Why does GCC generate 15-20% faster code if I optimize for size instead of speed?
I first noticed in 2009 that GCC (at least on my projects and on my machines) have the tendency to generate noticeably faster code if I optimize for size (-Os) instead of speed (-O2 or -O3), and I have been wondering ever since why.
I have managed to create (rather silly) code that shows this surprising behavior and is sufficiently small to be posted here.
const int LOOP_BOUND = 200000000;
__attribute__((noinline))
static int add(const int& x, const int& y) {
return x + y;
}
__attribute__((noinline))
static int work(int xval, int yval) {
int sum(0);
for (int i=0; i<LOOP_BOUND; ++i) {
int x(xval+sum);
int y(yval+sum);
int z = add(x, y);
sum += z;
}
return sum;
}
int main(int , char* argv[]) {
int result = work(*argv[1], *argv[2]);
return result;
}
If I compile it with -Os, it takes 0.38 s to execute this program, and 0.44 s if it is compiled with -O2 or -O3. These times are obtained consistently and with practically no noise (gcc 4.7.2, x86_64 GNU/Linux, Intel Core i5-3320M).
(Update: I have moved all assembly code to GitHub: They made the post bloated and apparently add very little value to the questions as the fno-align-* flags have the same effect.)
Here is the generated assembly with -Os and -O2.
Unfortunately, my understanding of assembly is very limited, so I have no idea whether what I did next was correct: I grabbed the assembly for -O2 and merged all its differences into the assembly for -Os except the .p2align lines, result here. This code still runs in 0.38s and the only difference is the .p2align stuff.
If I guess correctly, these are paddings for stack alignment. According to Why does GCC pad functions with NOPs? it is done in the hope that the code will run faster, but apparently this optimization backfired in my case.
Is it the padding that is the culprit in this case? Why and how?
The noise it makes pretty much makes timing micro-optimizations impossible.
How can I make sure that such accidental lucky / unlucky alignments are not interfering when I do micro-optimizations (unrelated to stack alignment) on C or C++ source code?
UPDATE:
Following Pascal Cuoq's answer I tinkered a little bit with the alignments. By passing -O2 -fno-align-functions -fno-align-loops to gcc, all .p2align are gone from the assembly and the generated executable runs in 0.38s. According to the gcc documentation:
-Os enables all -O2 optimizations [but] -Os disables the following optimization flags:
-falign-functions -falign-jumps -falign-loops -falign-labels -freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays
So, it pretty much seems like a (mis)alignment issue.
I am still skeptical about -march=native as suggested in Marat Dukhan's answer. I am not convinced that it isn't just interfering with this (mis)alignment issue; it has absolutely no effect on my machine. (Nevertheless, I upvoted his answer.)
UPDATE 2:
We can take -Os out of the picture. The following times are obtained by compiling with
-O2 -fno-omit-frame-pointer0.37s-O2 -fno-align-functions -fno-align-loops0.37s-S -O2then manually moving the assembly ofadd()afterwork()0.37s-O20.44s
It looks like to me the distance of add() from the call site matters a lot. I have tried perf, but the output of perf stat and perf report makes very little sense to me. However, I could only get one consistent result out of it:
-O2:
602,312,864 stalled-cycles-frontend # 0.00% frontend cycles idle
3,318 cache-misses
0.432703993 seconds time elapsed
[...]
81.23% a.out a.out [.] work(int, int)
18.50% a.out a.out [.] add(int const&, int const&) [clone .isra.0]
[...]
¦ __attribute__((noinline))
¦ static int add(const int& x, const int& y) {
¦ return x + y;
100.00 ¦ lea (%rdi,%rsi,1),%eax
¦ }
¦ ? retq
[...]
¦ int z = add(x, y);
1.93 ¦ ? callq add(int const&, int const&) [clone .isra.0]
¦ sum += z;
79.79 ¦ add %eax,%ebx
For fno-align-*:
604,072,552 stalled-cycles-frontend # 0.00% frontend cycles idle
9,508 cache-misses
0.375681928 seconds time elapsed
[...]
82.58% a.out a.out [.] work(int, int)
16.83% a.out a.out [.] add(int const&, int const&) [clone .isra.0]
[...]
¦ __attribute__((noinline))
¦ static int add(const int& x, const int& y) {
¦ return x + y;
51.59 ¦ lea (%rdi,%rsi,1),%eax
¦ }
[...]
