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[lld][MachO] Avoid quadratic iteration over already-folded symbols during ICF (#213339)

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`ConcatInputSection::foldIdentical()` clears the folded functions
`originalUnwindEntry`.
However, it cleared every symbol: with N members, the repeated clearing
is **O(N²)**.
This is redundant: we should only remove the incoming `copy->symbols`.

This patch moves the removing loop ahead and adds more clear comments on
why we need to skip the first element.
Also added a new `lld/test/MachO/icf-scale-same-class.s` with 500K
identical functions as a stress test, which would've taken minutes to
link, and less than a second with the patch.

Testing on real-world app (IRPGO instrumentation + ICF) find that we
achieved a 19x speed up (1:14:42 -> 3:55)
This commit is contained in:
Peter Rong 2026-08-01 12:44:24 -07:00 committed by GitHub
parent 278261413b
commit 84afdb4771
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2 changed files with 47 additions and 6 deletions
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16
lld/MachO/InputSection.cpp
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@ -204,16 +204,20 @@ void ConcatInputSection::foldIdentical(ConcatInputSection *copy,
for (auto &copySym : copy->symbols)
copySym->identicalCodeFoldingKind = foldKind;
symbols.insert(symbols.end(), copy->symbols.begin(), copy->symbols.end());
copy->symbols.clear();
// Remove duplicate compact unwind info for symbols at the same address.
if (symbols.empty())
if (copy->symbols.empty())
return;
for (auto it = symbols.begin() + 1; it != symbols.end(); ++it) {
auto *it = copy->symbols.begin();
// The first symbol in the merged section (symbols.front()) must keep its
// unwind entry. If this section is empty, the copy's first symbol becomes the
// new front, so we skip clearing it.
if (symbols.empty())
++it;
for (; it != copy->symbols.end(); ++it) {
assert((*it)->value == 0);
(*it)->originalUnwindEntry = nullptr;
}
symbols.insert(symbols.end(), copy->symbols.begin(), copy->symbols.end());
copy->symbols.clear();
}
void ConcatInputSection::writeTo(uint8_t *buf) {

37
lld/test/MachO/icf-scale-same-class.s Normal file
View file

@ -0,0 +1,37 @@
# REQUIRES: x86
# RUN: rm -rf %t*
# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t.o
# RUN: %lld -lSystem --icf=all -o %t %t.o
# RUN: llvm-objdump --macho --section-headers %t | FileCheck %s --check-prefix=SECT
# RUN: llvm-objdump --macho --syms %t | FileCheck %s --check-prefix=SYMS
## Every body folds into one, so __text holds a single 6-byte body plus _main
## rather than the 3 MiB (0x30000d) the unfolded copies occupy.
# SECT: Idx Name Size
# SECT-NEXT: 0 __text 00000009
## The sentinels bracket the group, so checking that the first and last members
## of the class share an address covers the whole run of folds.
# SYMS-DAG: [[#%.16x,F:]] g F __TEXT,__text _f_first
# SYMS-DAG: [[#F]] g F __TEXT,__text _f_last
.subsections_via_symbols
.text
.p2align 2
.globl _f_first
_f_first:; movl $7, %eax; ret
## 64 Ki identical function landing in one equivalence class.
.rept 65536
.globl _f\+
_f\+:; movl $$7, %eax; ret
.endr
.globl _f_last
_f_last:; movl $7, %eax; ret
.globl _main
_main:
ret