[DebugInfo][SelectionDAG] Fix position of salvaged 'dangling' DBG_VALUEs (#94458)

`SelectionDAGBuilder::handleDebugValue` has a parameter `Order` which
represents the insert-at position for the new DBG_VALUE. Prior to this patch
`SelectionDAGBuilder::SDNodeOrder` is used instead of the `Order` parameter.

The only code-paths where `Order != SDNodeOrder` are the two calls calls to
`handleDebugValue` from `salvageUnresolvedDbgValue`.
`salvageUnresolvedDbgValue` is called from `resolveOrClearDbgInfo` and
`dropDanglingDebugInfo`. The former is called after SelectionDAG completes one
block.

Some dbg.values can't be lowered to DBG_VALUEs right away. These get recorded
as 'dangling' - their order-number is saved - and get salvaged later through
`dropDanglingDebugInfo`, or if we've still got dangling debug info once the
whole block has been emitted, through `resolveOrClearDbgInfo`. Their saved
order-number is passed to `handleDebugValue`.

Prior to this patch, DBG_VALUEs inserted using these functions are inserted at
the "current" `SDNodeOrder` rather than the intended position that is passed to
the function.

Fix and add test.

Author: OCHyams

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -1684,7 +1684,7 @@ bool SelectionDAGBuilder::handleDebugValue(ArrayRef<const Value *> Values,
           if (!FragmentExpr)
             continue;
           SDDbgValue *SDV = DAG.getVRegDbgValue(
-              Var, *FragmentExpr, RegAndSize.first, false, DbgLoc, SDNodeOrder);
+              Var, *FragmentExpr, RegAndSize.first, false, DbgLoc, Order);
           DAG.AddDbgValue(SDV, false);
           Offset += RegisterSize;
         }
@@ -1699,11 +1699,10 @@ bool SelectionDAGBuilder::handleDebugValue(ArrayRef<const Value *> Values,
   }
 
   // We have created a SDDbgOperand for each Value in Values.
-  // Should use Order instead of SDNodeOrder?
   assert(!LocationOps.empty());
-  SDDbgValue *SDV = DAG.getDbgValueList(Var, Expr, LocationOps, Dependencies,
-                                        /*IsIndirect=*/false, DbgLoc,
-                                        SDNodeOrder, IsVariadic);
+  SDDbgValue *SDV =
+      DAG.getDbgValueList(Var, Expr, LocationOps, Dependencies,
+                          /*IsIndirect=*/false, DbgLoc, Order, IsVariadic);
   DAG.AddDbgValue(SDV, /*isParameter=*/false);
   return true;
 }

llvm/test/DebugInfo/X86/sdag-order.ll

@@ -0,0 +1,46 @@
+; RUN: llc %s --stop-after=finalize-isel -o - | FileCheck %s
+
+;; Check the DBG_VALUE which is salvaged from the dbg.value using an otherwised
+;; unused value is emitted at the correct position in the function.
+;; Prior (-) to patching (+), these DBG_VALUEs would sink to the bottom of the
+;; function:
+;; │   bb.1.if.then:
+;; │-    $rax = COPY %1
+;; │     DBG_VALUE 0, $noreg, !9, !DIExpression(DW_OP_plus_uconst, 4, DW_OP_stack_value)
+;; │+    $rax = COPY %1
+;; │     RET 0, $rax
+
+; CHECK: bb.1.if.then:
+; CHECK-NEXT: DBG_VALUE 0, $noreg, ![[#]], !DIExpression(DW_OP_plus_uconst, 4, DW_OP_stack_value)
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @badger(ptr sret(i64) %sret) !dbg !5 {
+entry:
+  %f.i = getelementptr i8, ptr null, i64 4
+  br label %if.then
+
+if.then:                                          ; preds = %entry
+  tail call void @llvm.dbg.value(metadata ptr %f.i, metadata !9, metadata !DIExpression()), !dbg !11
+  ret void
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!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: "test.ll", directory: "/")
+!2 = !{i32 3}
+!3 = !{i32 1}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = distinct !DISubprogram(name: "_ZNK1d1gEv", linkageName: "_ZNK1d1gEv", scope: null, file: !1, line: 1, type: !6, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !8)
+!6 = !DISubroutineType(types: !7)
+!7 = !{}
+!8 = !{!9}
+!9 = !DILocalVariable(name: "1", scope: !5, file: !1, line: 1, type: !10)
+!10 = !DIBasicType(name: "ty64", size: 64, encoding: DW_ATE_unsigned)
+!11 = !DILocation(line: 5, column: 1, scope: !5)