[DebugInfo] Normalize common kinds of DWARF sub-expressions.

[JDevlieghere]

Summary

Normalize common kinds of DWARF sub-expressions to make debug info encoding a bit more compact:

1. DW_OP_constu [X < 32] -> DW_OP_litX
2. DW_OP_constu [all ones] -> DW_OP_lit0, DW_OP_not

Addresses Adrian's comments in https://reviews.llvm.org/D48676.

[aprantl]

Can you add a testcase for the 0xffffffff case?

llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp

198

1. This is short one f.
2. I'd use some form of uint_max or ~0ULL so this isn't an issue ;-)

[JDevlieghere]

• Normalize more usages.
• Compare against 32-bit unsigned max.

[JDevlieghere]

I generated some unrelated files while fixing the tests that were included in the previous diff.

[aprantl]

llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp

31

These are 64 bit values. We either need ~0ULL or std::limits::max<uint64_t>().

[probinson]

llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp

31

The DWARF expression stack uses target-address-size values, and that isn't necessarily 64-bit. I'm not sure we can make this compaction work in all cases.

[aprantl]

llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp

31

But if we do implement the 64-bit case, it will be correct there, we just don't get the more compact representation on a 32-bit arch, so that would still be win an many targets without breaking anything?

[probinson]

llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp

31

Acknowledging we're not compacting -1 on 32-bit targets, then okay.

[aprantl]

LGTM, with outstanding changes addressed & perhaps add a note about how this doesn't trigger for 32-bit targets. Be sure to watch the bots afterwards, since we didn't emit DW_OP_LIT before AFAIK and LLDB may have bugs.

llvm/lib/CodeGen/AsmPrinter/DwarfExpression.h

protected:
   virtual void emitOp(uint8_t Op, const char *Comment = nullptr) = 0;
 
   /// Emit a raw signed value.
   virtual void emitSigned(int64_t Value) = 0;
 
   /// Emit a raw unsigned value.
   virtual void emitUnsigned(uint64_t Value) = 0;
 
+  /// Emit a normalized unsigned constant.
+  void emitConstu(uint64_t Value);
+
   /// Return whether the given machine register is the frame register in the
   /// current function.
   virtual bool isFrameRegister(const TargetRegisterInfo &TRI, unsigned MachineReg) = 0;
 
   /// Emit a DW_OP_reg operation. Note that this is only legal inside a DWARF
   /// register location description.
   void addReg(int DwarfReg, const char *Comment = nullptr);
 

llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp

 #include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <algorithm>
 #include <cassert>
 #include <cstdint>
 
 using namespace llvm;
 
+void DwarfExpression::emitConstu(uint64_t Value) {
+  if (Value < 32)
+    emitOp(dwarf::DW_OP_lit0 + Value);
+  else if (Value == std::numeric_limits<uint64_t>::max()) {
+    // Only do this for 64-bit values as the DWARF expression stack uses
+    // target-address-size values.
+    emitOp(dwarf::DW_OP_lit0);
+    emitOp(dwarf::DW_OP_not);
+  } else {
+    emitOp(dwarf::DW_OP_constu);
+    emitUnsigned(Value);
+  }
+}
+
 void DwarfExpression::addReg(int DwarfReg, const char *Comment) {
  assert(DwarfReg >= 0 && "invalid negative dwarf register number");
  assert((LocationKind == Unknown || LocationKind == Register) &&
         "location description already locked down");
  LocationKind = Register;
  if (DwarfReg < 32) {
    emitOp(dwarf::DW_OP_reg0 + DwarfReg, Comment);
   } else {
if (OffsetInBits > 0 || SizeInBits % SizeOfByte) {
     emitOp(dwarf::DW_OP_piece);
     unsigned ByteSize = SizeInBits / SizeOfByte;
     emitUnsigned(ByteSize);
   }
   this->OffsetInBits += SizeInBits;
 }
 
 void DwarfExpression::addShr(unsigned ShiftBy) {
-  emitOp(dwarf::DW_OP_constu);
-  emitUnsigned(ShiftBy);
+  emitConstu(ShiftBy);
   emitOp(dwarf::DW_OP_shr);
 }
 
 void DwarfExpression::addAnd(unsigned Mask) {
-  emitOp(dwarf::DW_OP_constu);
-  emitUnsigned(Mask);
+  emitConstu(Mask);
   emitOp(dwarf::DW_OP_and);
 }
 
