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

This fixes #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 fa7cd54 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.

Author: jcranmer-intel

clang/docs/ReleaseNotes.rst

@@ -207,6 +207,16 @@ Non-comprehensive list of changes in this release
 - ``__typeof_unqual__`` is available in all C modes as an extension, which behaves
   like ``typeof_unqual`` from C23, similar to ``__typeof__`` and ``typeof``.
 
+
+* Shared libraries linked with either the ``-ffast-math``, ``-Ofast``, or
+  ``-funsafe-math-optimizations`` flags will no longer enable flush-to-zero
+  floating-point mode by default. This decision can be overridden with use of
+  ``-mdaz-ftz``. This behavior now matches GCC's behavior.
+  (`#57589 <https://github.com/llvm/llvm-project/issues/57589>`_)
+
+* ``-fdenormal-fp-math=preserve-sign`` is no longer implied by ``-ffast-math``
+  on x86 systems.
+
 New Compiler Flags
 ------------------
 - ``-fsanitize=implicit-bitfield-conversion`` checks implicit truncation and

clang/docs/UsersManual.rst

@@ -1506,7 +1506,8 @@ floating point semantic models: precise (the default), strict, and fast.
 
    * ``-ffp-contract=fast``
 
-   Note: ``-ffast-math`` causes ``crtfastmath.o`` to be linked with code. See
+   Note: ``-ffast-math`` causes ``crtfastmath.o`` to be linked with code unless
+   ``-shared`` or ``-mno-daz-ftz`` is present. See
    :ref:`crtfastmath.o` for more details.
 
 .. option:: -fno-fast-math
@@ -1560,7 +1561,8 @@ floating point semantic models: precise (the default), strict, and fast.
      ``-ffp-contract``.
 
    Note: ``-fno-fast-math`` implies ``-fdenormal-fp-math=ieee``.
-   ``-fno-fast-math`` causes ``crtfastmath.o`` to not be linked with code.
+   ``-fno-fast-math`` causes ``crtfastmath.o`` to not be linked with code
+   unless ``-mdaz-ftz`` is present.
 
 .. option:: -fdenormal-fp-math=<value>
 
@@ -1938,10 +1940,13 @@ by using ``#pragma STDC FENV_ROUND`` with a value other than ``FE_DYNAMIC``.
 
 A note about ``crtfastmath.o``
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-``-ffast-math`` and ``-funsafe-math-optimizations`` cause ``crtfastmath.o`` to be
-automatically linked,  which adds a static constructor that sets the FTZ/DAZ
+``-ffast-math`` and ``-funsafe-math-optimizations`` without the ``-shared``
+option cause ``crtfastmath.o`` to be
+automatically linked, which adds a static constructor that sets the FTZ/DAZ
 bits in MXCSR, affecting not only the current compilation unit but all static
-and shared libraries included in the program.
+and shared libraries included in the program. This decision can be overridden
+by using either the flag ``-mdaz-ftz`` or ``-mno-daz-ftz`` to respectively
+link or not link ``crtfastmath.o``.
 
 .. _FLT_EVAL_METHOD:
 

clang/include/clang/Driver/Options.td

@@ -2615,6 +2615,11 @@ defm protect_parens : BoolFOption<"protect-parens",
           "floating-point expressions are evaluated">,
   NegFlag<SetFalse>>;
 
+defm daz_ftz : SimpleMFlag<"daz-ftz",
+  "Globally set", "Do not globally set",
+  " the denormals-are-zero (DAZ) and flush-to-zero (FTZ) bits in the "
+  "floating-point control register on program startup">;
+
 def ffor_scope : Flag<["-"], "ffor-scope">, Group<f_Group>;
 def fno_for_scope : Flag<["-"], "fno-for-scope">, Group<f_Group>;
 

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.

clang/lib/Driver/ToolChains/Linux.cpp

@@ -842,25 +842,6 @@ void Linux::addProfileRTLibs(const llvm::opt::ArgList &Args,
   ToolChain::addProfileRTLibs(Args, CmdArgs);
 }
 
-llvm::DenormalMode
-Linux::getDefaultDenormalModeForType(const llvm::opt::ArgList &DriverArgs,
-                                     const JobAction &JA,
-                                     const llvm::fltSemantics *FPType) const {
-  switch (getTriple().getArch()) {
-  case llvm::Triple::x86:
-  case llvm::Triple::x86_64: {
-    std::string Unused;
-    // DAZ and FTZ are turned on in crtfastmath.o
-    if (!DriverArgs.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles) &&
-        isFastMathRuntimeAvailable(DriverArgs, Unused))
-      return llvm::DenormalMode::getPreserveSign();
-    return llvm::DenormalMode::getIEEE();
-  }
-  default:
-    return llvm::DenormalMode::getIEEE();
-  }
-}
-
 void Linux::addExtraOpts(llvm::opt::ArgStringList &CmdArgs) const {
   for (const auto &Opt : ExtraOpts)
     CmdArgs.push_back(Opt.c_str());

clang/lib/Driver/ToolChains/Linux.h

@@ -59,10 +59,6 @@ class LLVM_LIBRARY_VISIBILITY Linux : public Generic_ELF {
 
   std::vector<std::string> ExtraOpts;
 
