Disable FTZ/DAZ when compiling shared libraries by default.

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.

Author: jcranmer-intel

clang/docs/ReleaseNotes.rst

@@ -117,6 +117,11 @@ C23 Feature Support
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 
+* Code compiled with ``-shared`` and ``-ffast-math`` 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>`_)
+
 New Compiler Flags
 ------------------
 

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>
 
@@ -1907,10 +1909,13 @@ 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

@@ -2554,6 +2554,11 @@ 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

@@ -1271,9 +1271,14 @@ 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.hasArg(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,
@@ -1282,8 +1287,14 @@ 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/test/Driver/linux-ld.c

@@ -1569,6 +1569,40 @@
 // RUN:        --gcc-toolchain="" \
 // 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:        --gcc-toolchain="" \
+// 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:        --gcc-toolchain="" \
+// 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 -no-pie -### %s -ffast-math -shared -mdaz-ftz \
+// RUN:        --gcc-toolchain="" \
+// 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:        --gcc-toolchain="" \
+// 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:        --gcc-toolchain="" \
+// 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 -shared -mno-daz-ftz \
+// RUN:        --gcc-toolchain="" \
+// 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:        --gcc-toolchain="" \
+// 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:        --gcc-toolchain="" \
+// 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