 bool DwarfExpression::addMachineReg(const TargetRegisterInfo &TRI,
                                     unsigned MachineReg, unsigned MaxSize) {
   if (!TRI.isPhysicalRegister(MachineReg)) {
     if (isFrameRegister(TRI, MachineReg)) {
       DwarfRegs.push_back({-1, 0, nullptr});
void DwarfExpression::addSignedConstant(int64_t Value) {
   LocationKind = Implicit;
   emitOp(dwarf::DW_OP_consts);
   emitSigned(Value);
 }
 
 void DwarfExpression::addUnsignedConstant(uint64_t Value) {
   assert(LocationKind == Implicit || LocationKind == Unknown);
   LocationKind = Implicit;
-  emitOp(dwarf::DW_OP_constu);
-  emitUnsigned(Value);
+  emitConstu(Value);
 }
 
 void DwarfExpression::addUnsignedConstant(const APInt &Value) {
   assert(LocationKind == Implicit || LocationKind == Unknown);
   LocationKind = Implicit;
 
   unsigned Size = Value.getBitWidth();
   const uint64_t *Data = Value.getRawData();
 
   // Chop it up into 64-bit pieces, because that's the maximum that
case dwarf::DW_OP_deref:
         // Turning this into a memory location description makes the deref
         // implicit.
         LocationKind = Memory;
       else
         emitOp(dwarf::DW_OP_deref);
       break;
     case dwarf::DW_OP_constu:
       assert(LocationKind != Register);
-      emitOp(dwarf::DW_OP_constu);
-      emitUnsigned(Op->getArg(0));
+      emitConstu(Op->getArg(0));
       break;
     case dwarf::DW_OP_stack_value:
       LocationKind = Implicit;
       break;
     case dwarf::DW_OP_swap:
       assert(LocationKind != Register);
       emitOp(dwarf::DW_OP_swap);
       break;

llvm/test/DebugInfo/AMDGPU/variable-locations.ll

 ; CHECK-NEXT: DW_AT_location [DW_FORM_block1] (DW_OP_addr 0x0)
 @GlobB = common addrspace(1) global i32 0, align 4, !dbg !6
 
 ; CHECK: {{.*}}DW_TAG_subprogram
 ; CHECK: DW_AT_frame_base [DW_FORM_block1]  (DW_OP_reg9 SGPR9)
 
 define amdgpu_kernel void @kernel1(
 ; CHECK: {{.*}}DW_TAG_formal_parameter
-; CHECK-NEXT: DW_AT_location [DW_FORM_block1] (DW_OP_fbreg +4, DW_OP_constu 0x1, DW_OP_swap, DW_OP_xderef)
+; CHECK-NEXT: DW_AT_location [DW_FORM_block1] (DW_OP_fbreg +4, DW_OP_lit1, DW_OP_swap, DW_OP_xderef)
 ; CHECK-NEXT: DW_AT_name {{.*}}"ArgN"
     i32 %ArgN,
 ; CHECK: {{.*}}DW_TAG_formal_parameter
-; CHECK-NEXT: DW_AT_location [DW_FORM_block1] (DW_OP_fbreg +8, DW_OP_constu 0x1, DW_OP_swap, DW_OP_xderef)
+; CHECK-NEXT: DW_AT_location [DW_FORM_block1] (DW_OP_fbreg +8, DW_OP_lit1, DW_OP_swap, DW_OP_xderef)
 ; CHECK-NEXT: DW_AT_name {{.*}}"ArgA"
     i32 addrspace(1)* %ArgA,
 ; CHECK: {{.*}}DW_TAG_formal_parameter
-; CHECK-NEXT: DW_AT_location [DW_FORM_block1] (DW_OP_fbreg +16, DW_OP_constu 0x1, DW_OP_swap, DW_OP_xderef)
+; CHECK-NEXT: DW_AT_location [DW_FORM_block1] (DW_OP_fbreg +16, DW_OP_lit1, DW_OP_swap, DW_OP_xderef)
 ; CHECK-NEXT: DW_AT_name {{.*}}"ArgB"
     i32 addrspace(1)* %ArgB) !dbg !13 {
 entry:
   %ArgN.addr = alloca i32, align 4, addrspace(5)
   %ArgA.addr = alloca i32 addrspace(1)*, align 4, addrspace(5)
   %ArgB.addr = alloca i32 addrspace(1)*, align 4, addrspace(5)
   store i32 %ArgN, i32 addrspace(5)* %ArgN.addr, align 4
   call void @llvm.dbg.declare(metadata i32 addrspace(5)* %ArgN.addr, metadata !22, metadata !23), !dbg !24

