[PATCH] D144535: [RISCV] Enable machine copy propagation for copy-like instructions

[Wang Pengcheng via Phabricator via llvm-commits]

pcwang-thead marked 2 inline comments as done.
pcwang-thead added inline comments.


================
Comment at: llvm/lib/Target/RISCV/RISCVTargetMachine.cpp:344
+
+  if (TM->getOptLevel() >= CodeGenOpt::Default && EnableRISCVCopyPropagation)
+    addPass(createMachineCopyPropagationPass(true));
----------------
craig.topper wrote:
> pcwang-thead wrote:
> > craig.topper wrote:
> > > Can this be before the two Expand passes? Especially the one that says "last possible moment"?
> > How about between the two Expand passes? we may produce MVs when expanding `PseudoCCMOVGPR`.
> Would optimizing them break the macrofusion that `PseudoCCMOVGPR` represents?
After thinking, I think MVs produced by `PseudoCCMOVGPR` won't be eliminatable.
```
dst = falsev
branch
  |   \
  |   TrueBB: 
  |     dst = truev
  |   /
MergeBB:
  usage of dst 
```
For these kinds of CFGs, `truev` could be propagated only if we do tail duplication and CFG becomes like below, but we won't run tail duplication again after expanding.
```
dst = falsev
branch
  |         \
MergeBB   TrueBB: 
  |         dst = truev
  |         duplicated MergeBB
  |         /
Successor of MergeBB
```
So I think it has no impact on the macrofusion.

----

Another reason why we inserted it after expanding pseudos is because some internal pseudos in our downstream will produce MVs.
  


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D144535/new/

https://reviews.llvm.org/D144535