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[CodeView] Add support for local S_CONSTANT records
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CodeView doesn't have the ability to represent variables
in other ways than as in registers or memory values, but
LLVM very often transforms simple values into constants,
consider this program:

int f () { int i = 123; return i; }

LLVM will transform `i` into a constant value and just
leave behind a llvm.dbg.value, this can't be represented
as a S_LOCAL record in CodeView. But we can represent it
as a S_CONSTANT record.

This patch checks if the location of a debug value is null,
then we will insert a S_CONSTANT record instead of a S_LOCAL
value with the flag "OptimizedAway".

In lld we then output the S_CONSTANT in the right scope, before
they where always inserted in the global stream, now we check
the scope before inserting it.

This has shown to improve debugging for our developers
internally.

Fixes to #55958

Reviewed By: aganea

Differential Revision: https://reviews.llvm.org/D138995
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@tru
tru committed Dec 6, 2022
1 parent 5b22c51 commit 2298a44
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Showing 7 changed files with 261 additions and 46 deletions.
6 changes: 3 additions & 3 deletions lld/COFF/PDB.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -442,7 +442,6 @@ static bool symbolGoesInModuleStream(const CVSymbol &sym,
unsigned symbolScopeDepth) {
switch (sym.kind()) {
case SymbolKind::S_GDATA32:
case SymbolKind::S_CONSTANT:
case SymbolKind::S_GTHREAD32:
// We really should not be seeing S_PROCREF and S_LPROCREF in the first place
// since they are synthesized by the linker in response to S_GPROC32 and
Expand All @@ -451,8 +450,9 @@ static bool symbolGoesInModuleStream(const CVSymbol &sym,
case SymbolKind::S_PROCREF:
case SymbolKind::S_LPROCREF:
return false;
// S_UDT records go in the module stream if it is not a global S_UDT.
// S_UDT and S_CONSTANT records go in the module stream if it is not a global record.
case SymbolKind::S_UDT:
case SymbolKind::S_CONSTANT:
return symbolScopeDepth > 0;
// S_GDATA32 does not go in the module stream, but S_LDATA32 does.
case SymbolKind::S_LDATA32:
Expand All @@ -465,7 +465,6 @@ static bool symbolGoesInModuleStream(const CVSymbol &sym,
static bool symbolGoesInGlobalsStream(const CVSymbol &sym,
unsigned symbolScopeDepth) {
switch (sym.kind()) {
case SymbolKind::S_CONSTANT:
case SymbolKind::S_GDATA32:
case SymbolKind::S_GTHREAD32:
case SymbolKind::S_GPROC32:
Expand All @@ -482,6 +481,7 @@ static bool symbolGoesInGlobalsStream(const CVSymbol &sym,
case SymbolKind::S_UDT:
case SymbolKind::S_LDATA32:
case SymbolKind::S_LTHREAD32:
case SymbolKind::S_CONSTANT:
return symbolScopeDepth == 0;
default:
return false;
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192 changes: 192 additions & 0 deletions lld/test/COFF/Inputs/pdb-local-constants.s
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Original file line number Diff line number Diff line change
@@ -0,0 +1,192 @@
.text
.def @feat.00;
.scl 3;
.type 0;
.endef
.globl @feat.00
.set @feat.00, 0
.file "local_constant.cpp"
.def main;
.scl 2;
.type 32;
.endef
.globl main # -- Begin function main
.p2align 4, 0x90
main: # @main
.Lfunc_begin0:
.cv_func_id 0
# %bb.0: # %entry
#DEBUG_VALUE: main:i <- 123
.cv_file 1 "/home/tobias/code/llvm-project/build/local_constant.cpp" "C33315002D9B48E67EB3E617E430BC02" 1
.cv_loc 0 1 7 0 # local_constant.cpp:7:0
movl $444, %eax # imm = 0x1BC
retq
.Ltmp0:
.Lfunc_end0:
# -- End function
.section .debug$S,"dr"
.p2align 2, 0x0
.long 4 # Debug section magic
.long 241
.long .Ltmp2-.Ltmp1 # Subsection size
.Ltmp1:
.short .Ltmp4-.Ltmp3 # Record length
.Ltmp3:
.short 4353 # Record kind: S_OBJNAME
.long 0 # Signature
.byte 0 # Object name
.p2align 2, 0x0
.Ltmp4:
.short .Ltmp6-.Ltmp5 # Record length
.Ltmp5:
.short 4412 # Record kind: S_COMPILE3
.long 1 # Flags and language
.short 208 # CPUType
.short 16 # Frontend version
.short 0
.short 0
.short 0
.short 16000 # Backend version
.short 0
.short 0
.short 0
