[clang] Fix -fno-unsafe-math-optimizations behavior (PR #89473)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-driver

Author: Andy Kaylor (andykaylor)

<details>
<summary>Changes</summary>

This changes the handling of -fno-unsafe-fp-math to stop having that
option imply -ftrapping-math. In gcc, -fno-unsafe-math-optimizations sets
-ftrapping-math, but that dependency is based on the fact the
-ftrapping-math is enabled by default in gcc. Because clang does not
enabled -ftrapping-math by default, there is no reason for
-fno-unsafe-math-optimizations to set it.

On the other hand, -funsafe-math-optimizations continues to imply
-fno-trapping-math because this option necessarily disables strict
exception semantics.

This fixes https://github.com/llvm/llvm-project/issues/87523


---
Full diff: https://github.com/llvm/llvm-project/pull/89473.diff


3 Files Affected:

- (modified) clang/docs/UsersManual.rst (-1) 
- (modified) clang/lib/Driver/ToolChains/Clang.cpp (-2) 
- (modified) clang/test/Driver/fast-math.c (+13-11) 


``````````diff
diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst
index c464bc3a69adc5..2b4155d4b65a48 100644
--- a/clang/docs/UsersManual.rst
+++ b/clang/docs/UsersManual.rst
@@ -1690,7 +1690,6 @@ floating point semantic models: precise (the default), strict, and fast.
    * ``-fno-associative-math``
    * ``-fno-reciprocal-math``
    * ``-fsigned-zeros``
-   * ``-ftrapping-math``
    * ``-ffp-contract=on``
    * ``-fdenormal-fp-math=ieee``
 
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 456ea74caadb00..0776d095327219 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -3137,8 +3137,6 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
       ReciprocalMath = false;
       SignedZeros = true;
       ApproxFunc = false;
-      TrappingMath = true;
-      FPExceptionBehavior = "strict";
 
       // The target may have opted to flush by default, so force IEEE.
       DenormalFPMath = llvm::DenormalMode::getIEEE();
diff --git a/clang/test/Driver/fast-math.c b/clang/test/Driver/fast-math.c
index 882e81fd14d34a..ef23f88dd817ea 100644
--- a/clang/test/Driver/fast-math.c
+++ b/clang/test/Driver/fast-math.c
@@ -271,11 +271,11 @@
 // RUN:   2>&1 | FileCheck --check-prefix=CHECK-NO-UNSAFE-MATH %s
 
 // RUN: %clang -### -funsafe-math-optimizations -fno-reciprocal-math -c %s \
-// RUN:   2>&1 | FileCheck --check-prefix=CHECK-NO-UNSAFE-MATH %s
+// RUN:   2>&1 | FileCheck --check-prefix=CHECK-REASSOC-NO-UNSAFE-MATH %s
 // RUN: %clang -### -funsafe-math-optimizations -fsigned-zeros -c %s 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-NO-UNSAFE-MATH %s
 // RUN: %clang -### -funsafe-math-optimizations -ftrapping-math -c %s 2>&1 \
-// RUN:   | FileCheck --check-prefix=CHECK-NO-UNSAFE-MATH %s
+// RUN:   | FileCheck --check-prefix=CHECK-TRAPPING-NO-UNSAFE-MATH %s
 // RUN: %clang -### -funsafe-math-optimizations -fno-unsafe-math-optimizations \
 // RUN:     -c %s 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-NO-UNSAFE-MATH %s
@@ -283,18 +283,20 @@
 // RUN:   | FileCheck --check-prefix=CHECK-NO-UNSAFE-MATH %s
 
 // RUN: %clang -### -ffast-math -fno-reciprocal-math -c %s 2>&1 \
-// RUN:   | FileCheck --check-prefix=CHECK-NO-UNSAFE-MATH %s
+// RUN:   | FileCheck --check-prefix=CHECK-REASSOC-NO-UNSAFE-MATH %s
 // RUN: %clang -### -ffast-math -fsigned-zeros -c %s 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-NO-UNSAFE-MATH %s
 // RUN: %clang -### -ffast-math -ftrapping-math -c %s 2>&1 \
-// RUN:   | FileCheck --check-prefix=CHECK-NO-UNSAFE-MATH %s
+// RUN:   | FileCheck --check-prefix=CHECK-TRAPPING-NO-UNSAFE-MATH %s
 // RUN: %clang -### -ffast-math -fno-unsafe-math-optimizations -c %s 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-NO-UNSAFE-MATH %s
 
 // CHECK-NO-UNSAFE-MATH: "-cc1"
 // CHECK-NO-UNSAFE-MATH-NOT: "-funsafe-math-optimizations"
-// CHECK-NO_UNSAFE-MATH-NOT: "-mreassociate"
+// CHECK-NO-UNSAFE-MATH-NOT: "-mreassociate"
 // CHECK-NO-UNSAFE-MATH: "-o"
+// CHECK-NO-UNSAFE-MATH-NOT: "-ffp-exception-behavior=strict"
+// CHECK-TRAPPING-NO-UNSAFE-MATH: "-ffp-exception-behavior=strict"
 
 
 // Reassociate is allowed because it does not require reciprocal-math.
@@ -304,8 +306,8 @@
 // RUN:   | FileCheck --check-prefix=CHECK-REASSOC-NO-UNSAFE-MATH %s
 
 // CHECK-REASSOC-NO-UNSAFE-MATH: "-cc1"
-// CHECK-REASSOC-NO_UNSAFE-MATH-NOT: "-funsafe-math-optimizations"
-// CHECK-REASSOC-NO_UNSAFE-MATH: "-mreassociate"
+// CHECK-REASSOC-NO-UNSAFE-MATH-NOT: "-funsafe-math-optimizations"
+// CHECK-REASSOC-NO-UNSAFE-MATH: "-mreassociate"
 // CHECK-REASSOC-NO-UNSAFE-MATH-NOT: "-funsafe-math-optimizations"
 // CHECK-REASSOC-NO-UNSAFE-MATH: "-o"
 
@@ -318,12 +320,12 @@
 
 // RUN: %clang -### -fassociative-math -freciprocal-math -fno-signed-zeros \
 // RUN:     -fno-trapping-math -ftrapping-math -c %s 2>&1 \
-// RUN:   | FileCheck --check-prefix=CHECK-NO-REASSOC-NO-UNSAFE-MATH %s
+// RUN:   | FileCheck --check-prefix=CHECK-NO-UNSAFE-MATH %s
 
 // CHECK-NO-REASSOC-NO-UNSAFE-MATH: "-cc1"
-// CHECK-NO-REASSOC-NO_UNSAFE-MATH-NOT: "-funsafe-math-optimizations"
-// CHECK-NO-REASSOC-NO_UNSAFE-MATH-NOT: "-mreassociate"
-// CHECK-NO-REASSOC-NO_UNSAFE-MATH-NOT: "-funsafe-math-optimizations"
+// CHECK-NO-REASSOC-NO-UNSAFE-MATH-NOT: "-funsafe-math-optimizations"
+// CHECK-NO-REASSOC-NO-UNSAFE-MATH-NOT: "-mreassociate"
+// CHECK-NO-REASSOC-NO-UNSAFE-MATH-NOT: "-funsafe-math-optimizations"
 // CHECK-NO-REASSOC-NO-UNSAFE-MATH: "-o"
 
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/89473