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c++gcccompiler-optimizationsimdauto-vectorization

Is `-ftree-slp-vectorize` not enabled by `-O2` in GCC?

From https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

It says "-ftree-slp-vectorize: Perform basic block vectorization on trees. This flag is enabled by default at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile."

However it seems I have to pass a flag explicitly to turn on SIMD. Did I mis undertand something here? It is enabled at -O3 though.

https://www.godbolt.org/z/1ffzdqMoT

Solution

  • Is -ftree-slp-vectorize not enabled by -O2 in GCC?

    Yes and no. It depends on the version of the compiler.

    From https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

    You have linked to the latest version of documentation. It applies to the version that is currently under development, which at the moment is version 12.

    However it seems I have to pass a flag explicitly to turn on SIMD.

    https://www.godbolt.org/z/1ffzdqMoT

    Your example uses GCC version 11.

    Did I mis undertand something here?

    You read the wrong version of documentation, or used wrong version of compiler and hence your assumption didn't hold.