Make sure PHIElimination doesn't copy debug locations across basic blocks.

PHIElimination may insert copy instructions in multiple basic
blocks. Moving debug locations across basic block boundaries would be
misleading as illustrated by the test case.

rdar://75463656

Differential Revision: https://reviews.llvm.org/D100886

llvm/lib/CodeGen/PHIElimination.cpp

@@ -475,9 +475,10 @@ void PHIElimination::LowerPHINode(MachineBasicBlock &MBB,
           if (DefMI->isImplicitDef())
             ImpDefs.insert(DefMI);
       } else {
-        NewSrcInstr =
-            TII->createPHISourceCopy(opBlock, InsertPos, MPhi->getDebugLoc(),
-                                     SrcReg, SrcSubReg, IncomingReg);
+        // Delete the debug location, since the copy is inserted into a
+        // different basic block.
+        NewSrcInstr = TII->createPHISourceCopy(opBlock, InsertPos, nullptr,
+                                               SrcReg, SrcSubReg, IncomingReg);
       }
     }
 

llvm/test/CodeGen/AArch64/PHIElimination-debugloc.mir

@@ -0,0 +1,41 @@
+# RUN: llc -mtriple=aarch64-linux-gnu -verify-machineinstrs -o - %s \
+# RUN:   -run-pass=livevars,phi-node-elimination,twoaddressinstruction \
+# RUN:   -no-phi-elim-live-out-early-exit=1 -phi-elim-split-all-critical-edges=1 \
+# RUN: | FileCheck %s
+
+--- |
+  define void @test() !dbg !7 {
+  entry:
+    ret void, !dbg !14
+  }
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!3, !4}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "Adrian", emissionKind: FullDebug)
+  !1 = !DIFile(filename: "/tmp/t.c", directory: "/")
+  !2 = !{}
+  !3 = !{i32 2, !"Dwarf Version", i32 2}
+  !4 = !{i32 2, !"Debug Info Version", i32 4}
+  !5 = !{i32 1, !"PIC Level", i32 2}
+  !7 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 1, type: !8, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: true, unit: !0)
+  !8 = !DISubroutineType(types: !{})
+  !14 = !DILocation(line: 1, column: 0, scope: !7)
+
+---
+name: test
+tracksRegLiveness: true
+body: |
+  bb.0:
+    liveins: $nzcv, $wzr
+    Bcc 8, %bb.2, implicit $nzcv
+
+  ; CHECK-LABEL: bb.1:
+  bb.1:
+    %x:gpr32 = COPY $wzr
+  ; Test that the debug location is not copied into bb1!
+  ; CHECK: %3:gpr32 = COPY killed %x{{$}}
+  ; CHECK-LABEL: bb.2:
+  bb.2:
+    %y:gpr32 = PHI %x:gpr32, %bb.1, undef %undef:gpr32, %bb.0, debug-location !14
+    $wzr = COPY %y:gpr32
+...