[DebugInfo][InstCombine] Do not overwrite prior DILocation for new Insts (#108565)

When InstCombine replaces an old instruction with a new instruction, it
copies !dbg and !annotation metadata from old to new. For some
InstCombine patterns we set a specific DILocation on the new instruction
prior to insertion, however, which more accurately reflects the new
instruction. This more specific DILocation may be overwritten on
insertion by a less appropriate one, resulting in a less correct line
mapping. This patch changes this behaviour to only copy the DILocation
from old to new if the new instruction has no existing DILocation (which
will always be the case for a new instruction unless InstCombine has
specifically set one).

Author: SLTozer

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -5170,8 +5170,13 @@ bool InstCombinerImpl::run() {
         LLVM_DEBUG(dbgs() << "IC: Old = " << *I << '\n'
                           << "    New = " << *Result << '\n');
 
-        Result->copyMetadata(*I,
-                             {LLVMContext::MD_dbg, LLVMContext::MD_annotation});
+        // We copy the old instruction's DebugLoc to the new instruction, unless
+        // InstCombine already assigned a DebugLoc to it, in which case we
+        // should trust the more specifically selected DebugLoc.
+        if (!Result->getDebugLoc())
+          Result->setDebugLoc(I->getDebugLoc());
+        // We also copy annotation metadata to the new instruction.
+        Result->copyMetadata(*I, LLVMContext::MD_annotation);
         // Everything uses the new instruction now.
         I->replaceAllUsesWith(Result);
 

llvm/test/Transforms/InstCombine/new-inst-dbgloc-overwrite.ll

@@ -0,0 +1,77 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --check-globals smart
+;; Tests that when InstCombine sets a DILocation on the new instruction when it
+;; is created, we do not try to overwrite that DILocation when we later insert
+;; the new instruction.
+;; In this test, InstCombine replaces two loads joined by a PHI into a PHI of
+;; the addresses followed by a load, and gives the new load a merge of the two
+;; incoming load DILocations. This test verifies that the new load keeps that
+;; DILocation, and doesn't have it overwritten by the DILocation of the original
+;; PHI. It will, however, receive the !annotation attached to the original PHI.
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+
+define void @test(ptr %xfA, ptr %xfB, i1 %cmp5) {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[CMP5:%.*]], label [[IF_ELSE:%.*]], label [[IF_THEN6:%.*]]
+; CHECK:       if.then6:
+; CHECK-NEXT:    br label [[IF_END11:%.*]]
+; CHECK:       if.else:
+; CHECK-NEXT:    br label [[IF_END11]]
+; CHECK:       if.end11:
+; CHECK-NEXT:    [[XFA_PN:%.*]] = phi ptr [ [[XFA:%.*]], [[IF_ELSE]] ], [ [[XFB:%.*]], [[IF_THEN6]] ]
+; CHECK-NEXT:    [[XF1_SROA_8_0_IN:%.*]] = getelementptr i8, ptr [[XFA_PN]], i64 4
+; CHECK-NEXT:    [[XF1_SROA_8_0:%.*]] = load float, ptr [[XF1_SROA_8_0_IN]], align 4, !dbg [[DBG3:![0-9]+]], !annotation [[META7:![0-9]+]]
+; CHECK-NEXT:    [[CMP_I:%.*]] = fcmp ugt float [[XF1_SROA_8_0]], 0.000000e+00
+; CHECK-NEXT:    br i1 [[CMP_I]], label [[IF_END_I:%.*]], label [[IF_THEN_I:%.*]]
+; CHECK:       if.then.i:
+; CHECK-NEXT:    br label [[IF_END_I]]
+; CHECK:       if.end.i:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br i1 %cmp5, label %if.else, label %if.then6
+
+if.then6:                                         ; preds = %entry
+  %xf1.sroa.8.0.xfB.sroa_idx = getelementptr i8, ptr %xfB, i64 4
+  %xf1.sroa.8.0.copyload = load float, ptr %xf1.sroa.8.0.xfB.sroa_idx, align 4, !dbg !3
+  br label %if.end11
+
+if.else:                                          ; preds = %entry
+  %xf1.sroa.8.0.xfA.sroa_idx = getelementptr i8, ptr %xfA, i64 4
+  %xf1.sroa.8.0.copyload494 = load float, ptr %xf1.sroa.8.0.xfA.sroa_idx, align 4, !dbg !7
+  br label %if.end11
+
+if.end11:                                         ; preds = %if.else, %if.then6
+  %xf1.sroa.8.0 = phi float [ %xf1.sroa.8.0.copyload494, %if.else ], [ %xf1.sroa.8.0.copyload, %if.then6 ], !dbg !8, !annotation !9
+  %cmp.i = fcmp ugt float %xf1.sroa.8.0, 0.000000e+00
+  br i1 %cmp.i, label %if.end.i, label %if.then.i
+
+if.then.i:                                        ; preds = %if.end11
+  br label %if.end.i
+
+if.end.i:                                         ; preds = %if.then.i, %if.end11
+  ret void
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 20.0.0git", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "test.cpp", directory: "/tmp")
+!2 = !{i32 2, !"Debug Info Version", i32 3}
+!3 = !DILocation(line: 13, column: 12, scope: !4)
+!4 = distinct !DISubprogram(name: "operator=", linkageName: "_ZN11btMatrix3x3aSERKS_", scope: null, file: !1, line: 61, type: !5, scopeLine: 62, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !6)
+!5 = distinct !DISubroutineType(types: !6)
+!6 = !{}
+!7 = !DILocation(line: 13, column: 15, scope: !4)
+!8 = !DILocation(line: 100, scope: !4)
+!9 = !{!"Test String"}
+;.
+; CHECK: [[META0:![0-9]+]] = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: [[META1:![0-9]+]], producer: "{{.*}}clang version {{.*}}", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+; CHECK: [[META1]] = !DIFile(filename: "test.cpp", directory: {{.*}})
+; CHECK: [[DBG3]] = !DILocation(line: 13, scope: [[META4:![0-9]+]])
+; CHECK: [[META4]] = distinct !DISubprogram(name: "operator=", linkageName: "_ZN11btMatrix3x3aSERKS_", scope: null, file: [[META1]], line: 61, type: [[META5:![0-9]+]], scopeLine: 62, spFlags: DISPFlagDefinition, unit: [[META0]], retainedNodes: [[META6:![0-9]+]])
+; CHECK: [[META5]] = distinct !DISubroutineType(types: [[META6]])
+; CHECK: [[META6]] = !{}
+; CHECK: [[META7]] = !{!"Test String"}
+;.