The following code:
void func();
void test(char *signature)
{
char ch = signature[0];
if (ch == 15 || ch == 3)
{
if (ch == 15) func();
}
}
is compiled in suboptimal way by gcc 4.4. Check for ch==3 can be completely
eliminated since func is only called if ch==15. gcc 4.3 is able to properly
infer this and eliminate the unneeded check, but gcc 4.4 fails to do this.
Althouth, found originally on ARM, this bug also reproduces on x86 as well.
gcc 4.3.1 (with -m32 -O2):
test:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %eax
movzbl (%eax), %eax
cmpb $15, %al // %al compared only with 15
jne .L8
popl %ebp
jmp func
.L8:
popl %ebp
ret
gcc 4.4.0:
test:
pushl %ebp
movl %esp, %ebp
subl $8, %esp
movl 8(%ebp), %eax
movzbl (%eax), %eax
cmpb $15, %al
sete %dl
cmpb $3, %al // compiler was not able to optimize ch==3
je .L4
testb %dl, %dl
jne .L8
.L4:
leave
ret
.p2align 4,,7
.p2align 3
.L8:
leave
.p2align 4,,8
.p2align 3
jmp func
Bisection shows that it was introduced by
http://gcc.gnu.org/viewcvs?view=rev&revision=140288
--
Summary: [4.4 regression] missing DCE
Product: gcc
Version: 4.4.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: alexvod at google dot com
GCC build triplet: x86_64-unknown-linux-gnu
GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86-unknown-linux-gnu, arm-eabi
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39874
