Bug 171075 – Air should support callsite-saves

• Status: RESOLVED
• Resolution: WONTFIX
• Component: JavaScriptCore

[Filip Pizlo]

Callsite-saves are an optimization that I found to be useful in a previous compiler project.  Basically, if you find that some register file needs callee-saves, but the ABI doesn't give them to you, you can fake it in the compiler: just designate some registers callsite-save and then do:

- Ignore those registers if they appear in the clobber set for any instruction during interference graph building.  Note that if an instruction defs a callsite-save, it still gets interference.

- After register allocation completes, emit save/restore code around any instruction that clobbers but does not def a callsite save.

I previously found this to be a >=30% speed-up on some floating-point code (some raytracer written in Java) on x86-32 when compiling to cdecl, which has no floating point callee-saves.  I used 3 callsite-saves in that project.

It's worth trying at B3 -O2.

[Filip Pizlo]

Created attachment 307636 [details]
it builds!

[Filip Pizlo]

Created attachment 307764 [details]
full attempt

This is neutral on everything except Kraken, where it's a 1% regression.

I guess we don't need it.

[Filip Pizlo]

It's not a progression, so we shouldn't do it.