Feature request: gas .prefalign directive for body-size-dependent function alignment

[Fangrui Song]

On Wed, Mar 11, 2026 at 6:58 PM Fangrui Song <i@maskray.me> wrote:
>
> Hi, I've filed a feature request
> (https://sourceware.org/bugzilla/show_bug.cgi?id=33943 ) proposing a
> new directive, `.prefalign`, for size-proportional function alignment.
> (LLVM's integrated assembler has already implemented a variant of this
> for the `-ffunction-sections` case, though its section
> sh_addralign-only semantics felt surprising to me, so I refined the
> design.)
>
> ## Proposed Syntax
>
>   .prefalign <pref_align>, <end_sym>, nop
>   .prefalign <pref_align>, <end_sym>, <fill_byte>
>
> - `pref_align`: preferred (maximum) alignment — must be a power of 2
> - `end_sym`: a symbol marking the end of the code body (forward reference)
> - Third operand: `nop` for target-appropriate variable-size NOP fill,
> or an integer fill byte [0, 255].
>   Expressions are not allowed.
>
> All three operands are required. Traditional `.align` implicitly uses
> NOPs in executable sections; `.prefalign` makes the fill intent
> explicit to avoid that ambiguity.
>
> There is a very minor code-size benefit. For example, both GCC and
> Clang prefer .p2align 4 for x86, which is wasteful for small
> functions.
> The assembler directive, once implemented, would be superior to GCC's
> -flimit-function-alignment.

I am modifying the syntax to

.prefalign <log2_align>, <end_sym>, <fill-byte> or nop

per  https://sourceware.org/bugzilla/show_bug.cgi?id=33943

E.g.

# Align to 2**4 = 16, pad byte 0x00
.prefalign 4, end, 00

# Align to 2**4 = 16, pad byte 0xcc
.prefalign 4, end, cc

Then if multi-byte fills are ever needed,

.prefalign 4, end, 0001
.prefalign 4, end, 00010203