Index: llvm/lib/CodeGen/TargetInstrInfo.cpp
===================================================================
--- llvm/lib/CodeGen/TargetInstrInfo.cpp
+++ llvm/lib/CodeGen/TargetInstrInfo.cpp
@@ -1147,6 +1147,11 @@
   if (auto DestSrc = isCopyInstr(MI)) {
     Register DestReg = DestSrc->Destination->getReg();
 
+    // If the copy destination is the forwarding reg, describe the forwarding
+    // reg using the copy source as the backup location. Example:
+    //
+    //   x0 = MOV x7
+    //   call callee(x0)      ; x0 described as x7
     if (Reg == DestReg)
       return ParamLoadedValue(*DestSrc->Source, Expr);
 
Index: llvm/lib/Target/ARM/ARMBaseInstrInfo.h
===================================================================
--- llvm/lib/Target/ARM/ARMBaseInstrInfo.h
+++ llvm/lib/Target/ARM/ARMBaseInstrInfo.h
@@ -105,6 +105,11 @@
   Optional<DestSourcePair>
   isCopyInstrImpl(const MachineInstr &MI) const override;
 
+  /// Specialization of \ref TargetInstrInfo::describeLoadedValue, used to
+  /// enhance debug entry value descriptions for ARM targets.
+  Optional<ParamLoadedValue> describeLoadedValue(const MachineInstr &MI,
+                                                 Register Reg) const override;
+
 public:
   // Return whether the target has an explicit NOP encoding.
   bool hasNOP() const;
Index: llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
===================================================================
--- llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
+++ llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
@@ -1027,6 +1027,36 @@
   return DestSourcePair{MI.getOperand(0), MI.getOperand(1)};
 }
 
+Optional<ParamLoadedValue>
+ARMBaseInstrInfo::describeLoadedValue(const MachineInstr &MI,
+                                      Register Reg) const {
+  if (auto DstSrcPair = isCopyInstrImpl(MI)) {
+    Register DstReg = DstSrcPair->Destination->getReg();
+
+    // TODO: We don't handle cases where the forwarding reg is narrower/wider
+    // than the copy registers. Consider for example:
+    //
+    //   s16 = VMOVS s0
+    //   s17 = VMOVS s1
+    //   call @callee(d0)
+    //
+    // We'd like to describe the call site value of d0 as d8, but this requires
+    // gathering and merging the descriptions for the two VMOVS instructions.
+    //
+    // We also don't handle the reverse situation, where the forwarding reg is
+    // narrower than the copy destination:
+    //
+    //   d8 = VMOVD d0
+    //   call @callee(s1)
+    //
+    // We need to produce a fragment description (the call site value of s1 is
+    // /not/ just d8).
+    if (DstReg != Reg)
+      return None;
+  }
+  return TargetInstrInfo::describeLoadedValue(MI, Reg);
+}
+
 const MachineInstrBuilder &
 ARMBaseInstrInfo::AddDReg(MachineInstrBuilder &MIB, unsigned Reg,
                           unsigned SubIdx, unsigned State,
Index: llvm/test/DebugInfo/MIR/ARM/call-site-info-vmovd.mir
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/MIR/ARM/call-site-info-vmovd.mir
@@ -0,0 +1,147 @@
+# RUN: llc -O1 -debug-entry-values -filetype=obj -mtriple thumbv7em-apple-unknown-macho -start-after=machineverifier %s -o %t.o
+# RUN: llvm-dwarfdump %t.o | FileCheck %s
+
+# Crash test, reduced from:
+#
+# a(float);
+# b(double c) {
+#   d();
+#   a(c);
+# }
+#
+# Some minor surgery was performed on the MIR to remove a call to a truncating
+# cast builtin between the calls to "d" and "a", and to force d0 as the copy dst
+# and s0 as the forwarding reg.
+#
+# This was done to test the case where the copy dst (d0) is a super-reg of the
+# forwarding reg (s0).