llvm/test/DebugInfo/ARM/PR26163.ll

 ; RUN: llc -filetype=obj -o - < %s | llvm-dwarfdump -v -debug-info - | FileCheck %s
 ;
 ; Checks that we're creating two ranges, one that terminates immediately
 ; and one that spans the rest of the function. This isn't necessarily the
 ; best thing to do here (and also not necessarily correct, since the first
 ; one has a bit_piece), but it is what is currently being emitted, any
 ; change here needs to be intentional, so the test is very specific.
 ;
 ; CHECK: DW_TAG_inlined_subroutine
 ; CHECK: DW_TAG_variable
 ; CHECK:   DW_AT_location [DW_FORM_sec_offset] ({{.*}}
-; CHECK:      [0x00000004, 0x00000004): DW_OP_constu 0x0, DW_OP_stack_value, DW_OP_piece 0x8
-; CHECK:      [0x00000004, 0x00000014): DW_OP_constu 0x0, DW_OP_stack_value, DW_OP_piece 0x4)
+; CHECK:      [0x00000004, 0x00000004): DW_OP_lit0, DW_OP_stack_value, DW_OP_piece 0x8
+; CHECK:      [0x00000004, 0x00000014): DW_OP_lit0, DW_OP_stack_value, DW_OP_piece 0x4)
 
 ; Created form the following test case (PR26163) with
 ; clang -cc1 -triple armv4t--freebsd11.0-gnueabi -emit-obj -debug-info-kind=standalone -O2 -x c test.c
 ;
 ; typedef unsigned int  size_t;
 ; struct timeval {
 ;   long long tv_sec;
 ;   int tv_usec;

llvm/test/DebugInfo/ARM/split-complex.ll

 ; RUN: %llc_dwarf -O0 -filetype=obj -o %t.o %s
 ; RUN: llvm-dwarfdump -v -debug-info %t.o | FileCheck %s
 ; REQUIRES: object-emission
 target datalayout = "e-m:o-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
 target triple = "thumbv7-apple-unknown-macho"
 
 ; generated from (-triple thumbv7-apple-unknown-macho -Os):
 ;   void f(_Complex double c) { c = 0; }
 
 ; Function Attrs: nounwind readnone
 define arm_aapcscc void @f([2 x i64] %c.coerce) #0 !dbg !4 {
 entry:
   tail call void @llvm.dbg.declare(metadata { double, double }* undef, metadata !14, metadata !15), !dbg !16
   ; The target has no native double type.
   ; SROA split the complex value into two i64 values.
   ; CHECK: DW_TAG_formal_parameter
-  ; CHECK-NEXT:  DW_AT_location [DW_FORM_block1]  (DW_OP_constu 0x0, DW_OP_piece 0x8)
+  ; CHECK-NEXT:  DW_AT_location [DW_FORM_block1]  (DW_OP_lit0, DW_OP_piece 0x8)
   ; CHECK-NEXT:  DW_AT_name {{.*}} "c"
   tail call void @llvm.dbg.value(metadata i64 0, metadata !14, metadata !17), !dbg !16
   ; Manually removed to disable location list emission:
   ; tail call void @llvm.dbg.value(metadata i64 0, metadata !14, metadata !18), !dbg !16
   ret void, !dbg !19
 }
 
 ; Function Attrs: nounwind readnone

llvm/test/DebugInfo/Generic/incorrect-variable-debugloc1.ll

 ; REQUIRES: object-emission
 ; This test is failing for powerpc64, because a location list for the
-; variable 'c' is not generated at all. Temporary marking this test as XFAIL 
+; variable 'c' is not generated at all. Temporary marking this test as XFAIL
 ; for powerpc, until PR21881 is fixed.
 ; XFAIL: powerpc64
 
