https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64119
Bug ID: 64119
Summary: asm triggers local register variable data corruption
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: inline-asm
Assignee: unassigned at gcc dot gnu.org
Reporter: adam at consulting dot net.nz
//asm triggers local register variable data corruption
#include <stdio.h>
__attribute__((noinline)) void fn(int inputa, int inputb, int inputc) {
register int a asm ("rax");
register int b asm ("rbx");
register int c asm ("rcx");
a = inputa; b = inputb; c = inputc;
printf("The values of a, b and c are %d, %d and %d respectively\n", a, b, c);
a = inputa; b = inputb; c = inputc;
printf("The values of a, b and c are %d, %d and %d respectively\n", a, b, c);
a = inputa; b = inputb; c = inputc;
asm ("" : "+r" (a), "+r" (b), "+r" (c));
printf("The values of a, b and c are %d, %d and %d respectively\n", a, b, c);
}
int main(void) {
fn(1, 2, 3);
return 0;
}
$ gcc-4.9 -O0 gcc-asm-reg-var-data-corruption.c && ./a.out
The values of a, b and c are 1, 2 and 3 respectively
The values of a, b and c are 1, 2 and 3 respectively
The values of a, b and c are 1, 2 and 3 respectively
$ gcc-4.9 -O1 gcc-asm-reg-var-data-corruption.c && ./a.out
The values of a, b and c are 1, 2 and 3 respectively
The values of a, b and c are 1, 2 and 3 respectively
The values of a, b and c are 53, 2 and 39 respectively
$ clang-3.6 -O0 gcc-asm-reg-var-data-corruption.c && ./a.out
The values of a, b and c are 1, 2 and 3 respectively
The values of a, b and c are 1, 2 and 3 respectively
The values of a, b and c are 1, 2 and 3 respectively
$ clang-3.6 -O1 gcc-asm-reg-var-data-corruption.c && ./a.out
The values of a, b and c are 1, 2 and 3 respectively
The values of a, b and c are 1, 2 and 3 respectively
The values of a, b and c are 1, 2 and 3 respectively
I think it's unfortunate that gcc allows library routines to clobber local
register variables (does clang save/restore values between library calls?)
Note: This code restores the variable values after each library call.
The asm statement should be harmless. At input it says: make sure rax, rbx and
rcx hold the values of a, b and c respectively. Since the inline assembly is
empty the asm output trivially satisfies the same constraint.
I upped the severity because I don't consider data corruption "normal" and this
kind of data corruption is difficult to isolate.