+
+# CHECK: DW_TAG_GNU_call_site
+# CHECK-NEXT: DW_AT_abstract_origin {{.*}} "a"
+# CHECK-NOT: call_site_parameter
+
+--- |
+  ; ModuleID = 'unreduced2.c'
+  source_filename = "unreduced2.c"
+  target datalayout = "e-m:o-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64"
+  target triple = "thumbv7em-apple-unknown-macho"
+  ; Function Attrs: nounwind optsize uwtable
+  define arm_aapcs_vfpcc i32 @b(double %c) local_unnamed_addr #0 !dbg !16 {
+  entry:
+    call void @llvm.dbg.value(metadata double %c, metadata !21, metadata !DIExpression()), !dbg !22
+    %call = tail call arm_aapcs_vfpcc i32 bitcast (i32 (...)* @d to i32 ()*)(), !dbg !23
+    %conv = fptrunc double %c to float, !dbg !24
+    %call1 = tail call arm_aapcs_vfpcc i32 @a(float %conv), !dbg !25
+    ret i32 undef, !dbg !26
+  }
+  declare arm_aapcs_vfpcc i32 @d(...) local_unnamed_addr #0
+  declare !dbg !4 arm_aapcs_vfpcc i32 @a(float) local_unnamed_addr #0
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+  declare void @llvm.stackprotector(i8*, i8**)
+
+  attributes #0 = { "disable-tail-calls"="false" "frame-pointer"="all" }
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!10, !11, !12, !13, !14}
+  !llvm.ident = !{!15}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 10.0.0 (git@github.com:apple/llvm-project.git 6203ec90d0bb0fd705fc0ad00ee263e5579aa709)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !3, nameTableKind: None)
+  !1 = !DIFile(filename: "jnyn_freeBSD.c", directory: "/Users/vsk/tmp/jnyn")
+  !2 = !{}
+  !3 = !{!4}
+  !4 = !DISubprogram(name: "a", scope: !5, file: !5, line: 1, type: !6, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized, retainedNodes: !2)
+  !5 = !DIFile(filename: "unreduced2.c", directory: "/Users/vsk/tmp/jnyn")
+  !6 = !DISubroutineType(types: !7)
+  !7 = !{!8, !9}
+  !8 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  !9 = !DIBasicType(name: "float", size: 32, encoding: DW_ATE_float)
+  !10 = !{i32 7, !"Dwarf Version", i32 4}
+  !11 = !{i32 2, !"Debug Info Version", i32 3}
+  !12 = !{i32 1, !"wchar_size", i32 4}
+  !13 = !{i32 1, !"min_enum_size", i32 4}
+  !14 = !{i32 7, !"PIC Level", i32 2}
+  !15 = !{!"clang version 10.0.0 (git@github.com:apple/llvm-project.git 6203ec90d0bb0fd705fc0ad00ee263e5579aa709)"}
+  !16 = distinct !DISubprogram(name: "b", scope: !5, file: !5, line: 2, type: !17, scopeLine: 2, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !20)
+  !17 = !DISubroutineType(types: !18)
+  !18 = !{!8, !19}
+  !19 = !DIBasicType(name: "double", size: 64, encoding: DW_ATE_float)
+  !20 = !{!21}
+  !21 = !DILocalVariable(name: "c", arg: 1, scope: !16, file: !5, line: 2, type: !19)
+  !22 = !DILocation(line: 0, scope: !16)
+  !23 = !DILocation(line: 3, column: 3, scope: !16)
+  !24 = !DILocation(line: 4, column: 5, scope: !16)
+  !25 = !DILocation(line: 4, column: 3, scope: !16)
+  !26 = !DILocation(line: 5, column: 1, scope: !16)
+
+...
