[clang] Disable FTZ/DAZ when compiling shared libraries by default. (PR #80475)
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Fri Feb 2 10:46:59 PST 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang
Author: Joshua Cranmer (jcranmer-intel)
<details>
<summary>Changes</summary>
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.
---
Full diff: https://github.com/llvm/llvm-project/pull/80475.diff
5 Files Affected:
- (modified) clang/docs/ReleaseNotes.rst (+5)
- (modified) clang/docs/UsersManual.rst (+10-5)
- (modified) clang/include/clang/Driver/Options.td (+5)
- (modified) clang/lib/Driver/ToolChain.cpp (+13-2)
- (modified) clang/test/Driver/linux-ld.c (+34)
``````````diff
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
``````````
</details>
https://github.com/llvm/llvm-project/pull/80475
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