[llvm-dev] Wide load/store optimization question

[陳韋任 via llvm-dev]

Hi Peter,

  For i64, our custom backend only support load/store instruction. We refer
to Sparc,
making load i64 as load v2i32 rather then two load i32 (LLVM default
Expand). I would
be happy to hear other's experience on this, too.

HTH,
chenwj


2017-06-13 23:44 GMT+08:00 Peter Bel via llvm-dev <llvm-dev at lists.llvm.org>:

> Hi,
>
> I'm trying to write an LLVM backend for Epiphany arch, and I wonder if
> someone can give me some advice on how to implement load/store
> optimization. The CPU itself is 32-bit, but it supports wider 64-bit loads
> and store. So the basic idea is to make use of those by combining narrow
> ones.
>
> I've checked how it is done in AArch64 and Hexagon, and my current code is
> very close to the AArch64 one (used it as a kick-off). The problem lies in
> constraints imposed by the platform.
>
> The main constraint is that regs used should be sequential, lower reg
> should be even/zero. And obviously frame offsets should be sequential to be
> merged, dword-aligned for the lower reg offset.
>
> Because of those constraints I'm currently running this pass on pre-emit,
> after RA and frame finalization. But at that point most of the choices made
> (RA, frame offsets), and those are obviously suboptimal. The most common
> issue can look somehow like this:
>     str r1, [fp, -4]
>     str r2, [fp, -8]
> Those two stores can't be merged because the lower reg (r1) is not even.
> To merge them, r1 should be changed to r0, and r2 to r1. Sometimes the same
> problem happens when the frame offset is misaligned, e.g. r0 will have
> offset aligned to word, not dword.
>
> Can someone please point me out in which direction should I move? And also
> - at which step should I apply such pass? If on PreRA - how to set reg
> constraints such as regsequence, as well as frame constraints? If before
> frame finalization - how to  set frame constraints? If on pre-emit like i'm
> doing now - how to optimize and rewrite frame offsets and regs?
>
> Thanks,
> Petr
>
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>


-- 
Wei-Ren Chen (陳韋任)
Homepage: https://people.cs.nctu.edu.tw/~chenwj
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