd(
 ; CHECK-NEXT:    [[TMP1:%.*]] = fadd <4 x float> [[A:%.*]], splat (float -1.000000e+00)

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

nikic · 2025-10-15T19:53:01Z

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

      if (auto *Splat = dyn_cast_or_null<ConstantFP>(FPC->getSplatValue()))
        if (Type *T = shrinkFPConstant(Splat, PreferBFloat))
-          return T;
+          return VectorType::get(T, VTy);


This looks right to me, but I don't really get how this worked previously. E.g. v4f32_fadd has a vector splat, why did that work?

Only the Instruction::FRem handling uses getMinimumFPType()'s return type and even then the bug only occurs when LHS is the vector splat of which there were no tests.

dtcxzyw · 2025-10-16T17:42:53Z

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

+  Type *ShrinkTy =
+      shrinkFPConstant(CFP->getContext(), CFP->getValueAPF(), PreferBFloat);
+  if (auto *VecTy = dyn_cast<VectorType>(Ty))
+    ShrinkTy = VectorType::get(ShrinkTy, VecTy);


ShrinkTy may be null.

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

… for unshrinkable constants.

nikic

LGTM

dtcxzyw

LG

llvm#163598) While my objective is to make the shrinkfp path safe for ConstantFP based splats I discovered the following issues also affect ConstantVector based splats: 1. PreferBFloat is not set for bfloat vectors. 2. getMinimumFPType() returns a scalar type for vector constants where getSplatValue() is successful.

paulwalker-arm requested a review from david-arm October 15, 2025 17:19

paulwalker-arm requested a review from nikic as a code owner October 15, 2025 17:19

llvmbot added llvm:instcombine Covers the InstCombine, InstSimplify and AggressiveInstCombine passes llvm:transforms labels Oct 15, 2025

nikic reviewed Oct 15, 2025

View reviewed changes

Use result of shrinkFPConstant directly and shorten PPC_FP128 check.

b6765f8

dtcxzyw reviewed Oct 16, 2025

View reviewed changes

Prevent APFloat copy when calling shrinkFPConstant, fix vector return…

db09782

… for unshrinkable constants.

nikic approved these changes Oct 17, 2025

View reviewed changes

dtcxzyw approved these changes Oct 17, 2025

View reviewed changes

paulwalker-arm merged commit bdcbc1e into llvm:main Oct 21, 2025
8 of 10 checks passed

paulwalker-arm deleted the vector-constants-shrink-fp branch October 21, 2025 10:55

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[LLVM][InstCombine] Preserve vector types when shrinking FP constants. #163598

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