[llvm-dev] [RFC][SLP] Let's turn -slp-vectorize-hor on by default

[Charlie Turner via llvm-dev]

I've done compile-time experiments for AArch64 over SPEC{2000,2006}
and of course the test-suite. I measure no significant compile-time
impact of enabling this feature by default.

I also ran the test-suite on an X86-64 machine. I can't imagine any
other targets being uniquely effected in terms of compile-time by
turning this on after testing both AArch64 and X86-64. I also timed
running the regression tests with -slp-vectorize-hor enabled and
disabled, no significant difference here either.

There are no significant performance regressions (or much
improvements) on AArch64 in night-test suite. I do see wins in third
party benchmarks when using this flag, which is why I'm asking if
there would be any objection from the community to making
-slp-vectorize-hor default on.

I have run the regression tests and looked through the bug tracker /
VC logs, I can't see any reason for not enabling it.

Thanks,
Charlie.