https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63641
Bug ID: 63641
Summary: Invalid shift leads to incorrect code on 32-bit system
Product: gcc
Version: 5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ian at airs dot com
Compile and run this program with -m32 -O2 on an x86 system.
#include <stdio.h>
int f (unsigned char) __attribute__ ((noinline));
int
f (unsigned char b)
{
if (0x0 <= b && b <= 0x8)
goto L;
if (b == 0x0b)
goto L;
if (0x0e <= b && b <= 0x1a)
goto L;
if (0x1c <= b && b <= 0x1f)
goto L;
return 0;
L:
return 1;
}
int
main ()
{
printf ("%d\n", f (' '));
}
The program should print 0. However, when compiled with -m32 -O2 with current
mainline (revision 216611) it prints 1.
The generated code for f is:
00000000 <f>:
0: 8b 4c 24 04 mov 0x4(%esp),%ecx
4: 31 c0 xor %eax,%eax
6: 80 f9 20 cmp $0x20,%cl
9: 77 0a ja 15 <f+0x15>
b: b8 ff c9 ff f7 mov $0xf7ffc9ff,%eax
10: d3 e8 shr %cl,%eax
12: 83 e0 01 and $0x1,%eax
15: f3 c3 repz ret
The bug is obvious: when the value in %cl is 0x20, the shr does nothing. The
ja needs to be a jae.
