Disable FTZ/DAZ when compiling shared libraries by default. (#80475)

This fixes https://github.com/llvm/llvm-project/issues/57589, and aligns
Clang with the behavior of current versions of gcc. There is a new
option, -mdaz-ftz, to control the linking of the file that sets FTZ/DAZ
on startup, and this flag is on by default if -ffast-math is present and
-shared isn't.

This also partially reverts fa7cd549d60 in that it disables the attempt
to set the IR denormal-fp-math attribute based on whether or not
-ffast-math is applied as it is insufficiently reliable.

1 files changed, 13 insertions, 2 deletions

diff --git a/clang/lib/Driver/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp
index 237092e..341d620 100644
--- a/clang/lib/Driver/ToolChain.cpp
+++ b/clang/lib/Driver/ToolChain.cpp
@@ -1307,9 +1307,14 @@ void ToolChain::AddCCKextLibArgs(const ArgList &Args,
 
 bool ToolChain::isFastMathRuntimeAvailable(const ArgList &Args,
                                            std::string &Path) const {
+  // Don't implicitly link in mode-changing libraries in a shared library, since
+  // this can have very deleterious effects. See the various links from
+  // https://github.com/llvm/llvm-project/issues/57589 for more information.
+  bool Default = !Args.hasArgNoClaim(options::OPT_shared);
+
   // Do not check for -fno-fast-math or -fno-unsafe-math when -Ofast passed
   // (to keep the linker options consistent with gcc and clang itself).
-  if (!isOptimizationLevelFast(Args)) {
+  if (Default && !isOptimizationLevelFast(Args)) {
     // Check if -ffast-math or -funsafe-math.
     Arg *A =
       Args.getLastArg(options::OPT_ffast_math, options::OPT_fno_fast_math,
@@ -1318,8 +1323,14 @@ bool ToolChain::isFastMathRuntimeAvailable(const ArgList &Args,
 
     if (!A || A->getOption().getID() == options::OPT_fno_fast_math ||
         A->getOption().getID() == options::OPT_fno_unsafe_math_optimizations)
-      return false;
+      Default = false;
   }
+
+  // Whatever decision came as a result of the above implicit settings, either
+  // -mdaz-ftz or -mno-daz-ftz is capable of overriding it.
+  if (!Args.hasFlag(options::OPT_mdaz_ftz, options::OPT_mno_daz_ftz, Default))
+    return false;
+
   // If crtfastmath.o exists add it to the arguments.
   Path = GetFilePath("crtfastmath.o");
   return (Path != "crtfastmath.o"); // Not found.