[PATCH] D138532: [SPARC][clang] Enable frame pointer optimization by default

Tue Nov 22 17:28:09 PST 2022

koakuma added a comment.

In D138532#3945422 <https://reviews.llvm.org/D138532#3945422>, @MaskRay wrote:

> I think we usually follow GCC instead of other targets.

GCC enables it by default too, for example see https://godbolt.org/z/brPMKnM17
(That link also has comparison with clang with `-fomit-frame-pointer`/`-fno-omit-frame-pointer`)

> I am unfamiliar with sparc. Can you show an example where `-fomit-frame-pointer`/`-fno-omit-frame-pointer` demonstrate a codegen difference? I tried some simple examples and cannot reproduce a difference.

Unfortunately on SPARC, due to the register window mechanics this optimization can only be done on leaf functions.
As for the concrete effects, it skips the `save` and `restore` instructions for register window management, which, among other things, does the saving/restoring of frame pointers, and changes the return instruction to `retl` (aka `jmp %o7+8`).

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https://reviews.llvm.org/D138532