compiler-rt: omit symbols that are not needed because of machine instructions

[andrewrk]

As an optimization, our compiler-rt can check if generating a given symbol would be redundant, because it would never be called due to a machine code instruction being available, and avoid calling @export for that symbol. This will make 2 small improvements:

• Faster compiler-rt build from cold cache (fewer functions to optimize)
• Faster link against compiler-rt (since the static archive will be smaller)

Similar issue: #11883

This issue is marked contributor friendly. As a first-time contributor, how would I go about this? I think the relevant file is lib/compiler_rt.zig but how would I know if a symbol is unnecessary?

[Vexu]

A compiler-rt symbol is unnecessary if the target architecture has a native machine code instruction for that operation. For example __multi3 multiplies two 128-bit integers producing a new 128-bit integer, so if the target architecture has a machine code instruction that also does this multiplication then __multi3 is unnecessary.

What I meant is how to know if the target architecture has an instruction.

[Vexu]

I'd recommend the architecture's Instruction Set Architecture manual or some equivalent document.

[andrewrk]

You could also try writing zig code and see if it lowers to a compiler-rt function call instead of machine code instructions. You do need to be mindful of CPU feature sets, however, and unfortunately we don't have a section in the language reference about this yet.

This issue seems to require too much knowledge of machine instructions for me.