.asciz "clang version 16.0.0 (git@github.com:llvm/llvm-project.git eef89bd2b3f4a13efcad176bb4c4dda1b1e202ce)" # Null-terminated compiler version string
.p2align 2, 0x0
.Ltmp6:
.Ltmp2:
.p2align 2, 0x0
.long 241 # Symbol subsection for main
.long .Ltmp8-.Ltmp7 # Subsection size
.Ltmp7:
.short .Ltmp10-.Ltmp9 # Record length
.Ltmp9:
.short 4423 # Record kind: S_GPROC32_ID
.long 0 # PtrParent
.long 0 # PtrEnd
.long 0 # PtrNext
.long .Lfunc_end0-main # Code size
.long 0 # Offset after prologue
.long 0 # Offset before epilogue
.long 4098 # Function type index
.secrel32 main # Function section relative address
.secidx main # Function section index
.byte 0 # Flags
.asciz "main" # Function name
.p2align 2, 0x0
.Ltmp10:
.short .Ltmp12-.Ltmp11 # Record length
.Ltmp11:
.short 4114 # Record kind: S_FRAMEPROC
.long 0 # FrameSize
.long 0 # Padding
.long 0 # Offset of padding
.long 0 # Bytes of callee saved registers
.long 0 # Exception handler offset
.short 0 # Exception handler section
.long 1056768 # Flags (defines frame register)
.p2align 2, 0x0
.Ltmp12:
.short .Ltmp14-.Ltmp13 # Record length
.Ltmp13:
.short 4359 # Record kind: S_CONSTANT
.long 116 # Type
.byte 0x7b, 0x00 # Value
.asciz "i" # Name
.p2align 2, 0x0
.Ltmp14:
.short 2 # Record length
.short 4431 # Record kind: S_PROC_ID_END
.Ltmp8:
.p2align 2, 0x0
.cv_linetable 0, main, .Lfunc_end0
.long 241 # Symbol subsection for globals
.long .Ltmp16-.Ltmp15 # Subsection size
.Ltmp15:
.short .Ltmp18-.Ltmp17 # Record length
.Ltmp17:
.short 4359 # Record kind: S_CONSTANT
.long 4099 # Type
.byte 0x41, 0x01 # Value
.asciz "g_const" # Name
.p2align 2, 0x0
.Ltmp18:
.Ltmp16:
.p2align 2, 0x0
.cv_filechecksums # File index to string table offset subsection
.cv_stringtable # String table
.long 241
.long .Ltmp20-.Ltmp19 # Subsection size
.Ltmp19:
.short .Ltmp22-.Ltmp21 # Record length
.Ltmp21:
.short 4428 # Record kind: S_BUILDINFO
.long 4103 # LF_BUILDINFO index
.p2align 2, 0x0
.Ltmp22:
.Ltmp20:
.p2align 2, 0x0
.section .debug$T,"dr"
.p2align 2, 0x0
.long 4 # Debug section magic
# ArgList (0x1000)
.short 0x6 # Record length
.short 0x1201 # Record kind: LF_ARGLIST
.long 0x0 # NumArgs
# Procedure (0x1001)
.short 0xe # Record length
.short 0x1008 # Record kind: LF_PROCEDURE
.long 0x74 # ReturnType: int
.byte 0x0 # CallingConvention: NearC
.byte 0x0 # FunctionOptions
.short 0x0 # NumParameters
.long 0x1000 # ArgListType: ()
# FuncId (0x1002)
.short 0x12 # Record length
.short 0x1601 # Record kind: LF_FUNC_ID
.long 0x0 # ParentScope
.long 0x1001 # FunctionType: int ()
.asciz "main" # Name
.byte 243
.byte 242
.byte 241
# Modifier (0x1003)
.short 0xa # Record length
.short 0x1001 # Record kind: LF_MODIFIER
.long 0x74 # ModifiedType: int
.short 0x1 # Modifiers ( Const (0x1) )
.byte 242
.byte 241
# StringId (0x1004)
.short 0x2e # Record length
.short 0x1605 # Record kind: LF_STRING_ID
.long 0x0 # Id
.asciz "/home/tobias/code/llvm-project/build" # StringData
.byte 243
.byte 242
.byte 241
# StringId (0x1005)
.short 0x1a # Record length
.short 0x1605 # Record kind: LF_STRING_ID
.long 0x0 # Id
.asciz "local_constant.cpp" # StringData
.byte 241
# StringId (0x1006)
.short 0xa # Record length
.short 0x1605 # Record kind: LF_STRING_ID
.long 0x0 # Id
.byte 0 # StringData
.byte 243
.byte 242
.byte 241
# BuildInfo (0x1007)
.short 0x1a # Record length
.short 0x1603 # Record kind: LF_BUILDINFO
.short 0x5 # NumArgs
.long 0x1004 # Argument: /home/tobias/code/llvm-project/build
.long 0x0 # Argument
.long 0x1005 # Argument: local_constant.cpp
.long 0x1006 # Argument
.long 0x0 # Argument
.byte 242
.byte 241
.addrsig
21 changes: 21 additions & 0 deletions lld/test/COFF/pdb-local-constants.test
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Original file line number Diff line number Diff line change
@@ -0,0 +1,21 @@
# REQUIRES: x86
# RUN: llvm-mc -filetype=obj %S/Inputs/pdb-local-constants.s -o %t.obj -triple x86_64-windows-msvc
# RUN: lld-link -entry:main -nodefaultlib %t.obj -out:%t.exe -pdb:%t.pdb -debug
# RUN: llvm-pdbutil dump -globals -symbols %t.pdb | FileCheck %s
#
# Check that LLD puts the S_CONSTANT records in the right scope
#
# Compiled from this C code, using
# clang t.cpp -g -gcodeview -S
#
# const int g_const = 321;
# int main() { int i = 123; return i + g_const; }
#

