[DebugInfo][SelectionDAG] Change order while transferring SDDbgValue to another node

SelectionDAG::transferDbgValues() can 'reattach' SDDbgValue from one to
another node, but doesn't change its source order. If the destination node has
the order greater than the SDDbgValue, there are two possible issues
revealed later:

* If debug info is attached to an instruction that is the first definition
of a register, this ends up with a def-after-use and the debug info
gets 'undef' later.

* If MIR has another definition of a register above the debug info,
the debug info may represent a source variable incorrectly because
it appears (significantly) before an instruction corresponded
to this debug info.

So, the patch changes the order of an SDDbgValue when it is moved
to a node with greater order.

Reviewers: dblaikie, jmorse, aprantl

Reviewed By: aprantl

Subscribers: aprantl, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71175

2 files changed, 67 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index c33233efb02..a808b399596 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -8152,9 +8152,9 @@ void SelectionDAG::transferDbgValues(SDValue From, SDValue To,
       Expr = *Fragment;
     }
     // Clone the SDDbgValue and move it to To.
-    SDDbgValue *Clone =
-        getDbgValue(Var, Expr, ToNode, To.getResNo(), Dbg->isIndirect(),
-                    Dbg->getDebugLoc(), Dbg->getOrder());
+    SDDbgValue *Clone = getDbgValue(
+        Var, Expr, ToNode, To.getResNo(), Dbg->isIndirect(), Dbg->getDebugLoc(),
+        std::max(ToNode->getIROrder(), Dbg->getOrder()));
     ClonedDVs.push_back(Clone);
 
     if (InvalidateDbg) {
diff --git a/llvm/test/DebugInfo/X86/sdag-transfer-dbgvalue.ll b/llvm/test/DebugInfo/X86/sdag-transfer-dbgvalue.ll
new file mode 100644
index 00000000000..772eb46941b
--- /dev/null
+++ b/llvm/test/DebugInfo/X86/sdag-transfer-dbgvalue.ll
@@ -0,0 +1,64 @@
+; RUN: llc -start-after=codegenprepare -stop-before finalize-isel -o - %s | FileCheck %s
+
+; This tests that transferDbgValues() changes order of SDDbgValue transferred
+; to another node and debug info for 'ADD32ri' appears *after* the instruction.
+;
+; This test case was generated from the following program
+; using: clang -g -O3 -S -emit-llvm test.c
+;
+; int foo(int a, int *b) {
+;   int c = a + 512;
+;   if (c != 0)
+;     *b = a;
+;   return c;
+; }
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK-LABEL: bb.0.entry:
+; CHECK:       %[[REG:[0-9]+]]:gr32 = ADD32ri %1, 512
+; CHECK-NEXT:  DBG_VALUE %[[REG]]
+
+; Function Attrs: nofree norecurse nounwind uwtable writeonly
+define dso_local i32 @foo(i32 %a, i32* nocapture %b) local_unnamed_addr !dbg !7 {
+entry:
+  %add = add nsw i32 %a, 512, !dbg !18
+  call void @llvm.dbg.value(metadata i32 %add, metadata !16, metadata !DIExpression()), !dbg !17
+  %cmp = icmp eq i32 %add, 0, !dbg !18
+  br i1 %cmp, label %if.end, label %if.then, !dbg !18
+
+if.then:                                          ; preds = %entry
+  store i32 %a, i32* %b, align 4, !dbg !18
+  br label %if.end, !dbg !18
+
+if.end:                                           ; preds = %entry, %if.then
+  ret i32 %add, !dbg !18
+}
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 10.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "test.c", directory: "/")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 10.0.0"}
+!7 = distinct !DISubprogram(name: "foo", scope: !8, file: !8, line: 1, type: !9, scopeLine: 1, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !13)
+!8 = !DIFile(filename: "test.c", directory: "/")
+!9 = !DISubroutineType(types: !10)
+!10 = !{!11, !11, !12}
+!11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!12 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !11, size: 64)
+!13 = !{!14, !15, !16}
+!14 = !DILocalVariable(name: "a", arg: 1, scope: !7, file: !8, line: 1, type: !11)
+!15 = !DILocalVariable(name: "b", arg: 2, scope: !7, file: !8, line: 1, type: !12)
+!16 = !DILocalVariable(name: "c", scope: !7, file: !8, line: 2, type: !11)
+!17 = !DILocation(line: 0, scope: !7)
+!18 = !DILocation(line: 2, column: 13, scope: !7)