In the following C source file test.c,
int compare(char const * p1, int n1, char const * p2, int n2) {
char const * q1 = p1 + n1;
char const * q2 = p2 + n2;
while (p1 < q1 && p2 < q2) {
int n = *--q1 - *--q2;
if (n) {
return n;
}
}
return n1 - n2;
}
int main(void) {
char str[1000];
int i;
for (i = 0; i < 1000000; ++i) {
compare(str, 1000, str, 1000);
}
}
compiled with
gcc -O2 test.c
the loop body within compare takes 14 instructions on GCC 3.4.3 Linux x86,
compared to only 11 instructions on GCC 3.2.2 Linux x86 (see disassemblies
below), and the GCC 3.4.3 output takes substantially longer to run than the GCC
3.2.2 output:
3.4.3> time ./a.out
4.690u 0.001s 0:04.71 99.5%
3.2.2> time ./a.out
3.533u 0.002s 0:03.55 99.4%
This seems to bite us on OpenOffice.org, which contains an oft-called function
similar to compare above, and new versions of OOo (built with GCC 3.4) run
slower than old versions (built with GCC 3.2). The question that remains for us
is whether this performance loss is specific to the given function, or could be
a general problem.
The dissasemblies:
08048360 <compare>: ! gcc 3.4.3
8048360: 55 push %ebp
8048361: 89 e5 mov %esp,%ebp
8048363: 57 push %edi
8048364: 56 push %esi
8048365: 53 push %ebx
8048366: 8b 45 0c mov 0xc(%ebp),%eax
8048369: 8b 7d 08 mov 0x8(%ebp),%edi
804836c: 8b 75 10 mov 0x10(%ebp),%esi
804836f: 8d 1c 07 lea (%edi,%eax,1),%ebx
8048372: 8b 45 14 mov 0x14(%ebp),%eax
8048375: 8d 0c 06 lea (%esi,%eax,1),%ecx
8048378: 90 nop
8048379: 8d b4 26 00 00 00 00 lea 0x0(%esi),%esi
8048380: 39 df cmp %ebx,%edi ! <-+
8048382: 0f 92 c0 setb %al ! |
8048385: 31 d2 xor %edx,%edx ! |
8048387: 39 ce cmp %ecx,%esi ! |
8048389: 0f 92 c2 setb %dl ! |
804838c: 85 d0 test %edx,%eax ! |
804838e: 74 13 je 80483a3 ! |
8048390: 4b dec %ebx ! |
8048391: 49 dec %ecx ! |
8048392: 0f be 01 movsbl (%ecx),%eax ! |
8048395: 0f be 13 movsbl (%ebx),%edx ! |
8048398: 29 c2 sub %eax,%edx ! |
804839a: 89 d0 mov %edx,%eax ! |
804839c: 74 e2 je 8048380 ! --+
804839e: 5b pop %ebx
804839f: 5e pop %esi
80483a0: 5f pop %edi
80483a1: 5d pop %ebp
80483a2: c3 ret
80483a3: 5b pop %ebx
80483a4: 8b 45 0c mov 0xc(%ebp),%eax
80483a7: 8b 55 14 mov 0x14(%ebp),%edx
80483aa: 5e pop %esi
80483ab: 29 d0 sub %edx,%eax
80483ad: 5f pop %edi
80483ae: 5d pop %ebp
80483af: c3 ret
08048370 <compare>: ! gcc 3.2.2
8048370: 55 push %ebp
8048371: 89 e5 mov %esp,%ebp
8048373: 57 push %edi
8048374: 8b 45 0c mov 0xc(%ebp),%eax
8048377: 56 push %esi
8048378: 8b 7d 08 mov 0x8(%ebp),%edi
804837b: 53 push %ebx
804837c: 8b 75 10 mov 0x10(%ebp),%esi
804837f: 8d 1c 38 lea (%eax,%edi,1),%ebx
8048382: 8b 45 14 mov 0x14(%ebp),%eax
8048385: 39 df cmp %ebx,%edi
8048387: 8d 0c 30 lea (%eax,%esi,1),%ecx
804838a: 73 1a jae 80483a6
804838c: 39 ce cmp %ecx,%esi
804838e: 73 16 jae 80483a6
8048390: 4b dec %ebx ! <-+
8048391: 49 dec %ecx ! |
8048392: 0f be 01 movsbl (%ecx),%eax ! |
8048395: 0f be 13 movsbl (%ebx),%edx ! |
8048398: 29 c2 sub %eax,%edx ! |
804839a: 89 d0 mov %edx,%eax ! |
804839c: 75 10 jne 80483ae ! |
804839e: 39 df cmp %ebx,%edi ! |
80483a0: 73 04 jae 80483a6 ! |
80483a2: 39 ce cmp %ecx,%esi ! |
80483a4: 72 ea jb 8048390 ! --+
80483a6: 8b 45 0c mov 0xc(%ebp),%eax
80483a9: 8b 55 14 mov 0x14(%ebp),%edx
80483ac: 29 d0 sub %edx,%eax
80483ae: 5b pop %ebx
80483af: 5e pop %esi
80483b0: 5f pop %edi
80483b1: 5d pop %ebp
80483b2: c3 ret
--
Summary: Performance regression in simple loop code
Product: gcc
Version: 3.4.3
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: regression
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: stephan dot bergmann at sun dot com
CC: gcc-bugs at gcc dot gnu dot org
GCC build triplet: i686-pc-linux-gnu
GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19672
