Re: [RFC PATCH] ARM: thumb1: Use LDMIA/STMIA for DI/DF loads/stores

[Siarhei Volkau <lis8215@gmail.com>]

пн, 17 июн. 2024 г. в 15:43, Richard Earnshaw (lists)
<Richard.Earnshaw@arm.com>:

> I like the idea behind this patch, but I think I'd try first doing this as a peephole2 rule to rewrite the address in this case.  That has the additional advantage that we then estimate the size of the instruction more accurately.

Indeed, I tried it and it seems to work, although sometimes it does
odd things that I can't explain, e.g:

define_insn patch                    define_peephole2 patch
...                                  ...
ldmia   r0!, {r4, r5}                movs    r3, r0
ldmia   r1!, {r2, r3}                ldmia   r3!, {r4, r5}
movs    r0, r7                       movs    r0, r7
...                                  ldr     r2, [r1, #0]
                                     ldr     r3, [r1, #4]
                                     # r1 unused later on
                                     ...

But in general it finds a little bit more cases where ldmia/stmia can
be applied.

> > 2. Might it be profitable for thumb2?

> I think it would then be easy to extend this to thumb2 as well if it looks like a win (perhaps only for -Os in the thumb2 case).

Sounds good, I'll look at it later.

> For testing, I'd start with something like gcc/testsuite/gcc.target/arm/thumb-andsi.c as a template and adapt that for your specific case.  Matching something like "ldmia\tr[0-7]!," should be enough.

I'll send the v2 patch with test case(s) soon.

BR, Siarhei