+---
+name:            b
+alignment:       2
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+hasWinCFI:       false
+registers:       []
+liveins:
+  - { reg: '$d0', virtual-reg: '' }
+frameInfo:
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       16
+  offsetAdjustment: 0
+  maxAlignment:    8
+  adjustsStack:    true
+  hasCalls:        true
+  stackProtector:  ''
+  maxCallFrameSize: 0
+  cvBytesOfCalleeSavedRegisters: 0
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+  localFrameSize:  0
+  savePoint:       ''
+  restorePoint:    ''
+fixedStack:      []
+stack:
+  - { id: 0, name: '', type: spill-slot, offset: -4, size: 4, alignment: 4,
+      stack-id: default, callee-saved-register: '$lr', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+  - { id: 1, name: '', type: spill-slot, offset: -8, size: 4, alignment: 4,
+      stack-id: default, callee-saved-register: '$r7', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+  - { id: 2, name: '', type: spill-slot, offset: -16, size: 8, alignment: 8,
+      stack-id: default, callee-saved-register: '$d8', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+callSites:
+  - { bb: 0, offset: 12, fwdArgRegs: [] }
+  - { bb: 0, offset: 15, fwdArgRegs:
+      - { arg: 0, reg: '$s0' } }
+constants:       []
+machineFunctionInfo: {}
+body:             |
+  bb.0.entry:
+    liveins: $d0, $lr, $d8
+
+    DBG_VALUE $d0, $noreg, !21, !DIExpression(), debug-location !22
+    frame-setup tPUSH 14, $noreg, $r7, killed $lr, implicit-def $sp, implicit $sp
+    frame-setup CFI_INSTRUCTION def_cfa_offset 8
+    frame-setup CFI_INSTRUCTION offset $lr, -4
+    frame-setup CFI_INSTRUCTION offset $r7, -8
+    $r7 = frame-setup tMOVr $sp, 14, $noreg
+    frame-setup CFI_INSTRUCTION def_cfa_register $r7
+    $sp = frame-setup VSTMDDB_UPD $sp, 14, $noreg, killed $d8
+    frame-setup CFI_INSTRUCTION offset $d8, -16
+    $s16 = VMOVS killed $s0, 14, $noreg
+    $s17 = VMOVS killed $s1, 14, $noreg, implicit-def $d8
+    DBG_VALUE $d8, $noreg, !21, !DIExpression(), debug-location !22
+    tBL 14, $noreg, @d, csr_aapcs, implicit-def dead $lr, implicit $sp, implicit-def $sp, implicit-def dead $r0, debug-location !23
+    $d0 = VMOVD killed $d8, 14, $noreg, debug-location !24
+    DBG_VALUE $d0, $noreg, !21, !DIExpression(), debug-location !22
+    tTAILJMPd @a, 14, $noreg, implicit $sp, implicit $sp, implicit killed $s0, debug-location !25
+
+...
Index: llvm/test/DebugInfo/MIR/ARM/call-site-info-vmovs.mir
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/MIR/ARM/call-site-info-vmovs.mir
@@ -0,0 +1,138 @@
+# RUN: llc -O1 -debug-entry-values -filetype=obj -mtriple thumbv7em-apple-unknown-macho -start-after=machineverifier %s -o %t.o
+# RUN: llvm-dwarfdump %t.o | FileCheck %s
+
+# Crash test, reduced from:
+#
+# a(double);
+# b(double c) {
+#   d();
+#   a(c);
+# }
+
+# CHECK: DW_TAG_GNU_call_site
+# CHECK-NEXT: DW_AT_abstract_origin {{.*}} "a"
+# CHECK-NOT: call_site_parameter
+
+--- |
+  ; ModuleID = 'unreduced.c'
+  source_filename = "unreduced.c"
+  target datalayout = "e-m:o-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64"
+  target triple = "thumbv7em-apple-unknown-macho"
+  ; Function Attrs: nounwind optsize uwtable
+  define arm_aapcs_vfpcc i32 @b(double %c) local_unnamed_addr #0 !dbg !16 {
+  entry:
+    call void @llvm.dbg.value(metadata double %c, metadata !18, metadata !DIExpression()), !dbg !19
+    %call = tail call arm_aapcs_vfpcc i32 bitcast (i32 (...)* @d to i32 ()*)(), !dbg !20
+    %call1 = tail call arm_aapcs_vfpcc i32 @a(double %c), !dbg !21
+    ret i32 undef, !dbg !22
+  }
+  declare arm_aapcs_vfpcc i32 @d(...) local_unnamed_addr #0
+  declare !dbg !4 arm_aapcs_vfpcc i32 @a(double) local_unnamed_addr #0
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+  declare void @llvm.stackprotector(i8*, i8**)
+
+  attributes #0 = { "disable-tail-calls"="false" "frame-pointer"="all" }
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!10, !11, !12, !13, !14}
+  !llvm.ident = !{!15}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 10.0.0 (git@github.com:apple/llvm-project.git 6203ec90d0bb0fd705fc0ad00ee263e5579aa709)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !3, nameTableKind: None)
+  !1 = !DIFile(filename: "jnyn_freeBSD.c", directory: "/Users/vsk/tmp/jnyn")
+  !2 = !{}
+  !3 = !{!4}
+  !4 = !DISubprogram(name: "a", scope: !5, file: !5, line: 1, type: !6, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized, retainedNodes: !2)
+  !5 = !DIFile(filename: "unreduced.c", directory: "/Users/vsk/tmp/jnyn")
+  !6 = !DISubroutineType(types: !7)
+  !7 = !{!8, !9}
+  !8 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  !9 = !DIBasicType(name: "double", size: 64, encoding: DW_ATE_float)
+  !10 = !{i32 7, !"Dwarf Version", i32 4}
+  !11 = !{i32 2, !"Debug Info Version", i32 3}
+  !12 = !{i32 1, !"wchar_size", i32 4}
+  !13 = !{i32 1, !"min_enum_size", i32 4}
+  !14 = !{i32 7, !"PIC Level", i32 2}
+  !15 = !{!"clang version 10.0.0 (git@github.com:apple/llvm-project.git 6203ec90d0bb0fd705fc0ad00ee263e5579aa709)"}
+  !16 = distinct !DISubprogram(name: "b", scope: !5, file: !5, line: 2, type: !6, scopeLine: 2, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !17)
+  !17 = !{!18}
+  !18 = !DILocalVariable(name: "c", arg: 1, scope: !16, file: !5, line: 2, type: !9)
+  !19 = !DILocation(line: 0, scope: !16)
+  !20 = !DILocation(line: 3, column: 3, scope: !16)
+  !21 = !DILocation(line: 4, column: 3, scope: !16)
+  !22 = !DILocation(line: 5, column: 1, scope: !16)
+
+...
