3 ms·

Did you use restrict? I made a simple test void nsum(float v, float acc, int n, int vc ) { int j, i; for(i = 0; i < n; i++) for(j = 0; j < vc; j++

Did you use restrict? I made a simple test void nsum(float v, float acc, int n, int vc ) { int j, i; for(i = 0; i < n; i++) for(j = 0; j < vc; j++) acc[i] += v[j][i]v[j][i]; } And then I tested the same function with a different declaration void nsum(float * restrict * v, float * restrict acc, int n, int vc ) The version without restrict qualifier had 1.01s runtime. Version with restrict had 0.45s runtime. Both were compiled with identical flags (just -O3) using the ancient gcc 4.4.5. (vectorizer is enabled by default at O3 even in this version). That's 2x speedup with a simple pointer definition.

nkurz 13y ago
Normally I'd use restrict and float pointers, but since I was trying to repeat what the original poster did, I used fixed arrays instead. Because of this, I did not see a difference with 'restrict'. But I might be missing something, or might have messed up with the array indexing. The generated GCC optimized function is 500 instructions long, and thus difficult to scan. I put my untested test code up here: http://pastebin.com/qB0DfkXN http://pastebin.com/qB0DfkXN
- sharpneli 13y ago
  At least on this ancient version of gcc restrict helps even with the fixed sized array argument. Without it the code of sum_of_squares_1 is as following: 400913: f3 0f 11 07 movss %xmm0,(%rdi) 400917: f3 0f 10 48 34 movss 0x34(%rax),%xmm1 40091c: f3 0f 59 c9 mulss %xmm1,%xmm1 400920: f3 0f 58 c8 addss %xmm0,%xmm1 400924: f3 0f 11 0f movss %xmm1,(%rdi) 400928: f3 0f 10 40 38 movss 0x38(%rax),%xmm0 40092d: f3 0f 59 c0 mulss %xmm0,%xmm0 400931: f3 0f 58 c1 addss %xmm1,%xmm0 400935: f3 0f 11 07 movss %xmm0,(%rdi) 400939: f3 0f 10 48 3c movss 0x3c(%rax),%xmm1 As you can see it stores the dst[y] on each iteration. With function definition of: void sum_of_squares_1(float dst[restrict ROWS], float src[restrict ROWS][COLS]) The disassembly becomes completely different. However the speed of the end result did not really change that much. Could you throw objdump -d of the best icc output to pastebin? I'm interested to see what kind of code it produces.
  - nkurz 13y ago
    icc -fno-alias -Wall -std=c99 -finline-functions -Ofast -march=native loop-optimization.c -o loop http://pastebin.com/qjEPy6Y0 http://pastebin.com/qjEPy6Y0 Late night here in California. Good night!

nkurz 13y ago

Normally I'd use restrict and float pointers, but since I was trying to repeat what the original poster did, I used fixed arrays instead. Because of this, I did not see a difference with 'restrict'. But I might be missing something, or might have messed up with the array indexing. The generated GCC optimized function is 500 instructions long, and thus difficult to scan. I put my untested test code up here: http://pastebin.com/qB0DfkXN http://pastebin.com/qB0DfkXN

sharpneli 13y ago
At least on this ancient version of gcc restrict helps even with the fixed sized array argument. Without it the code of sum_of_squares_1 is as following: 400913: f3 0f 11 07 movss %xmm0,(%rdi) 400917: f3 0f 10 48 34 movss 0x34(%rax),%xmm1 40091c: f3 0f 59 c9 mulss %xmm1,%xmm1 400920: f3 0f 58 c8 addss %xmm0,%xmm1 400924: f3 0f 11 0f movss %xmm1,(%rdi) 400928: f3 0f 10 40 38 movss 0x38(%rax),%xmm0 40092d: f3 0f 59 c0 mulss %xmm0,%xmm0 400931: f3 0f 58 c1 addss %xmm1,%xmm0 400935: f3 0f 11 07 movss %xmm0,(%rdi) 400939: f3 0f 10 48 3c movss 0x3c(%rax),%xmm1 As you can see it stores the dst[y] on each iteration. With function definition of: void sum_of_squares_1(float dst[restrict ROWS], float src[restrict ROWS][COLS]) The disassembly becomes completely different. However the speed of the end result did not really change that much. Could you throw objdump -d of the best icc output to pastebin? I'm interested to see what kind of code it produces.
- nkurz 13y ago
  icc -fno-alias -Wall -std=c99 -finline-functions -Ofast -march=native loop-optimization.c -o loop http://pastebin.com/qjEPy6Y0 http://pastebin.com/qjEPy6Y0 Late night here in California. Good night!