¦ __attribute__((noinline))
¦ static int work(int xval, int yval) {
¦ int sum(0);
¦ for (int i=0; i<LOOP_BOUND; ++i) {
¦ int x(xval+sum);
8.20 ¦ lea 0x0(%r13,%rbx,1),%edi
¦ int y(yval+sum);
¦ int z = add(x, y);
35.34 ¦ ? callq add(int const&, int const&) [clone .isra.0]
¦ sum += z;
39.48 ¦ add %eax,%ebx
¦ }
For -fno-omit-frame-pointer:
404,625,639 stalled-cycles-frontend # 0.00% frontend cycles idle
10,514 cache-misses
0.375445137 seconds time elapsed
[...]
75.35% a.out a.out [.] add(int const&, int const&) [clone .isra.0] ¦
24.46% a.out a.out [.] work(int, int)
[...]
¦ __attribute__((noinline))
¦ static int add(const int& x, const int& y) {
18.67 ¦ push %rbp
¦ return x + y;
18.49 ¦ lea (%rdi,%rsi,1),%eax
¦ const int LOOP_BOUND = 200000000;
¦
¦ __attribute__((noinline))
¦ static int add(const int& x, const int& y) {
¦ mov %rsp,%rbp
¦ return x + y;
¦ }
12.71 ¦ pop %rbp
¦ ? retq
[...]
¦ int z = add(x, y);
¦ ? callq add(int const&, int const&) [clone .isra.0]
¦ sum += z;
29.83 ¦ add %eax,%ebx
It looks like we are stalling on the call to add() in the slow case.
I have examined everything that perf -e can spit out on my machine; not just the stats that are given above.
For the same executable, the stalled-cycles-frontend shows linear correlation with the execution time; I did not notice anything else that would correlate so clearly. (Comparing stalled-cycles-frontend for different executables doesn't make sense to me.)
I included the cache misses as it came up as the first comment. I examined all the cache misses that can be measured on my machine by perf, not just the ones given above. The cache misses are very very noisy and show little to no correlation with the execution times.
Answer
By default compilers optimize for "average" processor. Since different processors favor different instruction sequences, compiler optimizations enabled by -O2 might benefit average processor, but decrease performance on your particular processor (and the same applies to -Os). If you try the same example on different processors, you will find that on some of them benefit from -O2 while other are more favorable to -Os optimizations.
Here are the results for time ./test 0 0 on several processors (user time reported):
Processor (System-on-Chip) Compiler Time (-O2) Time (-Os) Fastest
AMD Opteron 8350 gcc-4.8.1 0.704s 0.896s -O2
AMD FX-6300 gcc-4.8.1 0.392s 0.340s -Os
AMD E2-1800 gcc-4.7.2 0.740s 0.832s -O2
Intel Xeon E5405 gcc-4.8.1 0.603s 0.804s -O2
Intel Xeon E5-2603 gcc-4.4.7 1.121s 1.122s -
Intel Core i3-3217U gcc-4.6.4 0.709s 0.709s -
Intel Core i3-3217U gcc-4.7.3 0.708s 0.822s -O2
Intel Core i3-3217U gcc-4.8.1 0.708s 0.944s -O2
Intel Core i7-4770K gcc-4.8.1 0.296s 0.288s -Os
Intel Atom 330 gcc-4.8.1 2.003s 2.007s -O2
ARM 1176JZF-S (Broadcom BCM2835) gcc-4.6.3 3.470s 3.480s -O2
ARM Cortex-A8 (TI OMAP DM3730) gcc-4.6.3 2.727s 2.727s -
ARM Cortex-A9 (TI OMAP 4460) gcc-4.6.3 1.648s 1.648s -
ARM Cortex-A9 (Samsung Exynos 4412) gcc-4.6.3 1.250s 1.250s -
ARM Cortex-A15 (Samsung Exynos 5250) gcc-4.7.2 0.700s 0.700s -
Qualcomm Snapdragon APQ8060A gcc-4.8 1.53s 1.52s -Os
In some cases you can alleviate the effect of disadvantageous optimizations by asking gcc to optimize for your particular processor (using options -mtune=native or -march=native):
Processor Compiler Time (-O2 -mtune=native) Time (-Os -mtune=native)
AMD FX-6300 gcc-4.8.1 0.340s 0.340s
AMD E2-1800 gcc-4.7.2 0.740s 0.832s
Intel Xeon E5405 gcc-4.8.1 0.603s 0.803s
Intel Core i7-4770K gcc-4.8.1 0.296s 0.288s
Update: on Ivy Bridge-based Core i3 three versions of gcc (4.6.4, 4.7.3, and 4.8.1) produce binaries with significantly different performance, but the assembly code has only subtle variations. So far, I have no explanation of this fact.