+---
+name:            b
+alignment:       2
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+hasWinCFI:       false
+registers:       []
+liveins:
+  - { reg: '$d0', virtual-reg: '' }
+frameInfo:
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       16
+  offsetAdjustment: 0
+  maxAlignment:    8
+  adjustsStack:    true
+  hasCalls:        true
+  stackProtector:  ''
+  maxCallFrameSize: 0
+  cvBytesOfCalleeSavedRegisters: 0
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+  localFrameSize:  0
+  savePoint:       ''
+  restorePoint:    ''
+fixedStack:      []
+stack:
+  - { id: 0, name: '', type: spill-slot, offset: -4, size: 4, alignment: 4,
+      stack-id: default, callee-saved-register: '$lr', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+  - { id: 1, name: '', type: spill-slot, offset: -8, size: 4, alignment: 4,
+      stack-id: default, callee-saved-register: '$r7', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+  - { id: 2, name: '', type: spill-slot, offset: -16, size: 8, alignment: 8,
+      stack-id: default, callee-saved-register: '$d8', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+callSites:
+  - { bb: 0, offset: 12, fwdArgRegs: [] }
+  - { bb: 0, offset: 18, fwdArgRegs:
+      - { arg: 0, reg: '$d0' } }
+constants:       []
+machineFunctionInfo: {}
+body:             |
+  bb.0.entry:
+    liveins: $d0, $lr, $d8
+
+    DBG_VALUE $d0, $noreg, !18, !DIExpression(), debug-location !19
+    frame-setup tPUSH 14, $noreg, $r7, killed $lr, implicit-def $sp, implicit $sp
+    frame-setup CFI_INSTRUCTION def_cfa_offset 8
+    frame-setup CFI_INSTRUCTION offset $lr, -4
+    frame-setup CFI_INSTRUCTION offset $r7, -8
+    $r7 = frame-setup tMOVr $sp, 14, $noreg
+    frame-setup CFI_INSTRUCTION def_cfa_register $r7
+    $sp = frame-setup VSTMDDB_UPD $sp, 14, $noreg, killed $d8
+    frame-setup CFI_INSTRUCTION offset $d8, -16
+    $s16 = VMOVS killed $s0, 14, $noreg
+    $s17 = VMOVS killed $s1, 14, $noreg, implicit-def $d8
+    DBG_VALUE $d8, $noreg, !18, !DIExpression(), debug-location !19
+    tBL 14, $noreg, @d, csr_aapcs, implicit-def dead $lr, implicit $sp, implicit-def $sp, implicit-def dead $r0, debug-location !20
+    $s0 = VMOVS killed $s16, 14, $noreg, debug-location !21
+    $s1 = VMOVS killed $s17, 14, $noreg, implicit-def $d0, debug-location !21
+    DBG_VALUE $d0, $noreg, !18, !DIExpression(), debug-location !19
+    $sp = VLDMDIA_UPD $sp, 14, $noreg, def $d8, debug-location !21
+    $sp = t2LDMIA_UPD $sp, 14, $noreg, def $r7, def $lr, debug-location !21
+    tTAILJMPd @a, 14, $noreg, implicit $sp, implicit $sp, implicit killed $d0, debug-location !21
+
+...