[flang] Remove "unsafe-fp-math" attribute support

[paperchalice]

These global flags block furthur improvements for clang, users should always use fast-math flags
see also https://discourse.llvm.org/t/rfc-honor-pragmas-with-ffp-contract-fast/80797
Remove them incrementally, this is the flang part.

[llvmbot]

@llvm/pr-subscribers-flang-driver

@llvm/pr-subscribers-flang-fir-hlfir

Author: None (paperchalice)

Changes

These global flags block furthur improvements for clang, users should always use fast-math flags
see also https://discourse.llvm.org/t/rfc-honor-pragmas-with-ffp-contract-fast/80797
Remove them incrementally, this is the flang part.

---

Full diff: https://github.com/llvm/llvm-project/pull/162783.diff

4 Files Affected:

• (modified) flang/include/flang/Optimizer/Transforms/Passes.td (+1-1)
• (modified) flang/include/flang/Tools/CrossToolHelpers.h (+1-1)
• (modified) flang/lib/Optimizer/Transforms/FunctionAttr.cpp (-4)
• (modified) flang/test/Driver/func-attr-fast-math.f90 (+2-2)

diff --git a/flang/include/flang/Optimizer/Transforms/Passes.td b/flang/include/flang/Optimizer/Transforms/Passes.td
index e2ba9c3522837..0722bc91d3daa 100644
--- a/flang/include/flang/Optimizer/Transforms/Passes.td
+++ b/flang/include/flang/Optimizer/Transforms/Passes.td
@@ -436,7 +436,7 @@ def FunctionAttr : Pass<"function-attr", "mlir::func::FuncOp"> {
               "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.">,
+              "Set all fast-math flags on instructions in the module.">,
        Option<"reciprocals", "mrecip", "std::string", /*default=*/"",
               "Set the reciprocal-estimates attribute on functions in the "
               "module.">,
diff --git a/flang/include/flang/Tools/CrossToolHelpers.h b/flang/include/flang/Tools/CrossToolHelpers.h
index 850bd1f0940f7..e964882ef6dac 100644
--- a/flang/include/flang/Tools/CrossToolHelpers.h
+++ b/flang/include/flang/Tools/CrossToolHelpers.h
@@ -128,7 +128,7 @@ struct MLIRToLLVMPassPipelineConfig : public FlangEPCallBacks {
   bool ApproxFuncFPMath = false; ///< Set afn flag for instructions.
   bool NoSignedZerosFPMath =
       false; ///< Set no-signed-zeros-fp-math attribute for functions.
-  bool UnsafeFPMath = false; ///< Set unsafe-fp-math attribute for functions.
+  bool UnsafeFPMath = false; ///< Set all fast-math flags for instructions.
   std::string Reciprocals = ""; ///< Set reciprocal-estimate attribute for
                                 ///< functions.
   std::string PreferVectorWidth = ""; ///< Set prefer-vector-width attribute for
diff --git a/flang/lib/Optimizer/Transforms/FunctionAttr.cpp b/flang/lib/Optimizer/Transforms/FunctionAttr.cpp
index 9dfe26cbf5899..fea511cc63a37 100644
--- a/flang/lib/Optimizer/Transforms/FunctionAttr.cpp
+++ b/flang/lib/Optimizer/Transforms/FunctionAttr.cpp
@@ -99,10 +99,6 @@ void FunctionAttrPass::runOnOperation() {
     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));
   if (!reciprocals.empty())
     func->setAttr(
         mlir::LLVM::LLVMFuncOp::getReciprocalEstimatesAttrName(llvmFuncOpName),
diff --git a/flang/test/Driver/func-attr-fast-math.f90 b/flang/test/Driver/func-attr-fast-math.f90
index 3b6ce602c5373..3af641ea2db26 100644
--- a/flang/test/Driver/func-attr-fast-math.f90
+++ b/flang/test/Driver/func-attr-fast-math.f90
@@ -11,8 +11,8 @@ end subroutine func
 
 ! CHECK-OFAST-LABEL: define void @func_() local_unnamed_addr
 ! CHECK-OFAST-SAME: #[[ATTRS:[0-9]+]]
-! CHECK-OFAST: attributes #[[ATTRS]] = { {{.*}}"no-infs-fp-math"="true" {{.*}}"no-nans-fp-math"="true" {{.*}}"no-signed-zeros-fp-math"="true" {{.*}}"unsafe-fp-math"="true"{{.*}} }
+! CHECK-OFAST: attributes #[[ATTRS]] = { {{.*}}"no-infs-fp-math"="true" {{.*}}"no-nans-fp-math"="true" {{.*}}"no-signed-zeros-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]] = { {{.*}}"no-infs-fp-math"="true" {{.*}}"no-nans-fp-math"="true" {{.*}}"no-signed-zeros-fp-math"="true" {{.*}}"unsafe-fp-math"="true"{{.*}} }
+! CHECK-FFAST-MATH: attributes #[[ATTRS]] = { {{.*}}"no-infs-fp-math"="true" {{.*}}"no-nans-fp-math"="true" {{.*}}"no-signed-zeros-fp-math"="true"{{.*}} }

[vzakhari]

Thanks for the changes!

Can you please clarify whether all target CGs have been fixed in LLVM not to rely on this function attribute?

[paperchalice]

Thanks for the changes!

Can you please clarify whether all target CGs have been fixed in LLVM not to rely on this function attribute?

Now powerpc is the only backend that relies on "unsafe-fp-math".

[vzakhari]

Thanks for the changes!
Can you please clarify whether all target CGs have been fixed in LLVM not to rely on this function attribute?

Now powerpc is the only backend that relies on "unsafe-fp-math".

Thanks for the reply.

@kkwli @DanielCChen are you okay with removing the unsafe-fp-math function attribute in Flang, given that PPC backend still relies on it?

[paperchalice]

For reference, PowerPC part is in #154901,