[PATCH] D64393: [AMDGPU] Fix DPP combiner check for exec modification

[Valery Pykhtin via Phabricator via llvm-commits]

vpykhtin added inline comments.


================
Comment at: llvm/lib/Target/AMDGPU/SIInstrInfo.cpp:6159
+    }
+
     if (I->modifiesRegister(AMDGPU::EXEC, TRI))
----------------
foad wrote:
> vpykhtin wrote:
> > foad wrote:
> > > arsenm wrote:
> > > > foad wrote:
> > > > > vpykhtin wrote:
> > > > > > foad wrote:
> > > > > > > vpykhtin wrote:
> > > > > > > > vpykhtin wrote:
> > > > > > > > > Now I recall why I used the set originally - to detect use instruction fast. Matt, is there other way to detect whether the instruction use a register without scanning its operands?
> > > > > > > > and without scanning whether an instruction is in the use list for a register.
> > > > > > > We have to scan the operands of every instruction anyway, in order to test `I->modifiesRegister(AMDGPU::EXEC, TRI)`. Your original code did that too.
> > > > > > Good point. BTW you can probably use readsRegister instead of the loop.
> > > > > `readsRegister` doesn't appear to ignore debug operands.
> > > > debug uses should only appear on the debug pseudo-instructions, which you already skipped with the ->isDebugInstr check
> > > OK, if that is guaranteed (I wasn't sure) I could remove the `!Op.isDebug()` test. Do you want me to? But I still can't use `readsRegister` because I need to decrement `NumUse` multiple times if there are multiple operands that read the register.
> > > 
> > > I could also fold the check for modifying EXEC into the same loop, so that we don't have to do two separate loops over the operands of each instruction. But I don't know if it's worth it.
> > You can calculate NumUse as a number of use_nodbg_instructions in the first loop
> We've discussed that before. It only works if `use_nodbg_instructions` doesn't repeat instructions, which is only true if multiple uses by the same instruction are adjacent in the use list, which I'm not sure is necessarily true.
Again, there would be no sence if use_nodbg_instructions repeat instructions. Trust it and use :)


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https://reviews.llvm.org/D64393