[llvm] [DebugInfo][LoopLoadElim] Fix missing debug location updates (PR #91839)

[Orlando Cazalet-Hyams via llvm-commits]

================
@@ -0,0 +1,85 @@
+; RUN: opt -passes=loop-load-elim -S < %s | FileCheck %s
+
+; LoopLoadElimination's propagateStoredValueToLoadUsers() replaces the
+; `load` (`%a`) with an hoisted initial `load` and a `phi` that forwards
+; the stored value.
+; This test checks that the debug location is propagated to the new `phi`
+; from the original `load` it replaces in block `%for.body` and the debug
+; location drop of the hoisted `load` in block `%entry`.
+; Moreover, this test also checks the debug location update of the new
+; `bitcast` created when the `load` type is mismatched with the `store` type:
+;   store i32 ...
+;   %a = load float, ...
+; Because the `bitcast` casts the old `load` value, it has the same debug
+; location as the old `load` (ie., the same as the new `phi`).
+
+; If the store and the load use different types, but have the same
+; size then we should still be able to forward the value.
+;
+;   for (unsigned i = 0; i < 100; i++) {
+;     A[i+1] = B[i] + 2;
+;     C[i] = ((float*)A)[i] * 2;
+;   }
+
+define void @f(ptr noalias %A, ptr noalias %B, ptr noalias %C, i64 %N) !dbg !5 {
+; CHECK-LABEL: define void @f(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[LOAD_INITIAL:%.*]] = load float, ptr {{.*}}, align 4{{$}}
+; CHECK:       for.body:
+; CHECK-NEXT:    [[STORE_FORWARDED:%.*]] = phi float [ [[LOAD_INITIAL]], %entry ], [ [[STORE_FORWARD_CAST:%.*]], %for.body ], !dbg [[DBG9:![0-9]+]]
+; CHECK:         [[STORE_FORWARD_CAST]] = bitcast i32 {{.*}} to float, !dbg [[DBG9]]
+; CHECK:       [[DBG9]] = !DILocation(line: 11,
----------------
OCHyams wrote:

```suggestion
; CHECK:       ![[DBG9]] = !DILocation(line: 11,
```

https://github.com/llvm/llvm-project/pull/91839