[llvm-branch-commits] [llvm] Add constrained fadd to FP operations (PR #136499)

[Serge Pavlov via llvm-branch-commits]

https://github.com/spavloff created https://github.com/llvm/llvm-project/pull/136499

Constrained intrinsics are treated as regular intrinsicss from the perspective of FP operand bundle mechanism, meaning they can have FP operand bundles. The FP bundle API functions will report properties based on the operand bundles rather than metadata arguments.

These hybrid entities are temporary objects that exist while both alternative mechanisms remain available. Nevertheless, they can be useful for development:

* They can represent FP instructions with bundles. For example, while there is currently no intrinsic that directly represents 'fadd' instruction (and bundles cannot be attached to a plain 'fadd' since it is not a call), a corresponding constrained intrinsic call can represent the operation while still carrying operand bundles.

* Constrained intrinsic with bundles can be assigned properties that otherwise cannot be expressed, such as denormal behavior.

This change adds 'experimental.constrained.fadd' to the list of FP operations, enabling creation of tests for denormal behavior. It also modifies methods `getRoundingMode` and `getExceptionBehavior` so that they report information from metadata arguments if bundles are absent. This adjustment should help incorporating constrained intrinsics into algorithms based on operand bundles.

>From 1e6b6bfcc612d400761b8f8e0d56530cb6d695ed Mon Sep 17 00:00:00 2001
From: Serge Pavlov <sepavloff at gmail.com>
Date: Thu, 17 Apr 2025 15:28:01 +0700
Subject: [PATCH] Add constrained fadd to FP operations

Constrained intrinsics are treated as regular intrinsicss from the
perspective of FP operand bundle mechanism, meaning they can have FP
operand bundles. The FP bundle API functions will report properties
based on the operand bundles rather than metadata arguments.

These hybrid entities are temporary objects that exist while both
alternative mechanisms remain available. Nevertheless, they can be
useful for development:

* They can represent FP instructions with bundles. For example, while
  there is currently no intrinsic that directly represents 'fadd'
  instruction (and bundles cannot be attached to a plain 'fadd' since it
  is not a call), a corresponding constrained intrinsic call can
  represent the operation while still carrying operand bundles.

* Constrained intrinsic with bundles can be assigned properties that
  otherwise cannot be expressed, such as denormal behavior.

This change adds 'experimental.constrained.fadd' to the list of FP
operations, enabling creation of tests for denormal behavior. It also
modifies methods `getRoundingMode` and `getExceptionBehavior` so that
they report information from metadata arguments if bundles are absent.
This adjustment should help incorporating constrained intrinsics into
algorithms based on operand bundles.
---
 llvm/include/llvm/IR/FloatingPointOps.def |  1 +
 llvm/lib/IR/Instructions.cpp              | 10 ++++
 llvm/unittests/IR/IRBuilderTest.cpp       | 65 +++++++++++++++++++++++
 3 files changed, 76 insertions(+)

diff --git a/llvm/include/llvm/IR/FloatingPointOps.def b/llvm/include/llvm/IR/FloatingPointOps.def
index 8567b5dbac302..468227e648300 100644
--- a/llvm/include/llvm/IR/FloatingPointOps.def
+++ b/llvm/include/llvm/IR/FloatingPointOps.def
@@ -18,6 +18,7 @@
 // - intrinsic function name,
 // - DAG node corresponding to the intrinsic.
 
+FUNCTION(experimental_constrained_fadd,      FADD)
 FUNCTION(nearbyint,                          FNEARBYINT)
 FUNCTION(trunc,                              FTRUNC)
 
diff --git a/llvm/lib/IR/Instructions.cpp b/llvm/lib/IR/Instructions.cpp
index 0e362d79493c1..1073c2939a8a3 100644
--- a/llvm/lib/IR/Instructions.cpp
+++ b/llvm/lib/IR/Instructions.cpp
@@ -636,6 +636,11 @@ RoundingMode CallBase::getRoundingMode() const {
   if (RM)
     return *RM;
 
+  // If this is a constrained intrinsic, get rounding mode from its metadata
+  // arguments.
+  if (auto *CI = dyn_cast<ConstrainedFPIntrinsic>(this))
+    return CI->getRoundingMode().value_or(RoundingMode::Dynamic);
+
   // No FP bundle, try to guess from the current mode.
   if (getParent())
     if (auto *F = getFunction(); F)
@@ -658,6 +663,11 @@ fp::ExceptionBehavior CallBase::getExceptionBehavior() const {
   if (EB)
     return *EB;
 
