[PATCH] D72675: [Clang][Driver] Fix -ffast-math/-ffp-contract interaction

[Warren Ristow via Phabricator via cfe-commits]

wristow updated this revision to Diff 240305.
wristow added a comment.

Update a comment to remove the discussion about enabling the `__FAST_MATH__` preprocessor macro.  The handling of the setting of `__FAST_MATH__` is done in "clang/lib/Frontend/InitPreprocessor.cpp", and once we decide on the set of conditions that enable that definition, we can add an appropriate comment there.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D72675/new/

https://reviews.llvm.org/D72675

Files:
  clang/lib/Driver/ToolChains/Clang.cpp
  clang/test/Driver/fast-math.c


Index: clang/test/Driver/fast-math.c
===================================================================
--- clang/test/Driver/fast-math.c
+++ clang/test/Driver/fast-math.c
@@ -180,6 +180,21 @@
 // CHECK-FAST-MATH: "-ffast-math"
 // CHECK-FAST-MATH: "-ffinite-math-only"
 //
+// -ffp-contract=off and -ffp-contract=on must disable the fast-math umbrella,
+// and the unsafe-fp-math umbrella (-ffp-contract=fast leaves them enabled).
+// RUN: %clang -### -ffast-math -ffp-contract=off -c %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-NO-FAST-MATH %s
+// RUN: %clang -### -ffast-math -ffp-contract=off -c %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-NO-UNSAFE-MATH %s
+// RUN: %clang -### -ffast-math -ffp-contract=on -c %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-NO-FAST-MATH %s
+// RUN: %clang -### -ffast-math -ffp-contract=on -c %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-NO-UNSAFE-MATH %s
+// RUN: %clang -### -ffast-math -ffp-contract=fast -c %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-FAST-MATH %s
+// RUN: %clang -### -ffast-math -ffp-contract=fast -c %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-UNSAFE-MATH %s
+//
 // RUN: %clang -### -ffast-math -fno-fast-math -c %s 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-NO-FAST-MATH %s
 // RUN: %clang -### -ffast-math -fno-finite-math-only -c %s 2>&1 \
Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -2785,8 +2785,11 @@
   if (MathErrno)
     CmdArgs.push_back("-fmath-errno");
 
+  // If -ffp-contract=off/on has been specified on the command line, then we
+  // must suppress the emission of -ffast-math and -menable-unsafe-fp-math to
+  // cc1.
   if (!MathErrno && AssociativeMath && ReciprocalMath && !SignedZeros &&
-      !TrappingMath)
+      !TrappingMath && !(FPContract.equals("off") || FPContract.equals("on")))
     CmdArgs.push_back("-menable-unsafe-fp-math");
 
   if (!SignedZeros)
@@ -2831,12 +2834,17 @@
 
   ParseMRecip(D, Args, CmdArgs);
 
-  // -ffast-math enables the __FAST_MATH__ preprocessor macro, but check for the
-  // individual features enabled by -ffast-math instead of the option itself as
-  // that's consistent with gcc's behaviour.
+  // -ffast-math acts as an "umbrella", enabling a variety of individual
+  // floating-point transformations.  We check if the appropriate set of those
+  // transformations are enabled, and if they are, we pass that umbrella switch
+  // to cc1.  This also interacts with another "umbrella" switch, -ffp-model.
+  // We handle -ffp-contract somewhat specially here, to produce a warning in
+  // situations where -ffp-model=fast is overridden by the setting of
+  // -ffp-contract.
   if (!HonorINFs && !HonorNaNs && !MathErrno && AssociativeMath &&
       ReciprocalMath && !SignedZeros && !TrappingMath && !RoundingFPMath) {
-    CmdArgs.push_back("-ffast-math");
+    if (!(FPContract.equals("off") || FPContract.equals("on")))
+      CmdArgs.push_back("-ffast-math");
     if (FPModel.equals("fast")) {
       if (FPContract.equals("fast"))
         // All set, do nothing.


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