doc: document PCALIGN assembler directive

[mauri870]

The PCALIGN assembler directive is used to align instructions to a given boundary. On critical subroutines and critical innermost loops it can deliver a significant boost in performance.

Currently this pseudo instruction is available for ppc64, loong64, amd64, arm64 and soon for riscv64 once CL 541740 is merged. There is also ongoing work to optimize certain routines in #63678.

I propose adding a new section called "Instruction Alignment" to the https://go.dev/doc/asm that includes the following:

• Short introduction
• Supported architectures
• Usage
  • Tradeoffs
    • some cache space is lost to empty spaces before the aligned code entries
    • can lead to spurious alignment issues if not done correctly
  • Known places where adding it gets perf improvements (hot loops, critical sections)
  • Known boundary sizes that work (e.g 16 bytes on amd64, 32 for AVX2)

I'm happy to write such docs if we agree that it is relevant for the assembler documentation, preferably to be included in Go 1.22.

/cc @rsc @cherrymui @dmitshur

[mknyszek]

CC @golang/compiler

[cherrymui]

Sounds good to me for documenting this. I'd suggest a short description in the "Directives" section. The current https://go.dev/doc/asm documentation isn't performance-focused mostly, so I'd say let's keep the performance-related discussion short (a sentence or two). Thanks.

[gopherbot]

Change https://go.dev/cl/544115 mentions this issue: doc: document PCALIGN directive