[PATCH] D89415: [AArch64][GlobalISel] Introduce a new post-isel optimization pass

[Amara Emerson via Phabricator via llvm-commits]

aemerson added inline comments.


================
Comment at: llvm/lib/Target/AArch64/GISel/AArch64PostSelectOptimize.cpp:91
+    // If this instruction uses NZCV, then NZCV is live.
+    if (II->readsRegister(AArch64::NZCV)) {
+      NZCVIsLive = true;
----------------
paquette wrote:
> Maybe a sledgehammer, but would it make sense to use LiveRegUnits here?
> 
> e.g. something like this would probably work?
> 
> ```
> LiveRegUnits LRU(...);
> LRU.addLiveOuts(MBB);
> bool NZCVDead = LRU.available(AArch64::NZCV);
> for (...) {
>    LRU.stepBackward(*II);
> 
>    // Did this instruction define NZCV?
>    bool NZCVDeadAtCurrInstr = LRU.available(AArch64::NZCV);
>    if (NZCVDead && !NZCVDeadAtCurrInstr) {
>       // If we have a def and NZCV is dead...
>    }
> 
>    NZCVDead = NZCVDeadAtCurrInstr;
> }
> ```
This particular optimization isn't needed anymore, but yes I couldn't quite remember what that LiveRegUnits utility was called.


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  https://reviews.llvm.org/D89415/new/

https://reviews.llvm.org/D89415