CODEWITHSUNDEEP
cgccx86ssesimd
Genís
Genís

Reputation: 1508

SSE optimized code performs similar to plain version

I wanted to take my first steps with Intel's SSE so I followed the guide published here, with the difference that instead of developing for Windows and C++ I make it for Linux and C (therefore I don't use any _aligned_malloc but posix_memalign).

I also implemented one computing intensive method without making use of the SSE extensions. Surprisingly, when I run the program both pieces of code (that one with SSE and that one without) take similar amounts of time to run, usually being the time of the one using the SSE slightly higher than the other.

Is that normal? Could it be possible that GCC does already optimize with SSE (also using -O0 option)? I also tried the -mfpmath=387 option, but no way, still the same.

Upvotes: 3

Views: 1868

Answers (2)

Paul R
Paul R

Reputation: 212929

For floating point operations you may not see a huge benefit with SSE. Most modern x86 CPUs have two FPUs so double precision may only be about the same speed for SIMD vs scalar, and single precision might give you 2x for SIMD over scalar on a good day. For integer operations though, e.g. image or audio processing at 8 or 16 bits, you can still get substantial benefits with SSE.

Upvotes: 3

Necrolis
Necrolis

Reputation: 26171

GCC has a very good inbuilt code vectorizer, (which iirc kicks in at -O0 and above), so this means it will use SIMD in any place that it can in order to speed up scalar code (it will also optimize SIMD code a bit too, if its possible).

its pretty easy to confirm this is indeed whats happening here, just disassemble the output (or have gcc emit commented asm files).

Upvotes: 2

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