[flang] Set fast math related function attributes for -Ofast/-ffast-math (#79301)

The implemented logic matches the logic used for Clang in emitting these
attributes. Although it's hoped that function attributes won't be needed
in the future (vs using fast math flags in individual IR instructions),
there are codegen differences currently with/without these attributes,
as can be seen in issues like #79257 or by hacking Clang to avoid
producing these attributes and observing codegen changes.

7 files changed, 104 insertions, 8 deletions

diff --git a/flang/include/flang/Optimizer/Transforms/Passes.h b/flang/include/flang/Optimizer/Transforms/Passes.h
index aefb277..e1d22c8 100644
--- a/flang/include/flang/Optimizer/Transforms/Passes.h
+++ b/flang/include/flang/Optimizer/Transforms/Passes.h
@@ -92,7 +92,9 @@ struct FunctionAttrTypes {
 
 std::unique_ptr<mlir::Pass> createFunctionAttrPass();
 std::unique_ptr<mlir::Pass>
-createFunctionAttrPass(FunctionAttrTypes &functionAttr);
+createFunctionAttrPass(FunctionAttrTypes &functionAttr, bool noInfsFPMath,
+                       bool noNaNsFPMath, bool approxFuncFPMath,
+                       bool noSignedZerosFPMath, bool unsafeFPMath);
 
 // declarative passes
 #define GEN_PASS_REGISTRATION
diff --git a/flang/include/flang/Optimizer/Transforms/Passes.td b/flang/include/flang/Optimizer/Transforms/Passes.td
index 270b837..5fb576f 100644
--- a/flang/include/flang/Optimizer/Transforms/Passes.td
+++ b/flang/include/flang/Optimizer/Transforms/Passes.td
@@ -378,6 +378,21 @@ def FunctionAttr : Pass<"function-attr", "mlir::func::FuncOp"> {
            "mlir::LLVM::framePointerKind::FramePointerKind", 
            /*default=*/"mlir::LLVM::framePointerKind::FramePointerKind{}",
            "frame pointer">,
+    Option<"noInfsFPMath", "no-infs-fp-math",
+           "bool", /*default=*/"false",
+           "Set the no-infs-fp-math attribute on functions in the module.">,
+    Option<"noNaNsFPMath", "no-nans-fp-math",
+           "bool", /*default=*/"false",
+           "Set the no-nans-fp-math attribute on functions in the module.">,
+    Option<"approxFuncFPMath", "approx-func-fp-math",
+           "bool", /*default=*/"false",
+           "Set the approx-func-fp-math attribute on functions in the module.">,
+    Option<"noSignedZerosFPMath", "no-signed-zeros-fp-math",
+           "bool", /*default=*/"false",
+           "Set the no-signed-zeros-fp-math attribute on functions in the module.">,
+    Option<"unsafeFPMath", "unsafe-fp-math",
+           "bool", /*default=*/"false",
+           "Set the unsafe-fp-math attribute on functions in the module.">,
   ];
   let constructor = "::fir::createFunctionAttrPass()";
 }
diff --git a/flang/include/flang/Tools/CLOptions.inc b/flang/include/flang/Tools/CLOptions.inc
index 8dee307..68e504d 100644
--- a/flang/include/flang/Tools/CLOptions.inc
+++ b/flang/include/flang/Tools/CLOptions.inc
@@ -316,15 +316,22 @@ inline void createDefaultFIRCodeGenPassPipeline(
   // Add function attributes
   fir::FunctionAttrTypes functionAttrs;
 
-  if (config.FramePointerKind != llvm::FramePointerKind::None) {
+  if (config.FramePointerKind != llvm::FramePointerKind::None ||
+      config.NoInfsFPMath || config.NoNaNsFPMath || config.ApproxFuncFPMath ||
+      config.NoSignedZerosFPMath || config.UnsafeFPMath) {
     if (config.FramePointerKind == llvm::FramePointerKind::NonLeaf)
       functionAttrs.framePointerKind =
           mlir::LLVM::framePointerKind::FramePointerKind::NonLeaf;
-    else
+    else if (config.FramePointerKind == llvm::FramePointerKind::All)
       functionAttrs.framePointerKind =
           mlir::LLVM::framePointerKind::FramePointerKind::All;
+    else
+      functionAttrs.framePointerKind =
+          mlir::LLVM::framePointerKind::FramePointerKind::None;
 
-    pm.addPass(fir::createFunctionAttrPass(functionAttrs));
+    pm.addPass(fir::createFunctionAttrPass(functionAttrs, config.NoInfsFPMath,
+        config.NoNaNsFPMath, config.ApproxFuncFPMath,
+        config.NoSignedZerosFPMath, config.UnsafeFPMath));
   }
 
   fir::addFIRToLLVMPass(pm, config);
diff --git a/flang/include/flang/Tools/CrossToolHelpers.h b/flang/include/flang/Tools/CrossToolHelpers.h
index 5c59c99..b7d84af 100644
--- a/flang/include/flang/Tools/CrossToolHelpers.h
+++ b/flang/include/flang/Tools/CrossToolHelpers.h
@@ -13,6 +13,7 @@
 #ifndef FORTRAN_TOOLS_CROSS_TOOL_HELPERS_H
 #define FORTRAN_TOOLS_CROSS_TOOL_HELPERS_H
 
