[PATCH] D79391: [DSE] Remove noop stores in MSSA.

[Alina Sbirlea via Phabricator via llvm-commits]

asbirlea added inline comments.


================
Comment at: llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp:1815
+    if (auto *Load = dyn_cast<LoadInst>(SI->getOperand(0))) {
+      if (Load->getPointerOperand() == SI->getOperand(1)) {
+        State.deleteDeadInstruction(SI);
----------------
fhahn wrote:
> I think the check is not strict enough at the moment. I think we have to ensure that there are no memory-writes between the load and the store that may alias them.
> 
> In the simple example below, IIUC the patch would currently incorrectly eliminate  ` store i32 %a, i32* %Q`.
> 
> ```
> define void @test(i32* %Q) {
>   %a = load i32, i32* %Q
>   store i32 10, i32* %Q
>   store i32 %a, i32* %Q
>   ret void
> }```
> 
> In terms of MemorySSA, I think we are looking for loads that use the same MemoryDef as `KillingDef`. One simple way to check this would be getting the memoryDef of KillingDef and check if it dominates the load I think.
> 
+1.
Please add a testcase similar to the example fhahn@ gave.


Repository:
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  https://reviews.llvm.org/D79391/new/

https://reviews.llvm.org/D79391