Index: llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -5326,12 +5326,14 @@
     }
   }
 
-  if (!Op && N.getNode())
+  if (!Op && N.getNode()) {
     // Check if frame index is available.
-    if (LoadSDNode *LNode = dyn_cast<LoadSDNode>(N.getNode()))
+    SDValue LCandidate = peekThroughBitcasts(N);
+    if (LoadSDNode *LNode = dyn_cast<LoadSDNode>(LCandidate.getNode()))
       if (FrameIndexSDNode *FINode =
           dyn_cast<FrameIndexSDNode>(LNode->getBasePtr().getNode()))
         Op = MachineOperand::CreateFI(FINode->getIndex());
+  }
 
   if (!Op) {
     // Check if ValueMap has reg number.
Index: llvm/trunk/test/DebugInfo/ARM/float-stack-arg.ll
===================================================================
--- llvm/trunk/test/DebugInfo/ARM/float-stack-arg.ll
+++ llvm/trunk/test/DebugInfo/ARM/float-stack-arg.ll
@@ -0,0 +1,53 @@
+; RUN: llc -mtriple=armv4t-unknown-unknown -start-after=codegenprepare -stop-before=expand-isel-pseudos -o - %s | FileCheck %s
+
+; Verify that a stack-referencing DBG_VALUE is emitted for p5 at the start of
+; the function.
+;
+; Reproducer for PR40777.
+;
+; Based on the following C reproducer:
+;
+;   float fn1(int p1, int p2, int p3, int p4, float p5) {
+;    return p5;
+;   }
+;
+; that was compiled using -O1 -g -S -emit-llvm.
+;
+; Irrelevant metadata, e.g. information about %p[1-4], has been stripped.
+
+; CHECK: ![[P5:[0-9]*]] = !DILocalVariable(name: "p5"
+
+define arm_aapcscc float @fn1(i32 %p1, i32 %p2, i32 %p3, i32 %p4, float returned %p5) #0 !dbg !7 {
+; CHECK-LABEL: bb.0.entry:
+; CHECK-NEXT: DBG_VALUE %fixed-stack.0, 0, ![[P5]]
+entry:
+  call void @llvm.dbg.value(metadata float %p5, metadata !17, metadata !DIExpression()), !dbg !18
+  ret float %p5, !dbg !19
+}
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+attributes #0 = { norecurse nounwind readnone }
+attributes #1 = { nounwind readnone speculatable }
+
+!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 9.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "float.c", directory: "/")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"min_enum_size", i32 4}
+!6 = !{!"clang version 9.0.0"}
+!7 = distinct !DISubprogram(name: "fn1", scope: !1, file: !1, line: 1, type: !8, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !12)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10, !11, !11, !11, !11, !10}
+!10 = !DIBasicType(name: "float", size: 32, encoding: DW_ATE_float)
+!11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!12 = !{!17}
+!17 = !DILocalVariable(name: "p5", arg: 5, scope: !7, file: !1, line: 1, type: !10)
+!18 = !DILocation(line: 1, scope: !7)
+!19 = !DILocation(line: 2, scope: !7)