Assembly from gcc-4.6.4 -Os (executes in 0.709 secs):
00000000004004d2 <_ZL3addRKiS0_.isra.0>:
4004d2: 8d 04 37 lea eax,[rdi+rsi*1]
4004d5: c3 ret
00000000004004d6 <_ZL4workii>:
4004d6: 41 55 push r13
4004d8: 41 89 fd mov r13d,edi
4004db: 41 54 push r12
4004dd: 41 89 f4 mov r12d,esi
4004e0: 55 push rbp
4004e1: bd 00 c2 eb 0b mov ebp,0xbebc200
4004e6: 53 push rbx
4004e7: 31 db xor ebx,ebx
4004e9: 41 8d 34 1c lea esi,[r12+rbx*1]
4004ed: 41 8d 7c 1d 00 lea edi,[r13+rbx*1+0x0]
4004f2: e8 db ff ff ff call 4004d2 <_ZL3addRKiS0_.isra.0>
4004f7: 01 c3 add ebx,eax
4004f9: ff cd dec ebp
4004fb: 75 ec jne 4004e9 <_ZL4workii+0x13>
4004fd: 89 d8 mov eax,ebx
4004ff: 5b pop rbx
400500: 5d pop rbp
400501: 41 5c pop r12
400503: 41 5d pop r13
400505: c3 ret
Assembly from gcc-4.7.3 -Os (executes in 0.822 secs):
00000000004004fa <_ZL3addRKiS0_.isra.0>:
4004fa: 8d 04 37 lea eax,[rdi+rsi*1]
4004fd: c3 ret
00000000004004fe <_ZL4workii>:
4004fe: 41 55 push r13
400500: 41 89 f5 mov r13d,esi
400503: 41 54 push r12
400505: 41 89 fc mov r12d,edi
400508: 55 push rbp
400509: bd 00 c2 eb 0b mov ebp,0xbebc200
40050e: 53 push rbx
40050f: 31 db xor ebx,ebx
400511: 41 8d 74 1d 00 lea esi,[r13+rbx*1+0x0]
400516: 41 8d 3c 1c lea edi,[r12+rbx*1]
40051a: e8 db ff ff ff call 4004fa <_ZL3addRKiS0_.isra.0>
40051f: 01 c3 add ebx,eax
400521: ff cd dec ebp
400523: 75 ec jne 400511 <_ZL4workii+0x13>
400525: 89 d8 mov eax,ebx
400527: 5b pop rbx
400528: 5d pop rbp
400529: 41 5c pop r12
40052b: 41 5d pop r13
40052d: c3 ret
Assembly from gcc-4.8.1 -Os (executes in 0.994 secs):
00000000004004fd <_ZL3addRKiS0_.isra.0>:
4004fd: 8d 04 37 lea eax,[rdi+rsi*1]
400500: c3 ret
0000000000400501 <_ZL4workii>:
400501: 41 55 push r13
400503: 41 89 f5 mov r13d,esi
400506: 41 54 push r12
400508: 41 89 fc mov r12d,edi
40050b: 55 push rbp
40050c: bd 00 c2 eb 0b mov ebp,0xbebc200
400511: 53 push rbx
400512: 31 db xor ebx,ebx
400514: 41 8d 74 1d 00 lea esi,[r13+rbx*1+0x0]
400519: 41 8d 3c 1c lea edi,[r12+rbx*1]
40051d: e8 db ff ff ff call 4004fd <_ZL3addRKiS0_.isra.0>
400522: 01 c3 add ebx,eax
400524: ff cd dec ebp
400526: 75 ec jne 400514 <_ZL4workii+0x13>
400528: 89 d8 mov eax,ebx
40052a: 5b pop rbx
40052b: 5d pop rbp
40052c: 41 5c pop r12
40052e: 41 5d pop r13
400530: c3 ret