 ; RUN: %llc_dwarf -O2  -dwarf-version 2 -filetype=obj < %s | llvm-dwarfdump - | FileCheck %s  --check-prefix=DWARF23
 ; RUN: %llc_dwarf -O2  -dwarf-version 3 -filetype=obj < %s | llvm-dwarfdump - | FileCheck %s  --check-prefix=DWARF23
 ; RUN: %llc_dwarf -O2  -dwarf-version 4 -filetype=obj < %s | llvm-dwarfdump - | FileCheck %s  --check-prefix=DWARF4
 
 ; This is a test for PR21176.
-; DW_OP_const <const> doesn't describe a constant value, but a value at a constant address. 
+; DW_OP_const <const> doesn't describe a constant value, but a value at a constant address.
 ; The proper way to describe a constant value is DW_OP_constu <const>, DW_OP_stack_value.
+; For values < 32 we emit the canonical DW_OP_lit<const>.
 
 ; Generated with clang -S -emit-llvm -g -O2 test.cpp
 
 ; extern int func();
-; 
+;
 ; int main()
 ; {
 ;   volatile int c = 13;
 ;   c = func();
 ;   return c;
 ; }
 
 ; CHECK: DW_TAG_variable
 ; CHECK: DW_AT_location
 ; CHECK-NOT: DW_AT
-; DWARF23: DW_OP_constu 0xd{{$}}
-; DWARF4: DW_OP_constu 0xd, DW_OP_stack_value{{$}}
+; DWARF23: DW_OP_lit13{{$}}
+; DWARF4: DW_OP_lit13, DW_OP_stack_value{{$}}
 
 ; Function Attrs: uwtable
 define i32 @main() #0 !dbg !4 {
 entry:
   %c = alloca i32, align 4
   tail call void @llvm.dbg.value(metadata i32 13, metadata !10, metadata !16), !dbg !17
   store volatile i32 13, i32* %c, align 4, !dbg !18
   %call = tail call i32 @_Z4funcv(), !dbg !19

llvm/test/DebugInfo/X86/PR26148.ll

 ; int main() { return 0; }
 ;
 ; This is similar to the bug in test/DebugInfo/ARM/PR26163.ll, except that there is an
 ; extra non-overlapping range first. Thus, we make sure that the backend actually looks
 ; at all expressions when determining whether to merge ranges, not just the first one.
 ; AS in 26163, we expect two ranges (as opposed to one), the first one being zero sized
 ;
 ;
-; CHECK: [0x0000000000000004, 0x0000000000000004): DW_OP_constu 0x3, DW_OP_piece 0x4, DW_OP_reg5 RDI, DW_OP_piece 0x2
-; CHECK: [0x0000000000000004, 0x0000000000000014): DW_OP_constu 0x3, DW_OP_piece 0x4, DW_OP_constu 0x0, DW_OP_piece 0x4
+; CHECK: [0x0000000000000004, 0x0000000000000004): DW_OP_lit3, DW_OP_piece 0x4, DW_OP_reg5 RDI, DW_OP_piece 0x2
+; CHECK: [0x0000000000000004, 0x0000000000000014): DW_OP_lit3, DW_OP_piece 0x4, DW_OP_lit0, DW_OP_piece 0x4
 
 source_filename = "test/DebugInfo/X86/PR26148.ll"
 target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-apple-macosx10.11.0"
 
 %struct.S0 = type { i16, i32 }
 
 @a = common global %struct.S0 zeroinitializer, align 4, !dbg !0

llvm/test/DebugInfo/X86/constant-loclist.ll

 ; RUN: %llc_dwarf -filetype=obj %s -o - | llvm-dwarfdump -v -debug-info - | FileCheck %s
 
 ; A hand-written testcase to check 64-bit constant handling in location lists.
 
