[llvm] [IndVars] Teach widenLoopCompare to use sext if narrow IV is positive and other operand is already sext. (PR #142703)

[Philip Reames via llvm-commits]

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@@ -1630,6 +1630,12 @@ bool WidenIV::widenLoopCompare(WidenIV::NarrowIVDefUse DU) {
 
   // Widen the other operand of the compare, if necessary.
   if (CastWidth < IVWidth) {
+    // If the narrow IV is always postive and the other operand is sext, widen
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preames wrote:

The bit about not supporting the signed comparison was my key concern.  I was going to reason through the unsigned variants, but the alive2 proof nicely does that.  Can we add an assert here which checks the no-signed condition precondition?

@dtcxzyw - Your proof really makes me think we should have a instcombine transform which catches this form.  Recognizing the icmp zext nonneg, zext (sext) form and replacing the two extends with one seems like a generally useful transform isn't it?

https://github.com/llvm/llvm-project/pull/142703