DAGCombiner: don't do (zext (trunc x)) -> (and x, 65535) if !hasOneUse(trunc x) and zext is free

[escha]

Summary

This came up in our out of tree target and results in a lot of cases where 'x' ends up used by both (trunc x) and (and x, 65535), causing x to no longer have one use when it really should (and ending up materializing wasted instructions later).

[arsenm]

Testcase? This should apply to AMDGPU for i64/i32. For AMDGPU at least, I don't think we would want this even if there are multiple uses

[arsenm]

Actually shouldn't the and turn back into a trunc if it is free?

[escha]

ANDs can't turn back into truncs; trunc changes the width, AND doesn't; so I'm not *quite* sure what you mean. Either way, sounds like I need to reopen this with a (truncfree && zextfree) case, update the code to the latest revision, and find a test case.

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

   }
 
   // fold (zext (truncate x)) -> (and x, mask)
+  // Only do this if zext isn't free or (truncate x) has one use.
   if (N0.getOpcode() == ISD::TRUNCATE &&
-      (!LegalOperations || TLI.isOperationLegal(ISD::AND, VT))) {
+      (!LegalOperations || TLI.isOperationLegal(ISD::AND, VT)) &&
+      (N0.hasOneUse() || !TLI.isZExtFree(N0.getValueType(), VT))) {
 
     // fold (zext (truncate (load x))) -> (zext (smaller load x))
     // fold (zext (truncate (srl (load x), c))) -> (zext (smaller load (x+c/n)))