[llvm-dev] [RFC] Working around a PS2 hardware errata

[Cameron McInally via llvm-dev]

First off, awesome project! I love it. Comments inline...

On Tue, Nov 6, 2018 at 5:59 PM Dan Ravensloft via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> The MIPS R5900 core that powers the PlayStation 2's CPU has a hardware bug
> that affects very short loops of 6 or less instructions where the processor
> may fail to branch under "special conditions" (don't you love vague
> hardware manuals?)
>
> The obvious solution is to pad the loop with NOPs so that it is bigger
> than 6 instructions long.
>
> However, I have a few questions to ask about this.
>
> First, where would I go about adding a pass that detects and works around
> this hardware bug? Presumably it would have to go as a pass after
> instruction selection, but I'm unfamiliar with LLVM internals in this case.
>

Check out a similar pass in the X86
backend: llvm/lib/Target/X86/X86PadShortFunction.cpp

You'll likely need to use MachineLoopInfo to find the loops.

-Cameron
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