 ; CHECK: .debug_info contents:
 ; CHECK: DW_TAG_variable
 ; CHECK-NEXT: DW_AT_location [DW_FORM_data4]  (
 ; CHECK-NEXT:   [0x{{.*}}, 0x{{.*}}): DW_OP_constu 0x4000000000000000)
 ; CHECK-NEXT: DW_AT_name {{.*}}"d"
 ; CHECK: DW_TAG_variable
 ; CHECK-NEXT: DW_AT_location [DW_FORM_data4]  (
 ; CHECK-NEXT:   [0x{{.*}}, 0x{{.*}}): DW_OP_consts +0
 ; CHECK-NEXT:   [0x{{.*}}, 0x{{.*}}): DW_OP_consts +4611686018427387904)
 ; CHECK-NEXT: DW_AT_name {{.*}}"i"
 ; CHECK: DW_TAG_variable
 ; CHECK-NEXT: DW_AT_location [DW_FORM_data4]  (
-; CHECK-NEXT:   [0x{{.*}}, 0x{{.*}}): DW_OP_constu 0x0
+; CHECK-NEXT:   [0x{{.*}}, 0x{{.*}}): DW_OP_lit0
 ; CHECK-NEXT:   [0x{{.*}}, 0x{{.*}}): DW_OP_constu 0x4000000000000000)
 ; CHECK-NEXT: DW_AT_name {{.*}}"u"
 
 source_filename = "test.c"
 target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-apple-macosx"
 
 ; Function Attrs: nounwind ssp uwtable

llvm/test/DebugInfo/X86/dw_op_minus_direct.ll

 
 ; DWARF2: .debug_info
 ; DWARF2: DW_TAG_formal_parameter
 ; DWARF2-NEXT: DW_AT_name {{.*}}"i"
 ; DWARF2-NOT:      DW_AT_location
 
 ; CHECK: .debug_loc contents:
 ; CHECK: 0x00000000:
-; CHECK-NEXT:   [0x0000000000000000, 0x0000000000000004): DW_OP_breg0 RAX+0, DW_OP_constu 0xffffffff, DW_OP_and, DW_OP_constu 0x1, DW_OP_minus, DW_OP_stack_value
+; CHECK-NEXT:   [0x0000000000000000, 0x0000000000000004): DW_OP_breg0 RAX+0, DW_OP_constu 0xffffffff, DW_OP_and, DW_OP_lit1, DW_OP_minus, DW_OP_stack_value
 ;        rax+0, constu 0xffffffff, and, constu 0x00000001, minus, stack-value
 
 source_filename = "minus.c"
 target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-apple-macosx10.12.0"
 
 define i32 @inc(i32 %i) local_unnamed_addr #1 !dbg !7 {
 entry:

llvm/test/DebugInfo/X86/partial-constant.ll

 ; exit:
 ;   return result;
 ; }
 ;
 ; The constant should NOT be available for the entire function.
 ; CHECK-NOT: DW_AT_const_value
 ; CHECK: .debug_loc contents:
 ; CHECK-NEXT: 0x00000000:
-; CHECK-NEXT:   {{.*}}: DW_OP_constu 0x1, DW_OP_stack_value
+; CHECK-NEXT:   {{.*}}: DW_OP_lit1, DW_OP_stack_value
 
 source_filename = "test.ii"
 target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-apple-macosx10.12.0"
 
 ; Function Attrs: noimplicitfloat noredzone nounwind optsize
 define zeroext i1 @_Z5startv() local_unnamed_addr #0 !dbg !7 {
 entry:

llvm/test/DebugInfo/X86/pieces-4.ll

 ; CHECK:               #DEBUG_VALUE: bitpiece_spill:o <- [DW_OP_plus_uconst [[offs]], DW_OP_LLVM_fragment 0 32] [$rsp+0]
 ; CHECK:               #APP
 ; CHECK:               #NO_APP
 ; CHECK:               movl    [[offs]](%rsp), %eax          # 4-byte Reload
 ; CHECK:               retq
 
 ; DWARF: .debug_loc contents:
 ; DWARF-NEXT: 0x00000000:
-; DWARF-NEXT: {{.*}}: DW_OP_breg7 RSP+{{[0-9]+}}, DW_OP_piece 0x4, DW_OP_constu 0x0, DW_OP_stack_value, DW_OP_piece 0x4
+; DWARF-NEXT: {{.*}}: DW_OP_breg7 RSP+{{[0-9]+}}, DW_OP_piece 0x4, DW_OP_lit0, DW_OP_stack_value, DW_OP_piece 0x4
 
 ; ModuleID = 't.c'
 source_filename = "t.c"
 target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-pc-windows-msvc19.0.24210"
 
