[clang] [llvm] [LLVM][IRBuilder] Use NUW arithmetic for Create{ElementCount,TypeSize}. (PR #143532)

[Florian Hahn via llvm-commits]

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@@ -177,16 +177,16 @@ define void @add_unique_ind32(ptr noalias nocapture %a, i64 %n) {
 ; CHECK-LABEL: @add_unique_ind32(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
-; CHECK-NEXT:    [[TMP1:%.*]] = shl i64 [[TMP0]], 2
+; CHECK-NEXT:    [[TMP1:%.*]] = shl nuw i64 [[TMP0]], 2
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fhahn wrote:

I guess the code here already implicitly assumes that the multiply does not overflow, but it is not immedately clear how this follows from the definition of `llvm.vscale` in LangRef, which says only limits `vscale` to a positive constant:

```
vscale is a positive value that is constant throughout program execution, but is unknown at compile time. If the result value does not fit in the result type, then the result is a [poison value](https://llvm.org/docs/LangRef.html#poisonvalues).
```



https://github.com/llvm/llvm-project/pull/143532