https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92346
Bug ID: 92346
Summary: gcc mis-compute the side effect of inline assembly and
produce code with unexpected output
Product: gcc
Version: 8.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: 141242068 at smail dot nju.edu.cn
Target Milestone: ---
Code is attached:
```
#include <stdio.h>
int set_eax(int val) { return val; }
int main() {
int ax, tmp;
register int eax asm("eax") = set_eax(0x23232323);
asm volatile("movb $1, %%al\n" : "=r"(ax), "=r"(tmp)::"al");
printf("%08x\n", eax);
return 0;
}
```
This code will output `23232323` when compiled with clang and `gcc-8 -O2`,
however it output `23232301` in gcc with flags -O0. The inline assembly `movb
$1, $$al` will modify register `al`, this side effect has been informed to
compiler by clobber list, such inform help clang correctly compile the code,
but it seems that gcc ignores this in -O0 mode. `al` can be replaced by `ax`,
`eax`, gcc behaves samely.
gcc -v
```
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/8/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
8.3.0-6ubuntu1~18.04.1' --with-bugurl=file:///usr/share/doc/gcc-8/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-8
--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-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie
--with-system-zlib --with-target-system-zlib --enable-objc-gc=auto
--enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 8.3.0 (Ubuntu 8.3.0-6ubuntu1~18.04.1)
```
command and output:
```
$ cat a.c
#include <stdio.h>
int set_eax(int val) { return val; }
int main() {
int ax, tmp;
register int eax asm("eax") = set_eax(0x23232323);
asm volatile("movb $1, %%al\n" : "=r"(ax), "=r"(tmp)::"al");
printf("%08x\n", eax);
return 0;
}
$ gcc a.c -O0 -o a.out && ./a.out
23232301
$ gcc a.c -O1 -o a.out && ./a.out
23232323
$ clang a.c -O0 -o a.out && ./a.out
23232323
$ clang a.c -O1 -o a.out && ./a.out
23232323
```
