http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49057
Summary: benchmark of gcc. a piece of loop code compiled by
gcc-4.5.1 is slower compiled by gcc-4.4.2 when run on
cortex-a9.
Product: gcc
Version: 4.5.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: kun...@mediatek.com
The following C code is used to do a “integer add” test. The type of n, i, i1,
i2, loop_cnt are all ‘int’. the initial value: loop_cnt=5000000, i=0, i1=3,
i2=-3.
for (n = loop_cnt; n > 0; n--) { /* 0 x -x - initial value */
i += i1; /* x x -x */
i1 += i2; /* x 0 -x */
i1 += i2; /* x -x -x */
i2 += i; /* x -x 0 */
i2 += i; /* x -x x */
i += i1; /* 0 -x x */
i += i1; /* -x -x x */
i1 += i2; /* -x 0 x */
i1 += i2; /* -x x x */
i2 += i; /* -x x 0 */
i2 += i; /* -x x -x */
i += i1; /* 0 x -x */
/*
* Note that at loop end, i1 = -i2
*/
/*
* which is as we started. Thus,
*/
/*
* the values in the loop are stable
*/
}
I use gcc-4.4.2 and gcc-4.5.1 compile this C code, that will generate different
binary code.
Gcc-4.42:
284: e0800003 add r0, r0, r3
288: e2511001 subs r1, r1, #1 ; 0x1
28c: e0833082 add r3, r3, r2, lsl #1
290: e0822080 add r2, r2, r0, lsl #1
294: e0800083 add r0, r0, r3, lsl #1
298: e0833082 add r3, r3, r2, lsl #1
29c: e0822080 add r2, r2, r0, lsl #1
2a0: e0830000 add r0, r3, r0
2a4: 1afffff6 bne 284 <add_int+0x4c>
Gcc-4.5.1:
138: e0800003 add r0, r0, r3
13c: e0833082 add r3, r3, r2, lsl #1
140: e0822080 add r2, r2, r0, lsl #1
144: e2511001 subs r1, r1, #1
148: e0800083 add r0, r0, r3, lsl #1
14c: e0833082 add r3, r3, r2, lsl #1
150: e0822080 add r2, r2, r0, lsl #1
154: e0830000 add r0, r3, r0
158: 1afffff6 bne 138 <add_int+0x4c>
As you see, the only one difference is the position of “subs r1, r1, #1”,
and this difference has led to huge differences in performance. The performance
of the latter just has 80% of the former.
