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

Author: Apochens

llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp

@@ -448,6 +448,10 @@ class LoadEliminationForLoop {
         new LoadInst(Cand.Load->getType(), InitialPtr, "load_initial",
                      /* isVolatile */ false, Cand.Load->getAlign(),
                      PH->getTerminator()->getIterator());
+    // We don't give any debug location to Initial, because it is inserted
+    // into the loop's preheader. A debug location inside the loop will cause
+    // a misleading stepping when debugging. The test update-debugloc-store
+    // -forwarded.ll checks this.
 
     PHINode *PHI = PHINode::Create(Initial->getType(), 2, "store_forwarded");
     PHI->insertBefore(L->getHeader()->begin());
@@ -462,14 +466,20 @@ class LoadEliminationForLoop {
            "The type sizes should match!");
 
     Value *StoreValue = Cand.Store->getValueOperand();
-    if (LoadType != StoreType)
+    if (LoadType != StoreType) {
       StoreValue = CastInst::CreateBitOrPointerCast(StoreValue, LoadType,
                                                     "store_forward_cast",
                                                     Cand.Store->getIterator());
+      // Because it casts the old `load` value and is used by the new `phi`
+      // which replaces the old `load`, we give the `load`'s debug location
+      // to it.
+      cast<Instruction>(StoreValue)->setDebugLoc(Cand.Load->getDebugLoc());
+    }
 
     PHI->addIncoming(StoreValue, L->getLoopLatch());
 
     Cand.Load->replaceAllUsesWith(PHI);
+    PHI->setDebugLoc(Cand.Load->getDebugLoc());
   }
 
   /// Top-level driver for each loop: find store->load forwarding

llvm/test/Transforms/LoopLoadElim/update-debugloc-store-forwarded.ll

@@ -0,0 +1,85 @@
+; RUN: opt -passes=loop-load-elim -S < %s | FileCheck %s
+
+; LoopLoadElimination's propagateStoredValueToLoadUsers() replaces the
+; `load` (`%a`) with a 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,
+;
+entry:
+  br label %for.body, !dbg !8
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ], !dbg !9
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1, !dbg !10
+  %Aidx_next = getelementptr inbounds i32, ptr %A, i64 %indvars.iv.next, !dbg !11
+  %Bidx = getelementptr inbounds i32, ptr %B, i64 %indvars.iv, !dbg !12
+  %Cidx = getelementptr inbounds i32, ptr %C, i64 %indvars.iv, !dbg !13
+  %Aidx = getelementptr inbounds i32, ptr %A, i64 %indvars.iv, !dbg !14
+  %b = load i32, ptr %Bidx, align 4, !dbg !15
+  %a_p1 = add i32 %b, 2, !dbg !16
+  store i32 %a_p1, ptr %Aidx_next, align 4, !dbg !17
+  %a = load float, ptr %Aidx, align 4, !dbg !18
+  %c = fmul float %a, 2.000000e+00, !dbg !19
+  %c.int = fptosi float %c to i32, !dbg !20
+  store i32 %c.int, ptr %Cidx, align 4, !dbg !21
+  %exitcond = icmp eq i64 %indvars.iv.next, %N, !dbg !22
+  br i1 %exitcond, label %for.end, label %for.body, !dbg !23
+
+for.end:                                          ; preds = %for.body
+  ret void, !dbg !24
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!2, !3}
+!llvm.module.flags = !{!4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "type-mismatch.ll", directory: "/")
+!2 = !{i32 17}
+!3 = !{i32 0}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = distinct !DISubprogram(name: "f", linkageName: "f", scope: null, file: !1, line: 1, type: !6, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0)
+!6 = !DISubroutineType(types: !7)
+!7 = !{}
+!8 = !DILocation(line: 1, column: 1, scope: !5)
+!9 = !DILocation(line: 2, column: 1, scope: !5)
+!10 = !DILocation(line: 3, column: 1, scope: !5)
+!11 = !DILocation(line: 4, column: 1, scope: !5)
+!12 = !DILocation(line: 5, column: 1, scope: !5)
+!13 = !DILocation(line: 6, column: 1, scope: !5)
+!14 = !DILocation(line: 7, column: 1, scope: !5)
+!15 = !DILocation(line: 8, column: 1, scope: !5)
+!16 = !DILocation(line: 9, column: 1, scope: !5)
+!17 = !DILocation(line: 10, column: 1, scope: !5)
+!18 = !DILocation(line: 11, column: 1, scope: !5)
+!19 = !DILocation(line: 12, column: 1, scope: !5)
+!20 = !DILocation(line: 13, column: 1, scope: !5)
+!21 = !DILocation(line: 14, column: 1, scope: !5)
+!22 = !DILocation(line: 15, column: 1, scope: !5)
+!23 = !DILocation(line: 16, column: 1, scope: !5)
+!24 = !DILocation(line: 17, column: 1, scope: !5)