CHECK: Global Symbols
CHECK: 40 | S_CONSTANT [size = 20] `g_const`
CHECK-NEXT: type = 0x1002 (const int), value = 321

CHECK: Symbols
CHECK: 220 | S_CONSTANT [size = 12] `i`
CHECK-NEXT type = 0x0074 (int), value = 123
26 changes: 23 additions & 3 deletions llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1343,7 +1343,17 @@ void CodeViewDebug::calculateRanges(
Optional<DbgVariableLocation> Location =
DbgVariableLocation::extractFromMachineInstruction(*DVInst);
if (!Location)
{
// When we don't have a location this is usually because LLVM has
// transformed it into a constant and we only have an llvm.dbg.value. We
// can't represent these well in CodeView since S_LOCAL only works on
// registers and memory locations. Instead, we will pretend this to be a
// constant value to at least have it show up in the debugger.
auto Op = DVInst->getDebugOperand(0);
if (Op.isImm())
Var.ConstantValue = APSInt(APInt(64, Op.getImm()), false);
continue;
}

// CodeView can only express variables in register and variables in memory
// at a constant offset from a register. However, for variables passed
Expand Down Expand Up @@ -2788,9 +2798,19 @@ void CodeViewDebug::emitLocalVariableList(const FunctionInfo &FI,
emitLocalVariable(FI, *L);

// Next emit all non-parameters in the order that we found them.
for (const LocalVariable &L : Locals)
if (!L.DIVar->isParameter())
emitLocalVariable(FI, L);
for (const LocalVariable &L : Locals) {
if (!L.DIVar->isParameter()) {
if (L.ConstantValue) {
// If ConstantValue is set we will emit it as a S_CONSTANT instead of a
// S_LOCAL in order to be able to represent it at all.
const DIType *Ty = L.DIVar->getType();
APSInt Val(L.ConstantValue.value());
emitConstantSymbolRecord(Ty, Val, std::string(L.DIVar->getName()));
} else {
emitLocalVariable(FI, L);
}
}
}
}

void CodeViewDebug::emitLocalVariable(const FunctionInfo &FI,
Expand Down
1 change: 1 addition & 0 deletions llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.h
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Original file line number Diff line number Diff line change
Expand Up @@ -104,6 +104,7 @@ class LLVM_LIBRARY_VISIBILITY CodeViewDebug : public DebugHandlerBase {
SmallVector<std::pair<const MCSymbol *, const MCSymbol *>, 1>>
DefRanges;
bool UseReferenceType = false;
std::optional<APSInt> ConstantValue;
};

struct CVGlobalVariable {
Expand Down
12 changes: 4 additions & 8 deletions llvm/test/DebugInfo/COFF/local-constant.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -9,18 +9,14 @@
; useint(x);
; }

; FIXME: Find a way to describe variables optimized to constants.