 %struct.IntPair = type { i32, i32 }
 

llvm/test/DebugInfo/X86/split-global.ll

 ; CHECK-NEXT: DW_AT_name {{.*}}"point"
 ; CHECK-NOT: DW_TAG
 ; CHECK: DW_AT_location [DW_FORM_exprloc] (DW_OP_addr 0x4, DW_OP_piece 0x4, DW_OP_addr 0x0, DW_OP_piece 0x4)
 ; CHECK-NOT: DW_TAG
 ; CHECK: DW_TAG
 ; CHECK: DW_TAG_variable
 ; CHECK-NEXT: DW_AT_name {{.*}}"part_const"
 ; CHECK-NOT: DW_TAG
-; CHECK: DW_AT_location [DW_FORM_exprloc] (DW_OP_addr 0x8, DW_OP_piece 0x4, DW_OP_constu 0x2, DW_OP_stack_value, DW_OP_piece 0x4)
+; CHECK: DW_AT_location [DW_FORM_exprloc] (DW_OP_addr 0x8, DW_OP_piece 0x4, DW_OP_lit2, DW_OP_stack_value, DW_OP_piece 0x4)
 ;     [0x0000000000000008], piece 0x00000004, constu 0x00000002, stack-value, piece 0x00000004
 ; CHECK-NOT: DW_TAG
 ; CHECK: DW_TAG_variable
 ; CHECK-NEXT: DW_AT_name {{.*}}"full_const"
 ; CHECK-NOT: DW_TAG
-; CHECK: DW_AT_location [DW_FORM_exprloc] (DW_OP_constu 0x1, DW_OP_stack_value, DW_OP_piece 0x4, DW_OP_constu 0x2, DW_OP_stack_value, DW_OP_piece 0x4)
+; CHECK: DW_AT_location [DW_FORM_exprloc] (DW_OP_lit1, DW_OP_stack_value, DW_OP_piece 0x4, DW_OP_lit2, DW_OP_stack_value, DW_OP_piece 0x4)
 ; CHECK-NOT: DW_TAG
 @point.y = global i32 2, align 4, !dbg !13
 @point.x = global i32 1, align 4, !dbg !12
 
 @part_const.x = global i32 1, align 4, !dbg !14
 
 !llvm.dbg.cu = !{!1}
 !llvm.module.flags = !{!10, !11}

llvm/test/DebugInfo/X86/stack-value-dwarf4.ll

 ; RUN: llc -o - %s | FileCheck --check-prefix=CHECK-DWARF2 %s
 ; RUN: llc -dwarf-version=4 -o - %s | FileCheck --check-prefix=CHECK-DWARF4 %s
 
 ; Exercise DW_OP_stack_value on global constants.
 
 target datalayout = "e-p:64:64"
 target triple = "x86_64-unknown-linux-gnu"
 
-; CHECK-DWARF2: .byte 8                       # DW_AT_location
-; CHECK-DWARF2  .byte 16
-; CHECK-DWARF2  .byte 4
-; CHECK-DWARF2  .byte 147
-; CHECK-DWARF2  .byte 2
-; CHECK-DWARF2  .byte 16
-; CHECK-DWARF2  .byte 0
-; CHECK-DWARF2  .byte 147
-; CHECK-DWARF2  .byte 2
-
-; CHECK-DWARF4:       .byte 10                      # DW_AT_location
-; CHECK-DWARF4-NEXT:  .byte 16
-; CHECK-DWARF4-NEXT:  .byte 4
-; CHECK-DWARF4-NEXT:  .byte 159
-; CHECK-DWARF4-NEXT:  .byte 147
-; CHECK-DWARF4-NEXT:  .byte 2
-; CHECK-DWARF4-NEXT:  .byte 16
-; CHECK-DWARF4-NEXT:  .byte 0
-; CHECK-DWARF4-NEXT:  .byte 159
+; CHECK-DWARF2: .byte   6                       # DW_AT_location
+; CHECK-DWARF2-NEXT: .byte   52
+; CHECK-DWARF2-NEXT: .byte   147
+; CHECK-DWARF2-NEXT: .byte   2
+; CHECK-DWARF2-NEXT: .byte   48
+; CHECK-DWARF2-NEXT: .byte   147
+; CHECK-DWARF2-NEXT: .byte   2
+
+; CHECK-DWARF4: .byte   8                       # DW_AT_location
+; CHECK-DWARF4-NEXT:.byte   52
+; CHECK-DWARF4-NEXT:.byte   159
+; CHECK-DWARF4-NEXT:.byte   147
+; CHECK-DWARF4-NEXT:.byte   2
+; CHECK-DWARF4-NEXT:.byte   48
+; CHECK-DWARF4-NEXT:.byte   159
+; CHECK-DWARF4-NEXT:.byte   147
+; CHECK-DWARF4-NEXT:.byte   2
 