-  llvm::DenormalMode getDefaultDenormalModeForType(
-      const llvm::opt::ArgList &DriverArgs, const JobAction &JA,
-      const llvm::fltSemantics *FPType = nullptr) const override;
-
   const char *getDefaultLinker() const override;
 
 protected:

clang/test/Driver/default-denormal-fp-math.c

@@ -3,15 +3,6 @@
 
 // RUN: %clang -### -target x86_64-unknown-linux-gnu --sysroot=%S/Inputs/basic_linux_tree -c %s -v 2>&1 | FileCheck -check-prefix=CHECK-IEEE %s
 
-// crtfastmath enables ftz and daz
-// RUN: %clang -### -target x86_64-unknown-linux-gnu -ffast-math --sysroot=%S/Inputs/basic_linux_tree -c %s -v 2>&1 | FileCheck -check-prefix=CHECK-PRESERVESIGN %s
-
-// crt not linked in with nostartfiles
-// RUN: %clang -### -target x86_64-unknown-linux-gnu -ffast-math -nostartfiles --sysroot=%S/Inputs/basic_linux_tree -c %s -v 2>&1 | FileCheck -check-prefix=CHECK-IEEE %s
-
-// If there's no crtfastmath, don't assume ftz/daz
-// RUN: %clang -### -target x86_64-unknown-linux-gnu -ffast-math --sysroot=/dev/null -c %s -v 2>&1 | FileCheck -check-prefix=CHECK-IEEE %s
-
 // RUN: %clang -### -target x86_64-scei-ps4 -c %s -v 2>&1 | FileCheck -check-prefix=CHECK-PRESERVESIGN %s
 
 // Flag omitted for default

clang/test/Driver/linux-ld.c

@@ -1446,6 +1446,32 @@
 // RUN: %clang --target=i386-unknown-linux -no-pie -### %s -ffast-math \
 // RUN:        --sysroot=%S/Inputs/basic_linux_tree 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-NOCRTFASTMATH %s
+// Don't link crtfastmath.o with -shared
+// RUN: %clang --target=x86_64-unknown-linux -no-pie -### %s -ffast-math -shared \
+// RUN:        --sysroot=%S/Inputs/basic_linux_tree 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-NOCRTFASTMATH %s
+// RUN: %clang --target=x86_64-unknown-linux -no-pie -### %s -Ofast -shared \
+// RUN:        --sysroot=%S/Inputs/basic_linux_tree 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-NOCRTFASTMATH %s
+// Check for effects of -mdaz-ftz
+// RUN: %clang --target=x86_64-unknown-linux -### %s -ffast-math -shared -mdaz-ftz \
+// RUN:        --sysroot=%S/Inputs/basic_linux_tree 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-CRTFASTMATH %s
+// RUN: %clang --target=x86_64-unknown-linux -no-pie -### %s -ffast-math -mdaz-ftz \
+// RUN:        --sysroot=%S/Inputs/basic_linux_tree 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-CRTFASTMATH %s
+// RUN: %clang --target=x86_64-unknown-linux -no-pie -### %s -mdaz-ftz \
+// RUN:        --sysroot=%S/Inputs/basic_linux_tree 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-CRTFASTMATH %s
+// RUN: %clang --target=x86_64-unknown-linux -### %s -ffast-math -shared -mno-daz-ftz \
+// RUN:        --sysroot=%S/Inputs/basic_linux_tree 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-NOCRTFASTMATH %s
+// RUN: %clang --target=x86_64-unknown-linux -no-pie -### %s -ffast-math -mno-daz-ftz \
+// RUN:        --sysroot=%S/Inputs/basic_linux_tree 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-NOCRTFASTMATH %s
+// RUN: %clang --target=x86_64-unknown-linux -no-pie -### %s -mno-daz-ftz \
+// RUN:        --sysroot=%S/Inputs/basic_linux_tree 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-NOCRTFASTMATH %s
 // CHECK-CRTFASTMATH: usr/lib/gcc/x86_64-unknown-linux/10.2.0{{/|\\\\}}crtfastmath.o
 // CHECK-NOCRTFASTMATH-NOT: crtfastmath.o