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.