Variable properties lost when passed through aggregates - Githubissues

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Variable properties lost when passed through aggregates #54764

Open psiha opened 2 years ago

psiha commented 2 years ago

entry() has an aligned range (both the beginning and the end) specified with a pointer p and a size sz
entry() passes p and sz as separate arguments to foo_for( p, sz ) which loops on the range - it gets properly vectorized w/o any tail or alignment codegen
entry() passes p and sz as a std::span to foo_span({ p, sz }) which loops on the span - it gets suboptimally vectorized - as if the optimizer looses the alignment info

https://godbolt.org/z/xx7M15YYh

Notes:

on godbolt it looks as if at least the alignment of the size variable does get 'somewhat' preserved with Clang 14 but I cannot reproduce that locally with Clang-CL 14 (I get the same codegen as with Clang 13)
likely related to https://github.com/llvm/llvm-project/issues/54193

psiha commented 1 year ago

This seem to now finally be almost fixed with 'trunk' (whatever that currently is on godbolt), 16 is/was still broken. I say almost because the loop (the 'critical' part of the code) is now generated optimally however there are still some stray instructions being generated in the prologue (as if Clang still can't fully see through std::span).