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perf: Skip relaxation iteration early when no rescan candidates remain #1582

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Merged
lapla-cogito merged 1 commit into from
Feb 23, 2026
Merged

perf: Skip relaxation iteration early when no rescan candidates remain #1582

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43 changes: 32 additions & 11 deletions libwild/src/elf_riscv64.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,7 @@ use linker_utils::elf::riscv64_rel_type_to_string;
use linker_utils::elf::shf;
use linker_utils::relaxation::RelocationModifier;
use linker_utils::relaxation::SectionRelaxDeltas;
use linker_utils::riscv64::JAL_RANGE;
use linker_utils::riscv64::RelaxationKind;
use linker_utils::riscv64::distance_fits_jal;
use linker_utils::riscv64::relocation_type_from_raw;
Expand Down Expand Up @@ -281,9 +282,9 @@ pub(crate) fn collect_relaxation_deltas<R: Relocation>(
relocations: impl Iterator<Item = R>,
existing_deltas: Option<&SectionRelaxDeltas>,
mut resolve_symbol: impl FnMut(object::SymbolIndex) -> Option<RelaxSymbolInfo>,
) -> (Vec<(u64, u32)>, bool) {
) -> (Vec<(u64, u32)>, Option<u64>) {
let mut raw_deltas = Vec::new();
let mut has_unrelaxed_candidates = false;
let mut min_unrelaxed_margin: Option<u64> = None;
let mut prev_call: Option<(u64, object::SymbolIndex)> = None;
let mut prev_hi20: Option<(u64, object::SymbolIndex, i64)> = None;

Expand All @@ -307,15 +308,19 @@ pub(crate) fn collect_relaxation_deltas<R: Relocation>(
if let Some(info) = resolve_symbol(sym_idx)
&& !info.is_interposable
{
if distance_fits_jal(
info.output_address as i64
- (section_output_address + call_offset) as i64,
) {
let distance = info.output_address as i64
- (section_output_address + call_offset) as i64;
if distance_fits_jal(distance) {
// Delete the jalr instruction (4 bytes at call_offset + 4).
raw_deltas.push((call_offset + 4, 4));
} else {
// Distance might shrink on a later iteration.
has_unrelaxed_candidates = true;
// Record how far this candidate is from the JAL range boundary so the
// caller can decide whether a rescan is worthwhile given the total
// bytes deleted so far.
let jal_max = JAL_RANGE.end().unsigned_abs();
let margin = distance.unsigned_abs() - jal_max;
min_unrelaxed_margin =
Some(min_unrelaxed_margin.map_or(margin, |m| m.min(margin)));
}
}
} else if let Some((hi20_offset, sym_idx, addend)) = prev_hi20
Expand All @@ -335,8 +340,23 @@ pub(crate) fn collect_relaxation_deltas<R: Relocation>(
// Delete the last 2 bytes of the 4-byte lui instruction.
raw_deltas.push((hi20_offset + 2, 2));
} else {
// Value might change on a later iteration.
has_unrelaxed_candidates = true;
// Compute how far the hi20 value is from the c.lui immediate range
// ([-32, -1] ∪ [1, 31]). The hi20 field changes by 1 for every
// 0x1000 change in value.
let hi20 = value.wrapping_add(0x800) >> 12;
let hi20_signed = hi20 as i64;
let margin = if hi20_signed > 31 {
((hi20_signed - 31) as u64) * 0x1000
} else if hi20_signed < -32 {
// Address decreases from relaxation push hi20 further negative.
// This can never be fixed.
u64::MAX
} else {
// hi20 == 0: need to cross one page boundary.
0x1000u64
};
min_unrelaxed_margin =
Some(min_unrelaxed_margin.map_or(margin, |m| m.min(margin)));
}
}
}
Expand All @@ -350,5 +370,6 @@ pub(crate) fn collect_relaxation_deltas<R: Relocation>(
}
}
}
(raw_deltas, has_unrelaxed_candidates)

(raw_deltas, min_unrelaxed_margin)
}
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