+  // If this is a constrained intrinsic, get exception behavior from its
+  // metadata arguments.
+  if (auto *CI = dyn_cast<ConstrainedFPIntrinsic>(this))
+    return CI->getExceptionBehavior().value_or(fp::ebStrict);
+
   // No FP bundle, try to guess from the current mode.
   if (getParent())
     if (auto *F = getFunction(); F)
diff --git a/llvm/unittests/IR/IRBuilderTest.cpp b/llvm/unittests/IR/IRBuilderTest.cpp
index 48a76c9cd586d..660dc453a62fb 100644
--- a/llvm/unittests/IR/IRBuilderTest.cpp
+++ b/llvm/unittests/IR/IRBuilderTest.cpp
@@ -662,6 +662,71 @@ TEST_F(IRBuilderTest, FPBundlesStrict) {
   }
 }
 
+TEST_F(IRBuilderTest, FPBundlesConstrained) {
+  F->addFnAttr(Attribute::StrictFP);
+
+  IRBuilder<> Builder(BB);
+  LLVMContext &Context = Builder.getContext();
+  Builder.setDefaultConstrainedExcept(fp::ebStrict);
+  Builder.setDefaultConstrainedRounding(RoundingMode::TowardZero);
+  Builder.setIsFPConstrained(true);
+
+  GlobalVariable *GVDouble = new GlobalVariable(
+      *M, Type::getDoubleTy(Ctx), true, GlobalValue::ExternalLinkage, nullptr);
+  Value *FnArg = Builder.CreateLoad(GVDouble->getValueType(), GVDouble);
+  Function *Fn = Intrinsic::getOrInsertDeclaration(
+      M.get(), Intrinsic::experimental_constrained_fadd,
+      {Type::getDoubleTy(Ctx)});
+
+  const auto createConstrainedRounding = [&](RoundingMode RM) {
+    std::optional<StringRef> RoundingStr = convertRoundingModeToStr(RM);
+    assert(RoundingStr);
+    auto *RoundingMDS = MDString::get(Context, *RoundingStr);
+    return MetadataAsValue::get(Context, RoundingMDS);
+  };
+  const auto createConstrainedExcept = [&](fp::ExceptionBehavior EB) {
+    std::optional<StringRef> ExceptStr = convertExceptionBehaviorToStr(EB);
+    assert(ExceptStr);
+    auto *ExceptMDS = MDString::get(Context, *ExceptStr);
+    return MetadataAsValue::get(Context, ExceptMDS);
+  };
+
+  // Constrained function call without bundles.
+  {
+    Value *V = Builder.CreateFAdd(FnArg, FnArg);
+    auto *I = cast<IntrinsicInst>(V);
+    EXPECT_FALSE(I->getOperandBundle(LLVMContext::OB_fp_control).has_value());
+    EXPECT_FALSE(I->getOperandBundle(LLVMContext::OB_fp_except).has_value());
+    EXPECT_EQ(Intrinsic::experimental_constrained_fadd, I->getIntrinsicID());
+    EXPECT_EQ(RoundingMode::TowardZero, I->getRoundingMode());
+    EXPECT_EQ(fp::ebStrict, I->getExceptionBehavior());
+    MemoryEffects ME = I->getMemoryEffects();
+    EXPECT_TRUE(ME.doesAccessInaccessibleMem());
+  }
+
+  // Operand bundles have precedence over constrained intrinsic metadata
+  // arguments.
+  {
+    SmallVector<OperandBundleDef, 1> Bundles;
+    llvm::addFPRoundingBundle(Ctx, Bundles, RoundingMode::TowardNegative);
+    llvm::addFPExceptionBundle(Ctx, Bundles, fp::ebIgnore);
+    Value *V = Builder.CreateCall(
+        Fn,
+        {FnArg, FnArg, createConstrainedRounding(RoundingMode::TowardZero),
+         createConstrainedExcept(fp::ebStrict)},
+        Bundles);
+
+    auto *I = cast<IntrinsicInst>(V);
+    EXPECT_TRUE(I->getOperandBundle(LLVMContext::OB_fp_control).has_value());
+    EXPECT_TRUE(I->getOperandBundle(LLVMContext::OB_fp_except).has_value());
+    EXPECT_EQ(Intrinsic::experimental_constrained_fadd, I->getIntrinsicID());
+    EXPECT_EQ(RoundingMode::TowardNegative, I->getRoundingMode());
+    EXPECT_EQ(fp::ebIgnore, I->getExceptionBehavior());
+    MemoryEffects ME = I->getMemoryEffects();
+    EXPECT_TRUE(ME.doesAccessInaccessibleMem());
+  }
+}
+
 TEST_F(IRBuilderTest, Lifetime) {
   IRBuilder<> Builder(BB);
   AllocaInst *Var1 = Builder.CreateAlloca(Builder.getInt8Ty());