https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125916
Bug ID: 125916
Summary: [17 regression] Mesa maybe miscompiled with
__attribute__((const))
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: sjames at gcc dot gnu.org
Target Milestone: ---
I'm like 99% sure this is bogus but I want to check before I take it to them.
In the last few days, anything using llvmpipe in Mesa started to SIGFPE. This
ended up being because some property was 0.
The relevant function is:
```
static inline const
__attribute__((const))
struct util_cpu_caps_t *
util_get_cpu_caps (void)
{
extern void _util_cpu_detect_once (void);
extern struct _util_cpu_caps_state_t _util_cpu_caps_state;
/* On most CPU architectures, an atomic read is simply a regular memory
* load instruction with some extra compiler magic to prevent code
* re-ordering around it. The perf impact of doing this check should be
* negligible in most cases.
*
* Also, even though it looks like a bit of a lie, we've declared this
* function with ATTRIBUTE_CONST. The GCC docs say:
*
* "Calls to functions whose return value is not affected by changes to
* the observable state of the program and that have no observable
* effects on such state other than to return a value may lend
* themselves to optimizations such as common subexpression elimination.
* Declaring such functions with the const attribute allows GCC to avoid
* emitting some calls in repeated invocations of the function with the
* same argument values."
*
* The word "observable" is important here. With the exception of a
* llvmpipe debug flag behind an environment variable and a few unit tests,
* all of which emulate worse CPUs, this function neither affects nor is
* affected by any "observable" state. It has its own internal state for
* sure, but that state is such that it appears to return exactly the same
* value with the same internal data every time.
*/
if (unlikely (!p_atomic_read (&_util_cpu_caps_state.detect_done)))
call_once (&_util_cpu_caps_state.once_flag, _util_cpu_detect_once);
return &_util_cpu_caps_state.caps;
}
```
```
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/17/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-pc-linux-gnu
Configured with:
/var/tmp/portage.notmp/portage/sys-devel/gcc-17.0.9999/work/gcc-17.0.9999/configure
--host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --prefix=/usr
--bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/17
--includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/17/include
--datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/17
--mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/17/man
--infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/17/info
--with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/17/include/g++-v17
--disable-silent-rules --disable-dependency-tracking
--with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/17/python
--enable-libphobos --enable-objc-gc
--enable-languages=c,c++,d,objc,obj-c++,fortran,ada,algol68,m2,jit
--enable-obsolete --enable-secureplt --disable-werror --with-system-zlib
--enable-nls --without-included-gettext --disable-libunwind-exceptions
--enable-checking=yes,extra,rtl --with-bugurl=https://bugs.gentoo.org/
--with-pkgversion='Gentoo Hardened 17.0.9999 p, commit
364a6713b8046ab4d5dcdcffe8501432a4ec0632' --with-gcc-major-version-only
--enable-libstdcxx-time --enable-lto --disable-libstdcxx-pch
--enable-linker-build-id --enable-libada --enable-shared --enable-threads=posix
--enable-__cxa_atexit --enable-clocale=gnu --with-tls=gnu2 --enable-multilib
--with-multilib-list=m32,mx32,m64 --disable-fixed-point --with-abi=m64
--enable-targets=all --enable-offload-defaulted
--enable-offload-targets=nvptx-none --enable-libgomp --disable-libssp
--disable-cet --enable-systemtap --enable-valgrind-annotations
--enable-valgrind-interop --disable-vtable-verify --disable-libvtv --with-zstd
--with-isl --disable-isl-version-check --enable-default-pie --enable-host-pie
--enable-host-bind-now --enable-default-ssp --disable-fixincludes
--with-gxx-libcxx-include-dir=/usr/include/c++/v1 --enable-host-shared
--enable-libgdiagnostics --with-build-config='bootstrap-O3 bootstrap-lto-lean'
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 17.0.0 20260618 (experimental)
7374385c962b867eaf226d0e728aa10352a778bf (Gentoo Hardened 17.0.9999 p, commit
364a6713b8046ab4d5dcdcffe8501432a4ec0632)
```
