Index: llvm/trunk/lib/CodeGen/PrologEpilogInserter.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/PrologEpilogInserter.cpp
+++ llvm/trunk/lib/CodeGen/PrologEpilogInserter.cpp
@@ -168,6 +168,46 @@
 /// StackObjSet - A set of stack object indexes
 using StackObjSet = SmallSetVector<int, 8>;
 
+using SavedDbgValuesMap =
+    SmallDenseMap<MachineBasicBlock *, SmallVector<MachineInstr *, 4>, 4>;
+
+/// Stash DBG_VALUEs that describe parameters and which are placed at the start
+/// of the block. Later on, after the prologue code has been emitted, the
+/// stashed DBG_VALUEs will be reinserted at the start of the block.
+static void stashEntryDbgValues(MachineBasicBlock &MBB,
+                                SavedDbgValuesMap &EntryDbgValues) {
+  SmallVector<const MachineInstr *, 4> FrameIndexValues;
+
+  for (auto &MI : MBB) {
+    if (!MI.isDebugInstr())
+      break;
+    if (!MI.isDebugValue() || !MI.getDebugVariable()->isParameter())
+      continue;
+    if (MI.getOperand(0).isFI()) {
+      // We can only emit valid locations for frame indices after the frame
+      // setup, so do not stash away them.
+      FrameIndexValues.push_back(&MI);
+      continue;
+    }
+    const DILocalVariable *Var = MI.getDebugVariable();
+    const DIExpression *Expr = MI.getDebugExpression();
+    auto Overlaps = [Var, Expr](const MachineInstr *DV) {
+      return Var == DV->getDebugVariable() &&
+             Expr->fragmentsOverlap(DV->getDebugExpression());
+    };
+    // See if the debug value overlaps with any preceding debug value that will
+    // not be stashed. If that is the case, then we can't stash this value, as
+    // we would then reorder the values at reinsertion.
+    if (llvm::none_of(FrameIndexValues, Overlaps))
+      EntryDbgValues[&MBB].push_back(&MI);
+  }
+
+  // Remove stashed debug values from the block.
+  if (EntryDbgValues.count(&MBB))
+    for (auto *MI : EntryDbgValues[&MBB])
+      MI->removeFromParent();
+}
+
 /// runOnMachineFunction - Insert prolog/epilog code and replace abstract
 /// frame indexes with appropriate references.
 bool PEI::runOnMachineFunction(MachineFunction &MF) {
@@ -191,6 +231,11 @@
   // place all spills in the entry block, all restores in return blocks.
   calculateSaveRestoreBlocks(MF);
 
+  // Stash away DBG_VALUEs that should not be moved by insertion of prolog code.
+  SavedDbgValuesMap EntryDbgValues;
+  for (MachineBasicBlock *SaveBlock : SaveBlocks)
+    stashEntryDbgValues(*SaveBlock, EntryDbgValues);
+
   // Handle CSR spilling and restoring, for targets that need it.
   if (MF.getTarget().usesPhysRegsForPEI())
     spillCalleeSavedRegs(MF);
@@ -210,6 +255,10 @@
   if (!F.hasFnAttribute(Attribute::Naked))
     insertPrologEpilogCode(MF);
 
+  // Reinsert stashed debug values at the start of the entry blocks.
+  for (auto &I : EntryDbgValues)
+    I.first->insert(I.first->begin(), I.second.begin(), I.second.end());
+
   // Replace all MO_FrameIndex operands with physical register references
   // and actual offsets.
   //
Index: llvm/trunk/test/DebugInfo/COFF/register-variables.ll
===================================================================
--- llvm/trunk/test/DebugInfo/COFF/register-variables.ll
+++ llvm/trunk/test/DebugInfo/COFF/register-variables.ll
@@ -24,9 +24,9 @@
 ; ASM: f:                                      # @f
 ; ASM: .Lfunc_begin0:
 ; ASM: # %bb.0:                                 # %entry
+; ASM:         #DEBUG_VALUE: f:p <- $ecx
 ; ASM:         pushq   %rsi
 ; ASM:         subq    $32, %rsp
-; ASM:         #DEBUG_VALUE: f:p <- $ecx
 ; ASM:         movl    %ecx, %esi
 ; ASM: [[p_ecx_esi:\.Ltmp.*]]:
 ; ASM:         #DEBUG_VALUE: f:p <- $esi
Index: llvm/trunk/test/DebugInfo/X86/prolog-params.mir
===================================================================
--- llvm/trunk/test/DebugInfo/X86/prolog-params.mir
+++ llvm/trunk/test/DebugInfo/X86/prolog-params.mir
@@ -0,0 +1,156 @@
+# RUN: llc -mtriple=x86_64-unknown-linux-gnu -run-pass=prologepilog %s -o - | FileCheck %s
+
+# MIR test case produced by compiling:
+#
+# typedef struct { int a; long b; } S1;
+#
+# extern void bar(int *);
+#
+# void foo(int p1, S1 p2, int p3, int p4, int p5, int p6, int p7, int p8) {
