https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115552
Bug ID: 115552
Summary: wrong code at -O2 and above when strict-aliasing with
uint128 and double
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Keywords: alias, wrong-code
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: davidhu0903ex3 at gmail dot com
Target Milestone: ---
Created attachment 58469
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58469&action=edit
preproccessed file
=== Summary ===
When turning on -O2 optimization or above, the produced hash output is
different when without -O2, unless adding `-fno-strict-aliasing`. To my
investigation, the issue arise only when either:
1. output to a uint128_t*
2. input with a double*
=== Detail Explanation ===
The program produced the following output when compiled with -O1
60d8f0889f4b7d88
3bcf47a7eecabc5d
1b153029d058b5115490463dd8be0468
1b153029d058b5115490463dd8be0468
However, it produced different output with -O2
e09546989572a77c
95fe807a3fab3ad5
b8b1ef1e717bc0d4818be1e9343867f6
The first two lines are the case where the inputs are double* pointers. The
third line is the case where the output are written to a uint128_t* pointer.
The last line is the case of passing a uint128_t by value to `foo` before dump.
The warning didn't complain about strict-aliasing, but rather an ambiguous
message:
'd1' may be used uninitialized [-Wmaybe-uninitialized]
However, those variables are already initialized in `main`:
double d1 = 1;
double d2 = 2;
>From the generated assembly, the initialization of the inputs seems missing, so
perhaps that's the reason of the warning message. However, in production our
code didn't emit "-Wmaybe-uninitialized" at all, so that's what surprises us.
With Compiler Explorer (godbolt.org), I can roughly locate that this behavior
appear after version 10.5.0 and until 14.1.0.
=== Environment ===
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/12/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 12.2.0-14'
--with-bugurl=file:///usr/share/doc/gcc-12/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-12
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-gnu-unique-object --disable-vtable-verify --enable-plugin
--enable-default-pie --with-system-zlib --enable-libphobos-checking=release
--with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch
--disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-12-bTRWOB/gcc-12-12.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-12-bTRWOB/gcc-12-12.2.0/debian/tmp-gcn/usr
--enable-offload-defaulted --without-cuda-driver --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.2.0 (Debian 12.2.0-14)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O2' '-Wall' '-Wextra' '-o' 'poc'
'-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-linux-gnu/12/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu
poc.c -mtune=generic -march=x86-64 -Wall -Wextra -O2 -fpch-preprocess
-fasynchronous-unwind-tables -o poc.i
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/12/include-fixed"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/12/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-linux-gnu/12/include
/usr/local/include
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O2' '-Wall' '-Wextra' '-o' 'poc'
'-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-linux-gnu/12/cc1 -fpreprocessed poc.i -quiet -dumpbase
poc.c -dumpbase-ext .c -mtune=generic -march=x86-64 -O2 -Wall -Wextra -version
-fasynchronous-unwind-tables -o poc.s
GNU C17 (Debian 12.2.0-14) version 12.2.0 (x86_64-linux-gnu)
compiled by GNU C version 12.2.0, GMP version 6.2.1, MPFR version
4.1.1-p1, MPC version 1.3.1, isl version isl-0.25-GMP
warning: MPFR header version 4.1.1-p1 differs from library version 4.2.0.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
G
