[PATCH] D79916: Map -O to -O1 instead of -O2

Brandon Bergren via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Thu Sep 17 10:47:17 PDT 2020


Bdragon28 added a comment.

In D79916#2279045 <https://reviews.llvm.org/D79916#2279045>, @jrtc27 wrote:

> This has significantly regressed FreeBSD's performance with the new version of Clang. It seems Clang does not inline functions at -O1, unlike GCC, and since FreeBSD currently compiles its kernel with -O whenever debug symbols are enabled[1] (which, of course, is almost always true), this results in all its `static inline` helper functions not being inlined at all, a pattern that is common in the kernel, used for things like `get_curthread` and the atomics implementations.
>
> [1] This is a dubious decision made in r140400 in 2005 to provide "truer debugger stack traces" (well, before then there was ping-ponging between -O and -O2 based on concerns around correctness vs performance, but amd64 is an exception that has always used -O2 since r127180 it seems). Given that GCC will inline at -O, at least these days, the motivation seems to no longer exist, and compiling a kernel at anything other than -O2 (or maybe -O3) seems like a silly thing to do, but nevertheless it's what is currently done.
>
> Cc: @dim @trasz

This is actually SUCH a bad idea that a kernel built with -O will *not work at all* on 32 bit powerpc platforms (presumably due to allocating stack frames in the middle of assembly fragments in the memory management that are supposed to be inlined at all times.) I had to hack kern.pre.mk to rquest -O2 at all times just to get a functioning kernel.


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