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[lld-macho][nfc] Reduce size of icfEqClass hash 

... from a `uint64_t` to a `uint32_t`. (LLD-ELF uses a `uint32_t` too.)

About a 1.7% reduction in peak RSS when linking chromium_framework on my
3.2 GHz 16-Core Intel Xeon W Mac Pro, and no stat sig change in wall
time.

           </Users/jezng/test2.sh ["before"]>  </Users/jezng/test2.sh ["after"]>  difference (95% CI)
  RSS      1003036672.000 ± 9891065.259        985539505.231 ± 10272748.749       [  -2.3% ..   -1.2%]
  samples  27                                  26

             base           diff           difference (95% CI)
  sys_time   1.277 ± 0.023  1.277 ± 0.024  [  -0.9% ..   +0.9%]
  user_time  6.682 ± 0.046  6.598 ± 0.043  [  -1.6% ..   -0.9%]
  wall_time  5.904 ± 0.062  5.895 ± 0.063  [  -0.7% ..   +0.4%]
  samples    46             28

No appreciable change (~0.01%) in number of `equals` comparisons either:

Before:

  ld64.lld: ICF needed 8 iterations
  ld64.lld: equalsConstant() called 701643 times
  ld64.lld: equalsVariable() called 3438526 times

After:

  ld64.lld: ICF needed 8 iterations
  ld64.lld: equalsConstant() called 701729 times
  ld64.lld: equalsVariable() called 3438526 times

Reviewed By: #lld-macho, MaskRay, thakis

Differential Revision: https://reviews.llvm.org/D121052
This commit is contained in:
Jez Ng 2022-03-07 12:35:49 -05:00
parent 112135e774
commit ad1c32e9b3
3 changed files with 5 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
lld/MachO/ICF.cpp
View File

@ -275,7 +275,7 @@ void ICF::run() {
// Into each origin-section hash, combine all reloc referent section hashes.
for (icfPass = 0; icfPass < 2; ++icfPass) {
parallelForEach(icfInputs, [&](ConcatInputSection *isec) {
uint64_t hash = isec->icfEqClass[icfPass % 2];
uint32_t hash = isec->icfEqClass[icfPass % 2];
for (const Reloc &r : isec->relocs) {
if (auto *sym = r.referent.dyn_cast<Symbol *>()) {
if (auto *defined = dyn_cast<Defined>(sym)) {
@ -295,7 +295,7 @@ void ICF::run() {
}
}
// Set MSB to 1 to avoid collisions with non-hashed classes.
isec->icfEqClass[(icfPass + 1) % 2] = hash | (1ull << 63);
isec->icfEqClass[(icfPass + 1) % 2] = hash | (1ull << 31);
});
}
@ -391,7 +391,7 @@ void macho::foldIdenticalSections() {
assert(isec->icfEqClass[0] == 0); // don't overwrite a unique ID!
// Turn-on the top bit to guarantee that valid hashes have no collisions
// with the small-integer unique IDs for ICF-ineligible sections
isec->icfEqClass[0] = xxHash64(isec->data) | (1ull << 63);
isec->icfEqClass[0] = xxHash64(isec->data) | (1ull << 31);
});
// Now that every input section is either hashed or marked as unique, run the
// segregation algorithm to detect foldable subsections.

2
lld/MachO/InputSection.cpp
View File

@ -30,7 +30,7 @@ using namespace lld::macho;
// can differ based on STL debug levels (e.g. iterator debugging on MSVC's STL),
// so account for that.
static_assert(sizeof(void *) != 8 ||
sizeof(ConcatInputSection) == sizeof(std::vector<Reloc>) + 96,
sizeof(ConcatInputSection) == sizeof(std::vector<Reloc>) + 88,
"Try to minimize ConcatInputSection's size, we create many "
"instances of it");

2
lld/MachO/InputSection.h
View File

@ -117,7 +117,7 @@ public:
// Points to the surviving section after this one is folded by ICF
ConcatInputSection *replacement = nullptr;
// Equivalence-class ID for ICF
uint64_t icfEqClass[2] = {0, 0};
uint32_t icfEqClass[2] = {0, 0};
// With subsections_via_symbols, most symbols have their own InputSection,
// and for weak symbols (e.g. from inline functions), only the