[PATCH] D70157: Align branches within 32-Byte boundary

[James Y Knight via Phabricator via llvm-commits]

jyknight added a comment.

> .push_align_branch_boundary [N,] [instruction,]*

I'd like to raise again the possibility of using a more general region directive to denote "It is allowable to add prefixes/nops before instructions in this region if the assembler wants to", as I'd started discussing in https://reviews.llvm.org/D71238#1786885 (but let's move the discussion here).

Whether this is OK or not on a particular piece of assembly-code is likely to be a generic property of the code, regardless of the purpose of the optimization. If we're going to have multiple assembler optimizations that can make use of this, it would be nice to express the "OK to pad" "not OK to pad" property only once, rather than once for each kind of optimization which might make such modifications.

In particular, I'd like to look ahead towards the potential implementation of two other features:

1. Allowing the assembler to prefix-pad instructions in order to avoid having to emit a NOP for p2align directives.
2. Allowing the assembler to do other instruction-padding performance optimizations to avoid other DSB cacheline limits.

To be concrete, I propose:
".autopad", ".noautopad": allow/disallow the assembler to emit padding via inserting a nop or prefix before any instruction, as needed.
".align_branch_boundary [N,] [instruction,]": Enable branch-boundary padding (per previous description).

In this scheme, I'd generally expect an ".align_branch_boundary" directive to be specified once at the beginning of the file, and ".autopad"/".noautopad" directives to be sprinkled throughout the file as required.


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  https://reviews.llvm.org/D70157/new/

https://reviews.llvm.org/D70157