; OBJ: {{.*}}Proc{{.*}}Sym {
; OBJ: DisplayName: constant_var
; OBJ: }
; OBJ: LocalSym {
; OBJ-NEXT: Kind:
; OBJ: ConstantSym {
; OBJ-NEXT: Kind: S_CONSTANT
; OBJ-NEXT: Type: int (0x74)
; OBJ-NEXT: Flags [ (0x100)
; OBJ-NEXT: IsOptimizedOut (0x100)
; OBJ-NEXT: ]
; OBJ-NEXT: VarName: x
; OBJ-NEXT: Value: 42
; OBJ-NEXT: Name: x
; OBJ-NEXT: }
; OBJ-NOT: DefRange
; OBJ: ProcEnd
Expand Down
49 changes: 17 additions & 32 deletions llvm/test/DebugInfo/COFF/pieces.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -112,32 +112,21 @@

; ASM-LABEL: .short 4423 # Record kind: S_GPROC32_ID
; ASM: .asciz "loop_csr" # Function name
; ASM: .short 4414 # Record kind: S_LOCAL
; ASM: .short 4359 # Record kind: S_CONSTANT
; ASM: .long 4099 # Type
; ASM: .byte 0x00, 0x00 # Value
; ASM: .asciz "o"
; ASM: .cv_def_range [[oy_ox_start]] [[loopskip_start]], subfield_reg, 24, 0
; ASM: .cv_def_range [[oy_ox_start]] [[loopskip_start]], subfield_reg, 23, 4


; OBJ-LABEL: GlobalProcIdSym {
; OBJ: Kind: S_GPROC32_ID (0x1147)
; OBJ: DisplayName: loop_csr
; OBJ: }
; OBJ: LocalSym {
; OBJ: VarName: o
; OBJ: }
; OBJ: DefRangeSubfieldRegisterSym {
; OBJ: Register: EDI (0x18)
; OBJ: MayHaveNoName: 0
; OBJ: OffsetInParent: 0
; OBJ: LocalVariableAddrRange {
; OBJ: }
; OBJ: }
; OBJ: DefRangeSubfieldRegisterSym {
; OBJ: Register: ESI (0x17)
; OBJ: MayHaveNoName: 0
; OBJ: OffsetInParent: 4
; OBJ: LocalVariableAddrRange {
; OBJ: }
; OBJ: ConstantSym {
; OBJ: Kind: S_CONSTANT (0x1107)
; OBJ: Type: IntPair (0x1003)
; OBJ: Value: 0
; OBJ: Name: o
; OBJ: }
; OBJ: ProcEnd {
; OBJ: }
Expand Down Expand Up @@ -229,24 +218,20 @@

; ASM-LABEL: .short 4423 # Record kind: S_GPROC32_ID
; ASM: .asciz "bitpiece_spill" # Function name
; ASM: .short 4414 # Record kind: S_LOCAL
; ASM: .asciz "o"
; ASM: .cv_def_range [[spill_o_x_start]] .Lfunc_end4, reg_rel, 335, 65, 36
; ASM: .short 4359 # Record kind: S_CONSTANT
; ASM: .long 4099 # Type
; ASM: .byte 0x00, 0x00 # Value
; ASM: .asciz "o" # Name

; OBJ-LABEL: GlobalProcIdSym {
; OBJ: Kind: S_GPROC32_ID (0x1147)
; OBJ: DisplayName: bitpiece_spill
; OBJ: }
; OBJ: LocalSym {
; OBJ: VarName: o
; OBJ: }
; OBJ: DefRangeRegisterRelSym {
; OBJ: BaseRegister: RSP (0x14F)
; OBJ: HasSpilledUDTMember: Yes
; OBJ: OffsetInParent: 4
; OBJ: BasePointerOffset: 36
; OBJ: LocalVariableAddrRange {
; OBJ: }
; OBJ: ConstantSym {
; OBJ: Kind: S_CONSTANT (0x1107)
; OBJ: Type: IntPair (0x1003)
; OBJ: Value: 0
; OBJ: Name: o
; OBJ: }
; OBJ: ProcEnd {
; OBJ: }
Expand Down

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