[PATCH] D64766: [AMDGPU] Enable merging m0 initializations.

[Matt Arsenault via Phabricator via llvm-commits]

arsenm added inline comments.


================
Comment at: llvm/lib/Target/AMDGPU/SIFixSGPRCopies.cpp:458-459
+getFirstNonProlog(MachineBasicBlock *MBB, const TargetInstrInfo *TII) {
+  MachineBasicBlock::iterator I = MBB->getFirstNonPHI();
+  while (I != MBB->end() && TII->isBasicBlockPrologue(*I))
+    ++I;
----------------
rampitec wrote:
> arsenm wrote:
> > rampitec wrote:
> > > arsenm wrote:
> > > > Why does this need to care about the prolog instructions? (FYI I'm planning on eliminating the rarely followed concept)
> > > You actually cannot, it was created to prevent RA from insrting spills into endif/else block before we updated exec mask.
> > Yes, I'm working on splitting the blocks to avoid this
> I never found a way to properly split a block for that. Assume you split it and endif sequence is in the 1st block. What would prevent RA from inserting spill/restore there? You will need to mark a whole block as not suitable for spills and in addition prohibit RA from inserting any COPY instructions there.
> 
> Besides until there is another way m0 init must follow prologue anyway. If it is inserted before endif sequence that will break a contiguous sequence of prologue instructions and we will have bogus spilling again.
The details of the split aren't important here. Why does this scalar value initialization care?


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  https://reviews.llvm.org/D64766/new/

https://reviews.llvm.org/D64766