[PATCH] D121277: [MachineSink] Check block prologue does not clobber uses

[Carl Ritson via Phabricator via llvm-commits]

critson added a comment.

In D121277#3370730 <https://reviews.llvm.org/D121277#3370730>, @MatzeB wrote:

> What do you mean by "block prologue"? Can you tell me which of the 2 `COPY`s in your test is falsely moved with your test and which instruction clobbers the register?
>
> (Intuitively I would expect a live-in value to be available after all PHI instructions so I don't see why there can be a clobber when something is inserted at the beginning of the basic block...)

I have pre-committed the test, so the offending sink should now be obvious.

This occurs because `SkipPHIsAndLabels` skips block prologue instructions as determined by the target, something which is not obvious from the function name.
On AMDGPU the `S_AND_SAVEEXEC_B64` is considered part of the block prologue because it is setting up the correct execution mask for the instructions in the block; however RA has assigned the register `$sgpr4` to the destination of this instruction, effectively clobbering the source of the `COPY` being sunk.


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https://reviews.llvm.org/D121277