[clang] Make -fveclib={ArmPL,SLEEF} imply -fno-math-errno

These two veclibs are only available for AArch64 targets, and as
mentioned in https://discourse.llvm.org/t/rfc-should-fveclib-imply-fno-math-errno-for-all-targets/81384,
we (Arm) think that `-fveclib` should imply `-fno-math-errno`. By
setting `-fveclib` the user shows they intend to use the vector math
functions, which implies they don't care about errno. However,
currently, the vector mappings won't be used in many cases without
setting `-fno-math-errno` separately.

Making this change would also help resolve some inconsistencies in how
vector mappings are applied (see llvm#108980 (comment)).

Author: MacDue

clang/include/clang/Driver/Options.td

@@ -3410,7 +3410,8 @@ def fno_experimental_isel : Flag<["-"], "fno-experimental-isel">, Group<f_clang_
   Alias<fno_global_isel>;
 def fveclib : Joined<["-"], "fveclib=">, Group<f_Group>,
   Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>,
-    HelpText<"Use the given vector functions library">,
+    HelpText<"Use the given vector functions library."
+             "Note: -fveclib={ArmPL,SLEEF} implies -fno-math-errno">,
     Values<"Accelerate,libmvec,MASSV,SVML,SLEEF,Darwin_libsystem_m,ArmPL,AMDLIBM,none">,
     NormalizedValuesScope<"llvm::driver::VectorLibrary">,
     NormalizedValues<["Accelerate", "LIBMVEC", "MASSV", "SVML", "SLEEF",

clang/lib/Driver/ToolChains/Clang.cpp

@@ -2854,6 +2854,10 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
                                        bool OFastEnabled, const ArgList &Args,
                                        ArgStringList &CmdArgs,
                                        const JobAction &JA) {
+  // List of veclibs which when used with -fveclib imply -fno-math-errno.
+  constexpr std::array VecLibImpliesNoMathErrno{llvm::StringLiteral("ArmPL"),
+                                                llvm::StringLiteral("SLEEF")};
+
   // Handle various floating point optimization flags, mapping them to the
   // appropriate LLVM code generation flags. This is complicated by several
   // "umbrella" flags, so we do this by stepping through the flags incrementally
@@ -3125,6 +3129,10 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
       TrappingMathPresent = true;
       FPExceptionBehavior = "strict";
       break;
+    case options::OPT_fveclib:
+      if (llvm::is_contained(VecLibImpliesNoMathErrno, A->getValue()))
+        MathErrno = false;
+      break;
     case options::OPT_fno_trapping_math:
       if (!TrappingMathPresent && !FPExceptionBehavior.empty() &&
           FPExceptionBehavior != "ignore")

clang/test/Driver/fveclib.c

@@ -36,16 +36,23 @@
 /* Verify that the correct vector library is passed to LTO flags. */
 
 // RUN: %clang -### --target=x86_64-unknown-linux-gnu -fveclib=LIBMVEC -flto %s 2>&1 | FileCheck --check-prefix=CHECK-LTO-LIBMVEC %s
+// CHECK-LTO-LIBMVEC: "-fmath-errno"
 // CHECK-LTO-LIBMVEC: "-plugin-opt=-vector-library=LIBMVEC-X86"
 
 // RUN: %clang -### --target=powerpc64-unknown-linux-gnu -fveclib=MASSV -flto %s 2>&1 | FileCheck --check-prefix=CHECK-LTO-MASSV %s
+// CHECK-LTO-MASSV: "-fmath-errno"
 // CHECK-LTO-MASSV: "-plugin-opt=-vector-library=MASSV"
 
 // RUN: %clang -### --target=x86_64-unknown-linux-gnu -fveclib=SVML -flto %s 2>&1 | FileCheck --check-prefix=CHECK-LTO-SVML %s
+// CHECK-LTO-SVML: "-fmath-errno"
 // CHECK-LTO-SVML: "-plugin-opt=-vector-library=SVML"
 
 // RUN: %clang -### --target=aarch64-linux-gnu -fveclib=SLEEF -flto %s 2>&1 | FileCheck --check-prefix=CHECK-LTO-SLEEF %s
+// CHECK-LTO-SLEEF-NOT: "-fmath-errno"
 // CHECK-LTO-SLEEF: "-plugin-opt=-vector-library=sleefgnuabi"
+// CHECK-LTO-SLEEF-NOT: "-fmath-errno"
 
 // RUN: %clang -### --target=aarch64-linux-gnu -fveclib=ArmPL -flto %s 2>&1 | FileCheck --check-prefix=CHECK-LTO-ARMPL %s
+// CHECK-LTO-ARMPL-NOT: "-fmath-errno"
 // CHECK-LTO-ARMPL: "-plugin-opt=-vector-library=ArmPL"
+// CHECK-LTO-ARMPL-NOT: "-fmath-errno"