+#   int local1 = p1;
+#   int local2 = p6;
+#   int arr[2];
+#   bar(arr);
+# }
+#
+# using the following commands (with some manual stripping of metadata):
+#
+# $ clang -O1 -g -S -emit-llvm
+# $ llc -O1 -stop-before=prologepilog
+
+--- |
+  target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+  target triple = "x86_64-unknown-linux-gnu"
+
+  ; Function Attrs: nounwind uwtable
+  define dso_local void @foo(i32 %p1, i32 %p2.coerce0, i64 %p2.coerce1, i32 %p3, i32 %p4, i32 %p5, i32 %p6, i32 %p7, i32 %p8) local_unnamed_addr #0 !dbg !7 {
+  entry:
+    %arr = alloca [2 x i32], align 4
+    call void @llvm.dbg.value(metadata i32 %p1, metadata !18, metadata !DIExpression()), !dbg !28
+    call void @llvm.dbg.value(metadata i32 %p2.coerce0, metadata !19, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32)), !dbg !28
+    call void @llvm.dbg.value(metadata i64 %p2.coerce1, metadata !19, metadata !DIExpression(DW_OP_LLVM_fragment, 64, 64)), !dbg !28
+    call void @llvm.dbg.value(metadata i32 %p3, metadata !20, metadata !DIExpression()), !dbg !28
+    call void @llvm.dbg.value(metadata i32 %p4, metadata !21, metadata !DIExpression()), !dbg !28
+    call void @llvm.dbg.value(metadata i32 %p5, metadata !22, metadata !DIExpression()), !dbg !28
+    call void @llvm.dbg.value(metadata i32 %p6, metadata !23, metadata !DIExpression()), !dbg !28
+    call void @llvm.dbg.value(metadata i32 %p7, metadata !24, metadata !DIExpression()), !dbg !28
+    call void @llvm.dbg.value(metadata i32 %p8, metadata !25, metadata !DIExpression()), !dbg !28
+    call void @llvm.dbg.value(metadata i32 %p1, metadata !26, metadata !DIExpression()), !dbg !28
+    call void @llvm.dbg.value(metadata i32 %p6, metadata !27, metadata !DIExpression()), !dbg !28
+    %arraydecay1 = bitcast [2 x i32]* %arr to i32*, !dbg !28
+    call void @bar(i32* nonnull %arraydecay1), !dbg !28
+    ret void, !dbg !28
+  }
+
+  ; Function Attrs: nounwind readnone speculatable
+  declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
+
+  declare dso_local void @bar(i32*) local_unnamed_addr
+
+  ; Function Attrs: nounwind readnone speculatable
+  declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+  attributes #0 = { nounwind uwtable }
+  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: "params.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 9.0.0"}
+  !7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 5, type: !8, scopeLine: 5, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !17)
+  !8 = !DISubroutineType(types: !9)
+  !9 = !{null, !10, !11, !10, !10, !10, !10, !10, !10}
+  !10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  !11 = !DIDerivedType(tag: DW_TAG_typedef, name: "S1", file: !1, line: 1, baseType: !12)
+  !12 = distinct !DICompositeType(tag: DW_TAG_structure_type, file: !1, line: 1, size: 128, elements: !13)
+  !13 = !{!14, !15}
+  !14 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !12, file: !1, line: 1, baseType: !10, size: 32)
+  !15 = !DIDerivedType(tag: DW_TAG_member, name: "b", scope: !12, file: !1, line: 1, baseType: !16, size: 64, offset: 64)
+  !16 = !DIBasicType(name: "long int", size: 64, encoding: DW_ATE_signed)
+  !17 = !{!18, !19, !20, !21, !22, !23, !24, !25, !26, !27}
+  !18 = !DILocalVariable(name: "p1", arg: 1, scope: !7, file: !1, line: 5, type: !10)
+  !19 = !DILocalVariable(name: "p2", arg: 2, scope: !7, file: !1, line: 5, type: !11)
+  !20 = !DILocalVariable(name: "p3", arg: 3, scope: !7, file: !1, line: 5, type: !10)
+  !21 = !DILocalVariable(name: "p4", arg: 4, scope: !7, file: !1, line: 5, type: !10)
+  !22 = !DILocalVariable(name: "p5", arg: 5, scope: !7, file: !1, line: 5, type: !10)
+  !23 = !DILocalVariable(name: "p6", arg: 6, scope: !7, file: !1, line: 5, type: !10)
+  !24 = !DILocalVariable(name: "p7", arg: 7, scope: !7, file: !1, line: 5, type: !10)
+  !25 = !DILocalVariable(name: "p8", arg: 8, scope: !7, file: !1, line: 5, type: !10)
+  !26 = !DILocalVariable(name: "local1", scope: !7, file: !1, line: 6, type: !10)
+  !27 = !DILocalVariable(name: "local2", scope: !7, file: !1, line: 7, type: !10)
+  !28 = !DILocation(line: 5, column: 21, scope: !7)
+
+...
