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Correct and compact DWARF line-program encoding #9196

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0cb3245
Preserve DWARF line-zero rows
cpunion Oct 2, 2026
885be9e
Update DWARF snapshots for preserved line-zero rows
cpunion Oct 2, 2026
a2a4a1d
Correct and compact DWARF line-program encoding
cpunion Oct 2, 2026
b59bd3d
Regenerate snapshots for compact DWARF line encoding
cpunion Oct 2, 2026
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58 changes: 37 additions & 21 deletions src/wasm/wasm-debug.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -36,6 +36,7 @@
#endif

#include "llvm/ObjectYAML/DWARFYAML.h"
#include "llvm/Support/LEB128.h"
#include "llvm/include/llvm/DebugInfo/DWARFContext.h"

std::error_code dwarf2yaml(llvm::DWARFContext& DCtx, llvm::DWARFYAML::Data& Y);
Expand Down Expand Up @@ -289,9 +290,9 @@ struct LineState {
}

bool needToEmit() {
// Zero values imply we can ignore this line.
// A zero line means no source attribution, not a dead address.
// https://github.com/WebAssembly/debugging/issues/9#issuecomment-567720872
return line != 0 && addr != 0;
return addr != 0;
}

// Given an old state, emit the diff from it to this state into a new line
Expand All @@ -305,11 +306,20 @@ struct LineState {
// Try to use a special opcode TODO
}
if (addr != old.addr && !useSpecial) {
// len = 1 (subopcode) + 4 (wasm32 address)
// FIXME: look at AddrSize on the Unit.
auto item = makeItem(llvm::dwarf::DW_LNE_set_address, 5);
item.Data = addr;
newOpcodes.push_back(item);
// Most instruction boundaries are only a few bytes apart. A relative
// advance avoids a full address per source/generated-code transition.
if (sequenceId == old.sequenceId && addr > old.addr &&
table.MinInstLength == 1) {
auto item = makeItem(llvm::dwarf::DW_LNS_advance_pc);
item.Data = addr - old.addr;
newOpcodes.push_back(item);
} else {
// len = 1 (subopcode) + 4 (wasm32 address)
// FIXME: look at AddrSize on the Unit.
auto item = makeItem(llvm::dwarf::DW_LNE_set_address, 5);
item.Data = addr;
newOpcodes.push_back(item);
}
}
if (line != old.line && !useSpecial) {
auto item = makeItem(llvm::dwarf::DW_LNS_advance_line);
Expand All @@ -334,9 +344,10 @@ struct LineState {
item.Data = isa;
newOpcodes.push_back(item);
}
if (discriminator != old.discriminator) {
// len = 1 (subopcode) + 4 (wasm32 address)
auto item = makeItem(llvm::dwarf::DW_LNE_set_discriminator, 5);
// Unlike file/line/column, the discriminator resets after every row.
if (discriminator) {
auto item = makeItem(llvm::dwarf::DW_LNE_set_discriminator,
1 + llvm::getULEB128Size(discriminator));
item.Data = discriminator;
newOpcodes.push_back(item);
}
Expand Down Expand Up @@ -367,12 +378,19 @@ struct LineState {
}
}

// Some flags are automatically reset after each debug line.
void resetAfterLine() {
// These flags describe a row boundary, not the rest of its address range.
void clearRowMarkers() {
basicBlock = false;
prologueEnd = false;
epilogueBegin = false;
}

// Some flags are automatically reset after each debug line.
void resetAfterLine() {
clearRowMarkers();
discriminator = 0;
}

private:
llvm::DWARFYAML::LineTableOpcode
makeItem(llvm::dwarf::LineNumberOps opcode) const {
Expand Down Expand Up @@ -744,17 +762,14 @@ static void updateDebugLines(llvm::DWARFYAML::Data& data,
if (newAddr && state.needToEmit()) {
// LLVM sometimes emits the same address more than once. We should
// probably investigate that.
if (newAddrInfo.contains(newAddr)) {
continue;
if (!newAddrInfo.contains(newAddr)) {
newAddrs.push_back(newAddr);
newAddrInfo.emplace(newAddr, state);
newAddrInfo.at(newAddr).addr = newAddr;
}
newAddrs.push_back(newAddr);
newAddrInfo.emplace(newAddr, state);
auto& updatedState = newAddrInfo.at(newAddr);
// The only difference is the address TODO other stuff?
updatedState.addr = newAddr;
// Reset relevant state.
state.resetAfterLine();
}
// Reset even when a row is unmapped or duplicates another address.
state.resetAfterLine();
if (opcode.Opcode == 0 &&
opcode.SubOpcode == llvm::dwarf::DW_LNE_end_sequence) {
sequenceId++;
Expand Down Expand Up @@ -783,6 +798,7 @@ static void updateDebugLines(llvm::DWARFYAML::Data& data,
lastState = LineState(table, -1);
}
}
lastState.resetAfterLine();
// This line ends a sequence if there is no next line after it, or if
// the next line is in a different sequence.
bool endSequence =
Expand Down
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