https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123656
Bug ID: 123656
Summary: [16 Regression] wrong code with unsigned char [2]
vector multiplication by 210 at -Og
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: zsojka at seznam dot cz
Target Milestone: ---
Host: x86_64-pc-linux-gnu
Target: x86_64-pc-linux-gnu
Created attachment 63373
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=63373&action=edit
reduced testcase
Output:
$ x86_64-pc-linux-gnu-gcc -Og testcase.c
$ ./a.out
Aborted ./a.out
At the ASM level, the multiplication by 210 seems to be done as n*7*15*2:
"foo":
.LFB0:
.cfi_startproc
mov edx, edi # v, v
# testcase.c:6: return v * 210;
mov eax, edi # _4, v
sal ah, 3 # _4,
sal al, 3 # _4,
sub al, dil # _5, v
sub ah, dh # _5, v
mov edx, eax # _6, _5
sal dh, 4 # _6,
sal dl, 4 # _6,
sub al, dl # _7, _6
sub ah, dh # _7, _6
add ah, ah # _8
add al, al # _8
# testcase.c:7: }
ret
but the multiplication by 15 seems to multiply by -15 instead.
$ x86_64-pc-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=/repo/gcc-trunk/binary-latest-amd64/bin/x86_64-pc-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/repo/gcc-trunk/binary-trunk-20260118011631-r16-6881-gddda478641ffcc-checking-yes-rtl-df-extra-nobootstrap-amd64/bin/../libexec/gcc/x86_64-pc-linux-gnu/16.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /repo/gcc-trunk//configure --enable-languages=c,c++
--enable-valgrind-annotations --disable-nls --enable-checking=yes,rtl,df,extra
--disable-bootstrap --build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu
--target=x86_64-pc-linux-gnu --with-ld=/usr/bin/x86_64-pc-linux-gnu-ld
--with-as=/usr/bin/x86_64-pc-linux-gnu-as --enable-libsanitizer
--disable-libstdcxx-pch
--prefix=/repo/gcc-trunk//binary-trunk-20260118011631-r16-6881-gddda478641ffcc-checking-yes-rtl-df-extra-nobootstrap-amd64
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 16.0.1 20260118 (experimental) (GCC)