+---
+name:            foo
+tracksRegLiveness: true
+fixedStack:
+  - { id: 0, type: default, offset: 16, size: 4, alignment: 16, stack-id: 0,
+      isImmutable: true, isAliased: false, callee-saved-register: '', callee-saved-restored: true }
+  - { id: 1, type: default, offset: 8, size: 4, alignment: 8, stack-id: 0,
+      isImmutable: true, isAliased: false, callee-saved-register: '', callee-saved-restored: true }
+  - { id: 2, type: default, offset: 0, size: 4, alignment: 16, stack-id: 0,
+      isImmutable: true, isAliased: false, callee-saved-register: '', callee-saved-restored: true }
+stack:
+  - { id: 0, name: arr, type: default, offset: 0, size: 8, alignment: 4,
+      stack-id: 0, callee-saved-register: '', callee-saved-restored: true }
+body:             |
+  bb.0.entry:
+    DBG_VALUE $edi, $noreg, !18, !DIExpression(), debug-location !28
+    DBG_VALUE $esi, $noreg, !19, !DIExpression(DW_OP_LLVM_fragment, 0, 32), debug-location !28
+    DBG_VALUE $rdx, $noreg, !19, !DIExpression(DW_OP_LLVM_fragment, 64, 64), debug-location !28
+    DBG_VALUE $ecx, $noreg, !20, !DIExpression(), debug-location !28
+    DBG_VALUE $r8d, $noreg, !21, !DIExpression(), debug-location !28
+    DBG_VALUE %fixed-stack.2, 0, !23, !DIExpression(), debug-location !28
+    DBG_VALUE $r9d, $noreg, !22, !DIExpression(), debug-location !28
+    DBG_VALUE $r9d, $noreg, !23, !DIExpression(), debug-location !28
+    DBG_VALUE %fixed-stack.1, 0, !24, !DIExpression(), debug-location !28
+    DBG_VALUE %fixed-stack.0, 0, !25, !DIExpression(), debug-location !28
+    DBG_VALUE $edi, $noreg, !26, !DIExpression(), debug-location !28
+    DBG_VALUE %fixed-stack.2, 0, !27, !DIExpression(), debug-location !28
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp, debug-location !28
+    renamable $rdi = LEA64r %stack.0.arr, 1, $noreg, 0, $noreg
+    CALL64pcrel32 @bar, csr_64, implicit $rsp, implicit $ssp, implicit $rdi, implicit-def $rsp, implicit-def $ssp, debug-location !28
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp, debug-location !28
+    RET 0, debug-location !28
+
+...
+
+# CHECK: ![[P1:.*]] = !DILocalVariable(name: "p1"
+# CHECK: ![[P2:.*]] = !DILocalVariable(name: "p2"
+# CHECK: ![[P3:.*]] = !DILocalVariable(name: "p3"
+# CHECK: ![[P4:.*]] = !DILocalVariable(name: "p4"
+# CHECK: ![[P5:.*]] = !DILocalVariable(name: "p5"
+# CHECK: ![[P6:.*]] = !DILocalVariable(name: "p6"
+# CHECK: ![[P7:.*]] = !DILocalVariable(name: "p7"
+# CHECK: ![[P8:.*]] = !DILocalVariable(name: "p8"
+# CHECK: ![[LOCAL1:.*]] = !DILocalVariable(name: "local1"
+# CHECK: ![[LOCAL2:.*]] = !DILocalVariable(name: "local2"
+
+# Please note that an additional DBG_VALUE has been added for p6 to see that
+# FI- and register-described DBG_VALUEs for the same variable are not reordered.
+
+# Parameters passed in registers are emitted before the prologue code.
+# CHECK: bb.0.entry:
+# CHECK-NEXT: DBG_VALUE $edi, $noreg, ![[P1]], !DIExpression()
+# CHECK-NEXT: DBG_VALUE $esi, $noreg, ![[P2]], !DIExpression(DW_OP_LLVM_fragment, 0, 32)
+# CHECK-NEXT: DBG_VALUE $rdx, $noreg, ![[P2]], !DIExpression(DW_OP_LLVM_fragment, 64, 64)
+# CHECK-NEXT: DBG_VALUE $ecx, $noreg, ![[P3]], !DIExpression()
+# CHECK-NEXT: DBG_VALUE $r8d, $noreg, ![[P4]], !DIExpression()
+# CHECK-NEXT: DBG_VALUE $r9d, $noreg, ![[P5]], !DIExpression()
+# CHECK-NEXT: frame-setup PUSH64r
+
+# Locals and stack parameters are emitted after the prologue code.
+# CHECK: DBG_VALUE $rsp, 0, ![[P6]], !DIExpression(DW_OP_plus_uconst, 16)
+# CHECK-NEXT: DBG_VALUE $r9d, $noreg, ![[P6]], !DIExpression()
+# CHECK-NEXT: DBG_VALUE $rsp, 0, ![[P7]], !DIExpression(DW_OP_plus_uconst, 24)
+# CHECK-NEXT: DBG_VALUE $rsp, 0, ![[P8]], !DIExpression(DW_OP_plus_uconst, 32)
+# CHECK-NEXT: DBG_VALUE $edi, $noreg, ![[LOCAL1]], !DIExpression()
+# CHECK-NEXT: DBG_VALUE $rsp, 0, ![[LOCAL2]], !DIExpression(DW_OP_plus_uconst, 16)