3 ms·

Most of all, the compiler can't change the layout of your data. Many of the use cases for hand-crafted assembly involve SIMD instructions, which perform several

by codeflo 14y ago

Most of all, the compiler can't change the layout of your data. Many of the use cases for hand-crafted assembly involve SIMD instructions, which perform several operations in parallel. These vector instructions can be incredibly fast, but their limitations often mean that you need to carefully design the data structures of the whole program around the optimization of one single critical loop.

colanderman 14y ago
I've written lots of code that either GCC has autovectorized into SIMD instructions, or that I've represented using GCC's vector types which produce SIMD instructions. While it is true that a poor choice of data structure will preclude SIMD optimizations, use of a compiler does not preclude the same.
- pmr_ 14y ago
  At the same time I have seen autovectorization to fail on things that would look like obvious candidates or to generate still sub-optimal code. I would prefer autovectorization over hand-written SIMD code everyday but that key here is reliability. If I cannot guarantee that the performance is going to stay the same across different compiler releases (which is often that case with autovectorization) it is much more convenient to just write the optimization myself and be sure that it is going to happen regardless of the environment.
zurn 14y ago
> the compiler can't change the layout of your data Sure it can, this is the most promising optimization avenue currently. Not just on the micro level to make data layout more SIMD-friendly, but more cache friendly object layout etc. Modern systems are primarily memory bound, not ALU bound, so squeezing out clock cycles from computations is pretty far in the dimishing returns territory. C/C++ have forbidden these kinds of optimizations but it's a the future (tm) in high level languages.

3 ms·

Most of all, the compiler can't change the layout of your data. Many of the use cases for hand-crafted assembly involve SIMD instructions, which perform several

by codeflo 14y ago

colanderman 14y ago
I've written lots of code that either GCC has autovectorized into SIMD instructions, or that I've represented using GCC's vector types which produce SIMD instructions. While it is true that a poor choice of data structure will preclude SIMD optimizations, use of a compiler does not preclude the same.
- pmr_ 14y ago
  At the same time I have seen autovectorization to fail on things that would look like obvious candidates or to generate still sub-optimal code. I would prefer autovectorization over hand-written SIMD code everyday but that key here is reliability. If I cannot guarantee that the performance is going to stay the same across different compiler releases (which is often that case with autovectorization) it is much more convenient to just write the optimization myself and be sure that it is going to happen regardless of the environment.
zurn 14y ago
> the compiler can't change the layout of your data Sure it can, this is the most promising optimization avenue currently. Not just on the micro level to make data layout more SIMD-friendly, but more cache friendly object layout etc. Modern systems are primarily memory bound, not ALU bound, so squeezing out clock cycles from computations is pretty far in the dimishing returns territory. C/C++ have forbidden these kinds of optimizations but it's a the future (tm) in high level languages.