[PATCH] D14537: sanitizer: speedup coverage by 33%

[Dmitry Vyukov via llvm-commits]

dvyukov created this revision.
dvyukov added a reviewer: kcc.
dvyukov added a subscriber: llvm-commits.

Atomic RMW is not necessary in InitializeGuardArray. It is supposed to run when no user code runs. And if user code runs concurrently, then the atomic RMW won't help anyway. So replace it with non-atomic RMW.

InitializeGuardArray takes more than 50% of time during re2 fuzzing:

real	0m47.215s
 51.56%  a.out  a.out                [.] __sanitizer_reset_coverage
  6.68%  a.out  a.out                [.] __sanitizer_cov
  3.41%  a.out  a.out                [.] __sanitizer::internal_bzero_aligned16(void*, unsigned long)
  1.79%  a.out  a.out                [.] __asan::Allocator::Allocate(unsigned long, unsigned long,

With this change:

real 0m31.661s
 26.21%  a.out  a.out                [.] __sanitizer_reset_coverage
 10.12%  a.out  a.out                [.] __sanitizer_cov
  5.38%  a.out  a.out                [.] __sanitizer::internal_bzero_aligned16(void*, unsigned long)
  2.53%  a.out  a.out                [.] __asan::Allocator::Allocate(unsigned long, unsigned long,

That's 33% speedup.


http://reviews.llvm.org/D14537

Files:
  lib/sanitizer_common/sanitizer_coverage_libcdep.cc

Index: lib/sanitizer_common/sanitizer_coverage_libcdep.cc
===================================================================
--- lib/sanitizer_common/sanitizer_coverage_libcdep.cc
+++ lib/sanitizer_common/sanitizer_coverage_libcdep.cc
@@ -226,7 +226,8 @@
   Enable();  // Make sure coverage is enabled at this point.
   s32 n = guards[0];
   for (s32 j = 1; j <= n; j++) {
-    uptr idx = atomic_fetch_add(&pc_array_index, 1, memory_order_relaxed);
+    uptr idx = atomic_load_relaxed(&pc_array_index);
+    atomic_store_relaxed(&pc_array_index, idx + 1);
     guards[j] = -static_cast<s32>(idx + 1);
   }
 }


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