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[Thumb] Resolve FIXME: Use 'mov hi, $src; mov $dst, hi' #81908

Merged
merged 1 commit into from
Apr 5, 2024
Merged

[Thumb] Resolve FIXME: Use 'mov hi, $src; mov $dst, hi' #81908

merged 1 commit into from
Apr 5, 2024

Commits on Apr 2, 2024

  1. [Thumb] Resolve FIXME: Use 'mov hi, $src; mov $dst, hi' 

    Consider the following:

        ldr     r0, [r4]
        ldr     r7, [r0, #4]
        cmp     r7, r3
        bhi     .LBB0_6
        cmp     r0, r2
        push    {r0}
        pop     {r4}
        bne     .LBB0_3
        movs    r0, r6
        pop     {r4, r5, r6, r7}
        pop     {r1}
        bx      r1

Here is a snippet of the generated THUMB1 code of the K&R malloc function that clang currently compiles to.

push    {r0} ends up being popped to pop {r4}.

movs r4, r0 would destroy the flags set by cmp right above.

The compiler has no alternative in this case, except one:
the only alternative is to transfer through a high register.

However, it seems like LLVM does not consider that this is a valid approach, even though it is a free clobbering a high register.

This patch addresses the FIXME so the compiler can do that when it can in r10 or r11, or r12.
    @AtariDreams
    AtariDreams committed Apr 2, 2024
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