[PATCH] D41884: Fix thread race between SectionPiece's OutputOff and Live members

[Dimitry Andric via Phabricator via llvm-commits]

dim created this revision.
dim added reviewers: ruiu, rafael.

As reported in bug 35788, https://reviews.llvm.org/rL316280 reintroduces a race between two
members of SectionPiece, which share the same 64 bit memory location.

Reduce the Hash member by 1 bit, and move the Live member next to it,
instead, while making OuputOff use the full 64 bit again.

Adjust getShardId to cope with Hashes being 31 bits wide (I hope I
interpreted that function correctly... :)


Repository:
  rLLD LLVM Linker

https://reviews.llvm.org/D41884

Files:
  ELF/InputSection.h
  ELF/SyntheticSections.h


Index: ELF/SyntheticSections.h
===================================================================
--- ELF/SyntheticSections.h
+++ ELF/SyntheticSections.h
@@ -710,7 +710,7 @@
   // If we use lower bits, it significantly increases the probability of
   // hash collisons.
   size_t getShardId(uint32_t Hash) {
-    return Hash >> (32 - llvm::countTrailingZeros(NumShards));
+    return Hash >> (31 - llvm::countTrailingZeros(NumShards));
   }
 
   // Section size
Index: ELF/InputSection.h
===================================================================
--- ELF/InputSection.h
+++ ELF/InputSection.h
@@ -200,13 +200,13 @@
 // be found by looking at the next one).
 struct SectionPiece {
   SectionPiece(size_t Off, uint32_t Hash, bool Live)
-      : InputOff(Off), Hash(Hash), OutputOff(-1),
-        Live(Live || !Config->GcSections) {}
+      : InputOff(Off), Hash(Hash), Live(Live || !Config->GcSections),
+        OutputOff(-1) {}
 
   uint32_t InputOff;
-  uint32_t Hash;
-  int64_t OutputOff : 63;
-  uint64_t Live : 1;
+  uint32_t Hash : 31;
+  uint32_t Live : 1;
+  int64_t OutputOff;
 };
 
 static_assert(sizeof(SectionPiece) == 16, "SectionPiece is too big");


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