[PATCH] D121052: [lld-macho][nfc] Reduce size of icfEqClass hash

[Jez Ng via Phabricator via llvm-commits]

int3 created this revision.
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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


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D121052

Files:
  lld/MachO/ICF.cpp
  lld/MachO/InputSection.cpp
  lld/MachO/InputSection.h


Index: lld/MachO/InputSection.h
===================================================================
--- lld/MachO/InputSection.h
+++ lld/MachO/InputSection.h
@@ -117,7 +117,7 @@
   // 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
Index: lld/MachO/InputSection.cpp
===================================================================
--- lld/MachO/InputSection.cpp
+++ lld/MachO/InputSection.cpp
@@ -30,7 +30,7 @@
 // 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");
 
Index: lld/MachO/ICF.cpp
===================================================================
--- lld/MachO/ICF.cpp
+++ lld/MachO/ICF.cpp
@@ -273,15 +273,14 @@
   // 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 *dylibSym = dyn_cast<DylibSymbol>(sym))
-            hash += dylibSym->stubsHelperIndex;
-          else if (auto *defined = dyn_cast<Defined>(sym)) {
+          if (auto *defined = dyn_cast<Defined>(sym)) {
             if (defined->isec) {
-              if (auto isec = dyn_cast<ConcatInputSection>(defined->isec))
-                hash += defined->value + isec->icfEqClass[icfPass % 2];
+              if (auto referentIsec =
+                      dyn_cast<ConcatInputSection>(defined->isec))
+                hash += defined->value + referentIsec->icfEqClass[icfPass % 2];
               else
                 hash += defined->isec->kind() +
                         defined->isec->getOffset(defined->value);
@@ -293,7 +292,7 @@
         }
       }
       // 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);
     });
   }
 
@@ -400,7 +399,7 @@
     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.


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