https://github.com/jcranmer-intel created https://github.com/llvm/llvm-project/pull/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. >From 14e683e37854816b34105f8ff9d80dafb1ba4001 Mon Sep 17 00:00:00 2001 From: Joshua Cranmer <joshua.cran...@intel.com> Date: Fri, 2 Feb 2024 10:35:29 -0800 Subject: [PATCH] Disable FTZ/DAZ when compiling shared libraries by default. 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. --- clang/docs/ReleaseNotes.rst | 5 ++++ clang/docs/UsersManual.rst | 15 ++++++++---- clang/include/clang/Driver/Options.td | 5 ++++ clang/lib/Driver/ToolChain.cpp | 15 ++++++++++-- clang/test/Driver/linux-ld.c | 34 +++++++++++++++++++++++++++ 5 files changed, 67 insertions(+), 7 deletions(-) diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst index cd8a82f281f52..904e292a2eb4b 100644 --- a/clang/docs/ReleaseNotes.rst +++ b/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 ------------------ diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst index 7391e4cf3a9ae..257af3c8d4d39 100644 --- a/clang/docs/UsersManual.rst +++ b/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: diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td index 73071a6648541..68f806d46c9e0 100644 --- a/clang/include/clang/Driver/Options.td +++ b/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>; diff --git a/clang/lib/Driver/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp index 388030592b483..5c003e153fa52 100644 --- a/clang/lib/Driver/ToolChain.cpp +++ b/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. diff --git a/clang/test/Driver/linux-ld.c b/clang/test/Driver/linux-ld.c index b8efd64cd91f0..137d84cca883a 100644 --- a/clang/test/Driver/linux-ld.c +++ b/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 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
