[kleinesfilmroellchen]

Tracking issue: #125612

On thumbv8m.main-none-eabi with -Os, this reduces memcpy from 324 bytes to 30 bytes. The assembly is so short and cute _UwU, I can include it here:

0000152a <__aeabi_memcpy>:
    152a:           1883        adds    r3, r0, r2
    152c:           4298        cmp     r0, r3
    152e:           bf28        it      cs
    1530:           4770        bxcs    lr
    1532:           b580        push    {r7, lr}
    1534:           466f        mov     r7, sp
    1536:       ,-> f811 3b01   ldrb.w  r3, [r1], #1
    153a:       |   3a01        subs    r2, #1
    153c:       |   f800 3b01   strb.w  r3, [r0], #1
    1540:       '-- d1f9        bne.n   1536 <__aeabi_memcpy+0xc>
    1542:           e8bd 4080   ldmia.w sp!, {r7, lr}
    1546:           4770        bx      lr

memset is reduced from 100 to 26 bytes at least in simple tests.

Introducing this change requires introducing the optimize_for_size feature to compiler-builtins, and passing it from alloc (and possibly other places I have missed). This is why I have made the PR here and not in the compiler-builtins repo.

I’ve done some testing on multiple ARM embedded targets, but overall this code should be trivially correct. The previous if <size-dependent condition> are simply turned into if false with the flag, always using the bytewise copy/write branch.

r? @diondokter
(as per our discussion about this on wg-embedded zulip)

[rustbot]

compiler-builtins is developed in its own repository. If possible, consider making this change to rust-lang/compiler-builtins instead.

cc @tgross35

[rustbot]

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[kleinesfilmroellchen]

r? libs

[clarfonthey]

Considering how this can be done via a separate PR to compiler-builtins, I would prefer that over this. We can still enable the feature on this repo without making changes to subtrees that will have to be reconciled later.

[kleinesfilmroellchen]

Considering how this can be done via a separate PR to compiler-builtins, I would prefer that over this. We can still enable the feature on this repo without making changes to subtrees that will have to be reconciled later.

So that will require a separate PR here to pass the flag through alloc, right?

[kleinesfilmroellchen]

See rust-lang/compiler-builtins#1333

[clarfonthey]

So that will require a separate PR here to pass the flag through alloc, right?

Correct; I believe that this should be fine even if the subtree isn't merged yet, since cargo features just do nothing if they're not defined for the other crate.