[clang] Disable FTZ/DAZ when compiling shared libraries by default. (PR #80475)
Joshua Cranmer via cfe-commits
cfe-commits at lists.llvm.org
Thu Feb 22 11:05:28 PST 2024
https://github.com/jcranmer-intel updated https://github.com/llvm/llvm-project/pull/80475
>From fc0507f013f556cc7c49a38f22d14578311f1f42 Mon Sep 17 00:00:00 2001
From: Joshua Cranmer <joshua.cranmer at intel.com>
Date: Fri, 2 Feb 2024 10:35:29 -0800
Subject: [PATCH 1/3] 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 74bb9a07f0b13f..723a2bc859219b 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -145,6 +145,11 @@ Non-comprehensive list of changes in this release
- ``__builtin_addc``, ``__builtin_subc``, and the other sizes of those
builtins are now constexpr and may be used in constant expressions.
+* 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 7391e4cf3a9aeb..257af3c8d4d39a 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 3a028fadb25b18..c607e2e5c6ca5f 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -2581,6 +2581,11 @@ 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 f8c13c86daf9b0..08301b89ebb33e 100644
--- a/clang/lib/Driver/ToolChain.cpp
+++ b/clang/lib/Driver/ToolChain.cpp
@@ -1292,9 +1292,14 @@ 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,
@@ -1303,8 +1308,14 @@ 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 b3ce5ca307a6c3..e3d721e620e0d1 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
>From 2ec10523c725c0ed7f88e8f02dac9be975beb19c Mon Sep 17 00:00:00 2001
From: Joshua Cranmer <joshua.cranmer at intel.com>
Date: Fri, 2 Feb 2024 11:01:04 -0800
Subject: [PATCH 2/3] Address review comments.
---
clang/docs/ReleaseNotes.rst | 7 ++++---
clang/include/clang/Driver/Options.td | 2 +-
clang/test/Driver/linux-ld.c | 12 ++----------
3 files changed, 7 insertions(+), 14 deletions(-)
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 723a2bc859219b..67c163232ee6d7 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -145,9 +145,10 @@ Non-comprehensive list of changes in this release
- ``__builtin_addc``, ``__builtin_subc``, and the other sizes of those
builtins are now constexpr and may be used in constant expressions.
-* 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.
+* 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>`_)
New Compiler Flags
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index c607e2e5c6ca5f..ac49a758bb685f 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -2584,7 +2584,7 @@ defm protect_parens : BoolFOption<"protect-parens",
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.">;
+ "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/test/Driver/linux-ld.c b/clang/test/Driver/linux-ld.c
index e3d721e620e0d1..85b90d738ce340 100644
--- a/clang/test/Driver/linux-ld.c
+++ b/clang/test/Driver/linux-ld.c
@@ -1571,36 +1571,28 @@
// 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: %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: --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: %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: --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
>From 4202a865f787fadd66cce847a4d317ebb037676c Mon Sep 17 00:00:00 2001
From: Joshua Cranmer <joshua.cranmer at intel.com>
Date: Thu, 22 Feb 2024 10:28:15 -0800
Subject: [PATCH 3/3] Partially revert fa7cd549d60.
The guessing of whether or not FTZ/DAZ mode would be enabled is
insufficiently accurate.
---
clang/lib/Driver/ToolChains/Linux.cpp | 19 -------------------
clang/lib/Driver/ToolChains/Linux.h | 4 ----
clang/test/Driver/default-denormal-fp-math.c | 9 ---------
3 files changed, 32 deletions(-)
diff --git a/clang/lib/Driver/ToolChains/Linux.cpp b/clang/lib/Driver/ToolChains/Linux.cpp
index 4300a2bdff1791..31e4ef74442c02 100644
--- a/clang/lib/Driver/ToolChains/Linux.cpp
+++ b/clang/lib/Driver/ToolChains/Linux.cpp
@@ -834,25 +834,6 @@ 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());
diff --git a/clang/lib/Driver/ToolChains/Linux.h b/clang/lib/Driver/ToolChains/Linux.h
index 524391743090ba..2d9e674e50a630 100644
--- a/clang/lib/Driver/ToolChains/Linux.h
+++ b/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:
diff --git a/clang/test/Driver/default-denormal-fp-math.c b/clang/test/Driver/default-denormal-fp-math.c
index 5f87e151df49e4..c04ad5c08b8d0d 100644
--- a/clang/test/Driver/default-denormal-fp-math.c
+++ b/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
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