+#include "flang/Common/MathOptionsBase.h"
 #include "flang/Frontend/CodeGenOptions.h"
 #include "flang/Frontend/LangOptions.h"
 #include <cstdint>
@@ -28,7 +29,8 @@ struct MLIRToLLVMPassPipelineConfig {
     OptLevel = level;
   }
   explicit MLIRToLLVMPassPipelineConfig(llvm::OptimizationLevel level,
-      const Fortran::frontend::CodeGenOptions &opts) {
+      const Fortran::frontend::CodeGenOptions &opts,
+      const Fortran::common::MathOptionsBase &mathOpts) {
     OptLevel = level;
     StackArrays = opts.StackArrays;
     Underscoring = opts.Underscoring;
@@ -36,6 +38,15 @@ struct MLIRToLLVMPassPipelineConfig {
     DebugInfo = opts.getDebugInfo();
     AliasAnalysis = opts.AliasAnalysis;
     FramePointerKind = opts.getFramePointer();
+    // The logic for setting these attributes is intended to match the logic
+    // used in Clang.
+    NoInfsFPMath = mathOpts.getNoHonorInfs();
+    NoNaNsFPMath = mathOpts.getNoHonorNaNs();
+    ApproxFuncFPMath = mathOpts.getApproxFunc();
+    NoSignedZerosFPMath = mathOpts.getNoSignedZeros();
+    UnsafeFPMath = mathOpts.getAssociativeMath() &&
+        mathOpts.getReciprocalMath() && NoSignedZerosFPMath &&
+        ApproxFuncFPMath && mathOpts.getFPContractEnabled();
   }
 
   llvm::OptimizationLevel OptLevel; ///< optimisation level
@@ -49,6 +60,13 @@ struct MLIRToLLVMPassPipelineConfig {
       llvm::FramePointerKind::None; ///< Add frame pointer to functions.
   unsigned VScaleMin = 0; ///< SVE vector range minimum.
   unsigned VScaleMax = 0; ///< SVE vector range maximum.
+  bool NoInfsFPMath = false; ///< Set no-infs-fp-math attribute for functions.
+  bool NoNaNsFPMath = false; ///< Set no-nans-fp-math attribute for functions.
+  bool ApproxFuncFPMath =
+      false; ///< Set approx-func-fp-math attribute for functions.
+  bool NoSignedZerosFPMath =
+      false; ///< Set no-signed-zeros-fp-math attribute for functions.
+  bool UnsafeFPMath = false; ///< Set unsafe-fp-math attribute for functions.
 };
 
 struct OffloadModuleOpts {
diff --git a/flang/lib/Frontend/FrontendActions.cpp b/flang/lib/Frontend/FrontendActions.cpp
index 09e861d..18bc168 100644
--- a/flang/lib/Frontend/FrontendActions.cpp
+++ b/flang/lib/Frontend/FrontendActions.cpp
@@ -782,6 +782,7 @@ void CodeGenAction::generateLLVMIR() {
 
   CompilerInstance &ci = this->getInstance();
   auto opts = ci.getInvocation().getCodeGenOpts();
+  auto mathOpts = ci.getInvocation().getLoweringOpts().getMathOptions();
   llvm::OptimizationLevel level = mapToLevel(opts);
 
   fir::support::loadDialects(*mlirCtx);
@@ -794,7 +795,7 @@ void CodeGenAction::generateLLVMIR() {
   pm.addPass(std::make_unique<Fortran::lower::VerifierPass>());
   pm.enableVerifier(/*verifyPasses=*/true);
 
-  MLIRToLLVMPassPipelineConfig config(level, opts);
+  MLIRToLLVMPassPipelineConfig config(level, opts, mathOpts);
 
   if (auto vsr = getVScaleRange(ci)) {
     config.VScaleMin = vsr->first;
diff --git a/flang/lib/Optimizer/Transforms/FunctionAttr.cpp b/flang/lib/Optimizer/Transforms/FunctionAttr.cpp
index 55b908b..f54080f 100644
--- a/flang/lib/Optimizer/Transforms/FunctionAttr.cpp
+++ b/flang/lib/Optimizer/Transforms/FunctionAttr.cpp
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 #include "flang/Optimizer/Transforms/Passes.h"
 #include "mlir/Dialect/LLVMIR/LLVMAttrs.h"
+#include "mlir/Dialect/LLVMIR/LLVMDialect.h"
 
 namespace fir {
 #define GEN_PASS_DECL_FUNCTIONATTR
@@ -27,6 +28,11 @@ class FunctionAttrPass : public fir::impl::FunctionAttrBase<FunctionAttrPass> {
 public:
   FunctionAttrPass(const fir::FunctionAttrOptions &options) {
     framePointerKind = options.framePointerKind;
+    noInfsFPMath = options.noInfsFPMath;
+    noNaNsFPMath = options.noNaNsFPMath;
+    approxFuncFPMath = options.approxFuncFPMath;
+    noSignedZerosFPMath = options.noSignedZerosFPMath;
+    unsafeFPMath = options.unsafeFPMath;
   }
   FunctionAttrPass() {}
   void runOnOperation() override;
@@ -45,14 +51,43 @@ void FunctionAttrPass::runOnOperation() {
     func->setAttr("frame_pointer", mlir::LLVM::FramePointerKindAttr::get(
                                        context, framePointerKind));
 
