https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64294
Bug ID: 64294
Summary: invalid code, zero check gets optimized away
Product: gcc
Version: 4.9.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gcc at breakpoint dot cc
Created attachment 34272
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34272&action=edit
the testcase
The testcase is a minimized / cut-out of some code which basically does:
if (!backsize)
exit(11);
while(backsize--) {
*ddst=*(ddst+backbytes);
ddst++;
}
gcc somehow assumes that backsize can't get zero which it can. I added an 'asm
volatile("labele:");' statement so the check can be easy spotted. At -O2 gcc
produces:
0000020a <labele>:
20a: 8b 44 24 20 mov 0x20(%esp),%eax
20e: 66 90 xchg %ax,%ax
210: 0f b6 54 0d 00 movzbl 0x0(%ebp,%ecx,1),%edx
215: 83 c5 01 add $0x1,%ebp
218: 88 55 ff mov %dl,-0x1(%ebp)
21b: 39 e8 cmp %ebp,%eax
21d: 75 f1 jne 210 <labele+0x6>
So it copies the first byte before checking for equal/zero.
With -O1 instead:
0000028a <labele>:
28a: 85 f6 test %esi,%esi
28c: 75 0a jne 298 <labele+0xe>
28e: 83 ec 0c sub $0xc,%esp
291: 6a 0b push $0xb
293: e8 fc ff ff ff call 294 <labele+0xa>
294: R_386_PC32 exit
298: 8b 5c 24 10 mov 0x10(%esp),%ebx
29c: 8b 54 24 2c mov 0x2c(%esp),%edx
2a0: 0f b6 0c 13 movzbl (%ebx,%edx,1),%ecx
2a4: 88 0b mov %cl,(%ebx)
2a6: 83 c3 01 add $0x1,%ebx
2a9: 39 d8 cmp %ebx,%eax
2ab: 75 f3 jne 2a0 <labele+0x16>
There is the 0 check withint the first two opcodes including the exit(0)
statement.