 !0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, producer: "clang", file: !4, globals: !1, emissionKind: FullDebug)
 !1 = !{!2, !10}
 !2 = !DIGlobalVariableExpression(var: !8, expr: !3)
 !3 = !DIExpression(DW_OP_constu, 4, DW_OP_stack_value, DW_OP_LLVM_fragment, 0, 16)
 !4 = !DIFile(filename: "<stdin>", directory: "/")
 !5 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
 !6 = !{i32 2, !"Dwarf Version", i32 2}
 !7 = !{i32 2, !"Debug Info Version", i32 3}
 !8 = distinct !DIGlobalVariable(name: "a", scope: null, isLocal: false, isDefinition: true, type: !5)
 !9 = !DIExpression(DW_OP_constu, 0, DW_OP_stack_value, DW_OP_LLVM_fragment, 16, 16)
 !10 = !DIGlobalVariableExpression(var: !8, expr: !9)
 
 !llvm.dbg.cu = !{!0}
 !llvm.module.flags = !{!6, !7}

llvm/test/DebugInfo/X86/stack-value-piece.ll

 ;   return r;
 ; }
 
 ; CHECK: .debug_info contents:
 ; CHECK: DW_TAG_subprogram
 ; CHECK:   DW_AT_name {{.*}} "i"
 ; CHECK:   DW_TAG_variable
 ; CHECK-NEXT:   DW_AT_location {{.*}} ([[I:.*]]
-; CHECK-NEXT:     [{{.*}}, {{.*}}): DW_OP_reg5 RDI, DW_OP_piece 0x4, DW_OP_constu 0x0, DW_OP_stack_value, DW_OP_piece 0x4)
+; CHECK-NEXT:     [{{.*}}, {{.*}}): DW_OP_reg5 RDI, DW_OP_piece 0x4, DW_OP_lit0, DW_OP_stack_value, DW_OP_piece 0x4)
 ; CHECK-NEXT:   DW_AT_name {{.*}} "r"
 ;
 ; CHECK: DW_TAG_subprogram
 ; CHECK:   DW_AT_name {{.*}} "f"
 ; CHECK:   DW_TAG_variable
 ; CHECK-NEXT:   DW_AT_location {{.*}} ([[F:.*]]
-; CHECK-NEXT:     [{{.*}}, {{.*}}): DW_OP_reg17 XMM0, DW_OP_piece 0x4, DW_OP_constu 0x0, DW_OP_stack_value, DW_OP_piece 0x4)
+; CHECK-NEXT:     [{{.*}}, {{.*}}): DW_OP_reg17 XMM0, DW_OP_piece 0x4, DW_OP_lit0, DW_OP_stack_value, DW_OP_piece 0x4)
 ; CHECK-NEXT:   DW_AT_name {{.*}} "r"
 ;
 ; CHECK: .debug_loc contents:
 ; CHECK:      [[I]]:
-; CHECK-NEXT:   [{{.*}}, {{.*}}): DW_OP_reg5 RDI, DW_OP_piece 0x4, DW_OP_constu 0x0, DW_OP_stack_value, DW_OP_piece 0x4
+; CHECK-NEXT:   [{{.*}}, {{.*}}): DW_OP_reg5 RDI, DW_OP_piece 0x4, DW_OP_lit0, DW_OP_stack_value, DW_OP_piece 0x4
 ; CHECK:      [[F]]:
-; CHECK-NEXT:   [{{.*}}, {{.*}}): DW_OP_reg17 XMM0, DW_OP_piece 0x4, DW_OP_constu 0x0, DW_OP_stack_value, DW_OP_piece 0x4
+; CHECK-NEXT:   [{{.*}}, {{.*}}): DW_OP_reg17 XMM0, DW_OP_piece 0x4, DW_OP_lit0, DW_OP_stack_value, DW_OP_piece 0x4
 
 source_filename = "stack-value-piece.c"
 target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-apple-macosx10.12.0"
 
 %struct.I = type { i32, i32 }
 %struct.F = type { float, float }