The code snippet below compiles correctly without optimization, but with the
optimization flag turned on (at least with -O2) the compiler wrongly assumes
that the register %edx was not modified by the inline assembly statement.
The bug can be reproduced by issuing the following command.
To reproduce the bug, compile the code snippet bellow with the command
gcc -S -O2 file.c
and look the file file.s in the lines between the to #APP sections.
See bellow, after the source file, the assembly code produced with and without
optimization.
/********* file.c *************************************/
#define rsmpl8b(addr, d1, d2) __asm__ __volatile__(\
"outb %%al, %%dx\n\t" \
"movb %%cl, %%al\n\t" \
"addw $2, %%dx\n\t" \
"outb %%al, %%dx\n\t" \
: : "a" (d1), "c" (d2), "d" (addr))
void teste()
{
int i;
unsigned short base_addr = 0x378;
for (i = 0; i < 0x1000; i++) {
rsmpl8b(base_addr, 0xFF, 0xFE);
rsmpl8b(base_addr, 0xFD, 0xFC);
}
}
/*************** file.s compiled with gcc -S -O2 file.c *****/
.file "file.c"
.text
.globl teste
.type teste, @function
teste:
pushl %ebp
movl $888, %edx
movl %esp, %ebp
pushl %edi
movl $255, %edi
pushl %esi
movl $254, %esi
pushl %ebx
movl $253, %ebx
subl $4, %esp
movl $0, -16(%ebp)
.p2align 4,,15
.L2:
movl %edi, %eax
movl %esi, %ecx
#APP
outb %al, %dx
movb %cl, %al
addw $2, %dx
outb %al, %dx
#NO_APP
movl $252, %ecx
movl %ebx, %eax
#APP
outb %al, %dx
movb %cl, %al
addw $2, %dx
outb %al, %dx
#NO_APP
incl -16(%ebp)
cmpl $4096, -16(%ebp)
jne .L2
popl %eax
popl %ebx
popl %esi
popl %edi
popl %ebp
ret
.size teste, .-teste
.ident "GCC: (GNU) 4.1.2 20070214 ( (gdc 0.24, using dmd
1.020)(Gentoo 4.1.2p1.0.2)"
.section .note.GNU-stack,"",@progbits
/*************** file.s compiled with gcc -S file.c *********/
.file "file.c"
.text
.globl teste
.type teste, @function
teste:
pushl %ebp
movl %esp, %ebp
subl $16, %esp
movw $888, -2(%ebp)
movl $0, -8(%ebp)
jmp .L2
.L3:
movl $255, %eax
movl $254, %ecx
movzwl -2(%ebp), %edx
#APP
outb %al, %dx
movb %cl, %al
addw $2, %dx
outb %al, %dx
#NO_APP
movl $253, %eax
movl $252, %ecx
movzwl -2(%ebp), %edx
#APP
outb %al, %dx
movb %cl, %al
addw $2, %dx
outb %al, %dx
#NO_APP
incl -8(%ebp)
.L2:
cmpl $4095, -8(%ebp)
jle .L3
leave
ret
.size teste, .-teste
.ident "GCC: (GNU) 4.1.2 20070214 ((gdc 0.24, using dmd 1.020))
(Gentoo 4.1.2p1.0.2)"
.section .note.GNU-stack,"",@progbits
--
Summary: wrong assumption about registers used in asm statements
Product: gcc
Version: 4.1.2
Status: UNCONFIRMED
Severity: critical
Priority: P3
Component: inline-asm
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: jlima at sim dot ul dot pt
GCC host triplet: i386-linux-gnu
GCC target triplet: i386-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36051
