[llvm] [LV] Don't skip instrs with side-effects in reg pressure computation. (PR #126415)

[Florian Hahn via llvm-commits]

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@@ -5253,7 +5253,7 @@ LoopVectorizationCostModel::calculateRegisterUsage(ArrayRef<ElementCount> VFs) {
       OpenIntervals.erase(ToRemove);
 
     // Ignore instructions that are never used within the loop.
-    if (!Ends.count(I))
+    if (!Ends.count(I) && !I->mayHaveSideEffects())
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fhahn wrote:

Yes, any instruction that is not explicitly used and has side-effects. So here we don't skip intrinsics any more or calls that return void. 

Instructions that should not count as 'real' users, e.g. llvm.assume() calls are skipped below (via ValuesToIgnore).

https://github.com/llvm/llvm-project/pull/126415