[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

lld/MachO/ICF.cpp

@@ -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.

lld/MachO/InputSection.cpp

@@ -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");
 

lld/MachO/InputSection.h

@@ -117,7 +117,7 @@ class ConcatInputSection final : public InputSection {
   // 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