When using -O>=1, gcc uses the wrong register for the inline assembly below. In
my actual usecase, it even does
bswap %eax
xorl %eax, %eax
so it instantly threw away the value. When using -m32, it seems to generate
correct code.
== CODE FILES ==
asgard:/tmp$ cat x.c
#include <stdint.h>
uint32_t
x()
{
return 0x11223344;
}
asgard:/tmp$ cat test.c
#include <stdint.h>
#include <stdio.h>
extern uint32_t x();
static inline __attribute__((always_inline))
bswap32(uint32_t i)
{
asm("bswap %0" : "=q"(i) : "q"(i));
return i;
}
int
main()
{
printf("%08X\n", bswap32(x()));
return 0;
}
== OUTPUT ==
asgard:/tmp$ gcc test.c x.c
asgard:/tmp$ ./a.out
44332211
asgard:/tmp$ gcc -O1 test.c x.c
asgard:/tmp$ ./a.out
381E625D
== ASSEMBLY OUTPUT (only important parts) ==
asgard:/tmp$ gcc -S test.c
asgard:/tmp$ cat test.s
[
]
movl $0, %eax
call x
movl %eax, -4(%rbp)
movl -4(%rbp), %eax
#APP
# 9 "test.c" 1
bswap %eax
# 0 "" 2
#NO_APP
movl %eax, -4(%rbp)
movl -4(%rbp), %eax
movl %eax, %edx
movl $.LC0, %eax
movl %edx, %esi
movq %rax, %rdi
movl $0, %eax
call printf
[
]
asgard:/tmp$ gcc -S -O1 test.c
asgard:/tmp$ cat test.s
[
]
movl $0, %eax
call x
#APP
# 9 "test.c" 1
bswap %edx
# 0 "" 2
#NO_APP
movl $.LC0, %esi
movl $1, %edi
movl $0, %eax
call __printf_chk
[
]
--
Summary: gcc generates broken code with -O>=1 for x86_64 after an
asm call
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: inline-asm
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: js-gcc at webkeks dot org
GCC build triplet: x86_64-linux-gnu
GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42214
