[llvm] [WIP][IndVarSimplify] Fix Masking Issue by Adding nsw/nuw Flags to Trunc Instruction (PR #150179)

[Luke Lau via llvm-commits]

================
@@ -1049,9 +1049,28 @@ linearFunctionTestReplace(Loop *L, BasicBlock *ExitingBB,
     if (Extended) {
       bool Discard;
       L->makeLoopInvariant(ExitCnt, Discard);
-    } else
+    } else{
       CmpIndVar = Builder.CreateTrunc(CmpIndVar, ExitCnt->getType(),
                                       "lftr.wideiv");
+
+      // Set the correct wrap flag to avoid the masking issue.
+      Instruction *TruncInst = dyn_cast<Instruction>(CmpIndVar);
+      
+      // The TruncatedIV is incrementing.
+      if (const SCEVAddRecExpr *TruncAR =
+              dyn_cast<SCEVAddRecExpr>(TruncatedIV)) {
+        // If TruncIV does not cause self-wrap, explicitly add the nsw and nuw
+        // flags to TruncInst.
+        if (TruncAR->hasNoSelfWrap()) {
+          TruncInst->setHasNoSignedWrap();
+          TruncInst->setHasNoUnsignedWrap();
----------------
lukel97 wrote:

I think NUW and NSW imply NW, not the other way around, so I don't think we can apply NUW/NSW on trunc here

https://github.com/llvm/llvm-project/pull/150179