+  auto llvmFuncOpName =
+      mlir::OperationName(mlir::LLVM::LLVMFuncOp::getOperationName(), context);
+  if (noInfsFPMath)
+    func->setAttr(
+        mlir::LLVM::LLVMFuncOp::getNoInfsFpMathAttrName(llvmFuncOpName),
+        mlir::BoolAttr::get(context, true));
+  if (noNaNsFPMath)
+    func->setAttr(
+        mlir::LLVM::LLVMFuncOp::getNoNansFpMathAttrName(llvmFuncOpName),
+        mlir::BoolAttr::get(context, true));
+  if (approxFuncFPMath)
+    func->setAttr(
+        mlir::LLVM::LLVMFuncOp::getApproxFuncFpMathAttrName(llvmFuncOpName),
+        mlir::BoolAttr::get(context, true));
+  if (noSignedZerosFPMath)
+    func->setAttr(
+        mlir::LLVM::LLVMFuncOp::getNoSignedZerosFpMathAttrName(llvmFuncOpName),
+        mlir::BoolAttr::get(context, true));
+  if (unsafeFPMath)
+    func->setAttr(
+        mlir::LLVM::LLVMFuncOp::getUnsafeFpMathAttrName(llvmFuncOpName),
+        mlir::BoolAttr::get(context, true));
+
   LLVM_DEBUG(llvm::dbgs() << "=== End " DEBUG_TYPE " ===\n");
 }
 
-std::unique_ptr<mlir::Pass>
-fir::createFunctionAttrPass(fir::FunctionAttrTypes &functionAttr) {
+std::unique_ptr<mlir::Pass> fir::createFunctionAttrPass(
+    fir::FunctionAttrTypes &functionAttr, bool noInfsFPMath, bool noNaNsFPMath,
+    bool approxFuncFPMath, bool noSignedZerosFPMath, bool unsafeFPMath) {
   FunctionAttrOptions opts;
   // Frame pointer
   opts.framePointerKind = functionAttr.framePointerKind;
+  opts.noInfsFPMath = noInfsFPMath;
+  opts.noNaNsFPMath = noNaNsFPMath;
+  opts.approxFuncFPMath = approxFuncFPMath;
+  opts.noSignedZerosFPMath = noSignedZerosFPMath;
+  opts.unsafeFPMath = unsafeFPMath;
 
   return std::make_unique<FunctionAttrPass>(opts);
 }
diff --git a/flang/test/Driver/func-attr-fast-math.f90 b/flang/test/Driver/func-attr-fast-math.f90
new file mode 100644
index 0000000..05824a6
--- /dev/null
+++ b/flang/test/Driver/func-attr-fast-math.f90
@@ -0,0 +1,18 @@
+! RUN: %flang -O1 -emit-llvm -S -o - %s 2>&1| FileCheck %s --check-prefix=CHECK-NOFASTMATH
+! RUN: %flang -Ofast -emit-llvm -S -o - %s 2>&1| FileCheck %s --check-prefix=CHECK-OFAST
+! RUN: %flang -O1 -ffast-math -emit-llvm -S -o - %s 2>&1| FileCheck %s --check-prefix=CHECK-FFAST-MATH
+
+subroutine func
+end subroutine func
+
+! CHECK-NOFASTMATH-LABEL: define void @func_() local_unnamed_addr
+! CHECK-NOFASTMATH-SAME: #[[ATTRS:[0-9]+]]
+! CHECK-NOT fp-math"=
+
+! CHECK-OFAST-LABEL: define void @func_() local_unnamed_addr
+! CHECK-OFAST-SAME: #[[ATTRS:[0-9]+]]
+! CHECK-OFAST: attributes #[[ATTRS]] = { {{.*}}"approx-func-fp-math"="true" {{.*}}"no-infs-fp-math"="true" {{.*}}"no-nans-fp-math"="true" {{.*}}"no-signed-zeros-fp-math"="true" {{.*}}"unsafe-fp-math"="true"{{.*}} }
+
+! CHECK-FFAST-MATH-LABEL: define void @func_() local_unnamed_addr
+! CHECK-FFAST-MATH-SAME: #[[ATTRS:[0-9]+]]
+! CHECK-FFAST-MATH: attributes #[[ATTRS]] = { {{.*}}"approx-func-fp-math"="true" {{.*}}"no-infs-fp-math"="true" {{.*}}"no-nans-fp-math"="true" {{.*}}"no-signed-zeros-fp-math"="true" {{.*}}"unsafe-fp-math"="true"{{.*}} }