[SanitizerCommon] Reduce wasting memory in LowLevelAllocator.

[ikudrin]

Summary

MmapOrDie allocates memory multiple to page size. LowLevelAllocator should use all that memory for the internal buffer because there are chances that subsequent allocations may be small enough to be placed in that space.

[eugenis]

LGTM, thank you!

compiler-rt/lib/sanitizer_common/sanitizer_allocator.cpp

 constexpr uptr kLowLevelAllocatorDefaultAlignment = 8;
 static uptr low_level_alloc_min_alignment = kLowLevelAllocatorDefaultAlignment;
 static LowLevelAllocateCallback low_level_alloc_callback;
 
 void *LowLevelAllocator::Allocate(uptr size) {
   // Align allocation size.
   size = RoundUpTo(size, low_level_alloc_min_alignment);
   if (allocated_end_ - allocated_current_ < (sptr)size) {
-    uptr size_to_allocate = Max(size, GetPageSizeCached());
+    uptr size_to_allocate = RoundUpTo(size, GetPageSizeCached());
     allocated_current_ =
         (char*)MmapOrDie(size_to_allocate, __func__);
     allocated_end_ = allocated_current_ + size_to_allocate;
     if (low_level_alloc_callback) {
       low_level_alloc_callback((uptr)allocated_current_,
                                size_to_allocate);
     }
   }

compiler-rt/lib/sanitizer_common/tests/sanitizer_allocator_test.cpp

TEST(SanitizerCommon, ThreadedTwoLevelByteMap) {
   }
   EXPECT_EQ((uptr)TestMapUnmapCallback::map_count, m.size1());
   EXPECT_EQ((uptr)TestMapUnmapCallback::unmap_count, 0UL);
   m.TestOnlyUnmap();
   EXPECT_EQ((uptr)TestMapUnmapCallback::map_count, m.size1());
   EXPECT_EQ((uptr)TestMapUnmapCallback::unmap_count, m.size1());
 }
 
+TEST(SanitizerCommon, LowLevelAllocatorShouldRoundUpSizeOnAlloc) {
+  // When allocating a memory block slightly bigger than a memory page and
+  // LowLevelAllocator calls MmapOrDie for the internal buffer, it should round
+  // the size up to the page size, so that subsequent calls to the allocator
+  // can use the remaining space in the last allocated page.
+  static LowLevelAllocator allocator;
+  char *ptr1 = (char *)allocator.Allocate(GetPageSizeCached() + 16);
+  char *ptr2 = (char *)allocator.Allocate(16);
+  EXPECT_EQ(ptr2, ptr1 + GetPageSizeCached() + 16);
+}
+
 #endif  // #if !SANITIZER_DEBUG