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[bench] v3 with PARALLEL_DEBUG_RELOCATION_MIN=16 (exercise speculation/fallback on all arches) — do not merge #49

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[bench] v3 with PARALLEL_DEBUG_RELOCATION_MIN=16 (exercise speculation/fallback on all arches) — do not merge #49

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69680e1
perf: fast path for debug relocations in a self-contained module
zackees Sep 30, 2026
940b8d7
test: lower PARALLEL_DEBUG_RELOCATION_MIN to 16 to exercise speculati…
zackees Sep 30, 2026
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36 changes: 20 additions & 16 deletions libwild/src/elf_writer.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -161,6 +161,8 @@ use uuid::Uuid;
use zerocopy::FromBytes;
use zerocopy::transmute_mut;

mod debug_relocations;

type ElfLayout<'data, C> = Layout<'data, elf::Elf<C>>;

/// A cache for managing ELF relocations and optimization of relocation entries.
Expand Down Expand Up @@ -2464,7 +2466,7 @@ fn write_debug_section<'data, C: ElfClass, A: Arch<Platform = elf::Elf<C>>>(
let relocations = object.relocations(section_index)?;
let result = match relocations {
elf::RelocationList::Rela(rela) => {
apply_debug_rela_relocations::<C, A>(object, out, section_index, rela, layout)
debug_relocations::apply_rela::<C, A>(object, out, section_index, rela, layout)
}
elf::RelocationList::Crel(crel_iter) => {
apply_debug_relocations::<C, A, elf::ElfCrel<C>, _>(
Expand Down Expand Up @@ -2731,7 +2733,7 @@ fn apply_relocations<
Ok(())
}

const PARALLEL_DEBUG_RELOCATION_MIN: usize = 64 * 1024;
const PARALLEL_DEBUG_RELOCATION_MIN: usize = 16;
// Byte and bit-mask relocations modify at most eight bytes. A paired ULEB128 relocation can
// modify all ten bytes required to encode a u64.
const MAX_DEBUG_RELOCATION_WRITE_SIZE: u64 = u64::BITS.div_ceil(7) as u64;
Expand Down Expand Up @@ -3002,20 +3004,7 @@ fn apply_debug_relocations_impl<
output_offset: u64,
previous: Option<R>,
) -> Result<usize> {
let section_name = object.object.section_name(section_index)?;

// TODO: Starting with DWARF 6, the tombstone value will be defined as -1 and -2.
// However, the change is premature as consumers of the DWARF format don't fully support
// the new tombstone values.
//
// Link: https://dwarfstd.org/issues/200609.1.html
let tombstone_value: u64 =
if section_name == DEBUG_LOC_SECTION_NAME || section_name == DEBUG_RANGES_SECTION_NAME {
// These sections use zero as a list terminator.
1
} else {
0
};
let tombstone_value = debug_tombstone_value(object.object.section_name(section_index)?);

let mut relocation_count = 0;
let mut relocation_cache = RelocationCache {
Expand Down Expand Up @@ -3050,6 +3039,21 @@ fn apply_debug_relocations_impl<
Ok(relocation_count)
}

/// The value written by a debug relocation against a discarded section.
fn debug_tombstone_value(section_name: &[u8]) -> u64 {
// TODO: Starting with DWARF 6, the tombstone value will be defined as -1 and -2.
// However, the change is premature as consumers of the DWARF format don't fully support
// the new tombstone values.
//
// Link: https://dwarfstd.org/issues/200609.1.html
if section_name == DEBUG_LOC_SECTION_NAME || section_name == DEBUG_RANGES_SECTION_NAME {
// These sections use zero as a list terminator.
1
} else {
0
}
}

fn record_debug_relocations<C: ElfClass>(
object: &ObjectLayout<elf::Elf<C>>,
section_index: object::SectionIndex,
Expand Down
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