4 ms·

I think that by non-optimized the chart refers to not hand-optimized. The code is likely still compiled via -O3. In theory the GCC is capable to do all these o

I think that by non-optimized the chart refers to not hand-optimized. The code is likely still compiled via -O3. In theory the GCC is capable to do all these optimizations, even loop tiling, vectorization and autopar as it indeed has a polyhedral representation of loops. In practice it might be another story of course.

qayxc 5y ago
> The code is likely still compiled via -O3. That doesn't matter. You have to specify the target platform as well. Just take the naive code and specify -O3 in godbolt.org for example. Assembly exerpt of the inner loop: L5: movss xmm0, DWORD PTR [rax] mulss xmm0, DWORD PTR [rdx] add rax, 4 add rdx, rdi addss xmm1, xmm0 movss DWORD PTR [rcx], xmm1 cmp rsi, rax jne .L5 No AVX in sight. Now specify -mavx2 -mfma and compare the inner loop: .L5: vmovss xmm2, DWORD PTR [rax] vfmadd231ss xmm0, xmm2, DWORD PTR [rdx] add rax, 4 add rdx, rdi vmovss DWORD PTR [rcx], xmm0 cmp rsi, rax jne .L5 Case in point: without providing the compiler flags used for compilation, charts are misleading at best and outright wrong at worst.
- gpderetta 5y ago
  I believe the flags are in the compile.sh script in the repo: OMP_NUM_THREADS=8 g++ -Wall -std=c++11 -fopenmp -D_USE_OPENBLAS -O3 -L/usr/lib -lopenblas -I /usr/include/openblas -march=native -mavx2 -mfma -funroll-loops compare_blas.cpp -lopenblas -o matmul Of course it is possible that different flags were used to benchmark the naive version. edit: and of course those f*ss suffixes show that the compiler is not actually vectorizing the loops, just using the avx encoding of scalar instructions (plus fma). edit2: likely -ffast-math is also required for vectorization as it will change the sequence of operations, but after experimenting with it a bit it seems that in this case is still not sufficient.

qayxc 5y ago

> The code is likely still compiled via -O3. That doesn't matter. You have to specify the target platform as well. Just take the naive code and specify -O3 in godbolt.org for example. Assembly exerpt of the inner loop: L5: movss xmm0, DWORD PTR [rax] mulss xmm0, DWORD PTR [rdx] add rax, 4 add rdx, rdi addss xmm1, xmm0 movss DWORD PTR [rcx], xmm1 cmp rsi, rax jne .L5 No AVX in sight. Now specify -mavx2 -mfma and compare the inner loop: .L5: vmovss xmm2, DWORD PTR [rax] vfmadd231ss xmm0, xmm2, DWORD PTR [rdx] add rax, 4 add rdx, rdi vmovss DWORD PTR [rcx], xmm0 cmp rsi, rax jne .L5 Case in point: without providing the compiler flags used for compilation, charts are misleading at best and outright wrong at worst.

gpderetta 5y ago
I believe the flags are in the compile.sh script in the repo: OMP_NUM_THREADS=8 g++ -Wall -std=c++11 -fopenmp -D_USE_OPENBLAS -O3 -L/usr/lib -lopenblas -I /usr/include/openblas -march=native -mavx2 -mfma -funroll-loops compare_blas.cpp -lopenblas -o matmul Of course it is possible that different flags were used to benchmark the naive version. edit: and of course those f*ss suffixes show that the compiler is not actually vectorizing the loops, just using the avx encoding of scalar instructions (plus fma). edit2: likely -ffast-math is also required for vectorization as it will change the sequence of operations, but after experimenting with it a bit it seems that in this case is still not sufficient.