[clang] [llvm] [InstCombine] Infer nsw/nuw for trunc (PR #87910)

[via cfe-commits]

================
@@ -897,7 +897,20 @@ Instruction *InstCombinerImpl::visitTrunc(TruncInst &Trunc) {
     }
   }
 
-  return nullptr;
+  bool Changed = false;
+  if (!Trunc.hasNoSignedWrap() &&
+      ComputeMaxSignificantBits(Src, /*Depth=*/0, &Trunc) <= DestWidth) {
+    Trunc.setHasNoSignedWrap(true);
+    Changed = true;
+  }
+  if (!Trunc.hasNoUnsignedWrap() &&
+      MaskedValueIsZero(Src, APInt::getBitsSetFrom(SrcWidth, DestWidth),
+                        /*Depth=*/0, &Trunc)) {
+    Trunc.setHasNoUnsignedWrap(true);
+    Changed = true;
+  }
----------------
goldsteinn wrote:

>From an efficiency POV, I think you should probably startout with a `computeKnownBits` on src and see if you get lucky about leading ones / zeros. Then if you don't, do a call to `computeMaxSignificantBits`.

https://github.com/llvm/llvm-project/pull/87910