skip unnecessary check during double-check SizeClass intergrity

On initialization step, tcmalloc double-checks SizeClass integrity with
all possible size values, 0 to kMaxSize. This causes tremendous overhead
for short-lived applications.

For example, consider following command.
'find -exec grep something {} \;'

Actual work of each grep is really small, but double-check requires
more work. To reduce this overhead, it is best to remove double-check
entirely. But we cannot be sure the integrity without double-checking,
so alternative is needed.

This patch doesn't remove double-check, instead, try to skip unnecessary
check based on ClassIndex() implementation. This reduce much overhead and
the code has same coverage as previous double-check. Following is
the result of this patch.

time LD_PRELOAD=libtcmalloc_minimal.so find ./ -exec grep "SOMETHING" {} \;

* Before
real	0m3.675s
user	0m1.000s
sys	0m0.640s

* This patch
real	0m2.833s
user	0m0.056s
sys	0m0.220s

* Remove double-check entirely
real	0m2.675s
user	0m0.072s
sys	0m0.184s

1 files changed, 6 insertions, 1 deletions

diff --git a/src/common.cc b/src/common.cc
index 50fbcf2..bd4f6ed 100644
--- a/src/common.cc
+++ b/src/common.cc
@@ -188,7 +188,7 @@ void SizeMap::Init() {
   }
 
   // Double-check sizes just to be safe
-  for (size_t size = 0; size <= kMaxSize; size++) {
+  for (size_t size = 0; size <= kMaxSize;) {
     const int sc = SizeClass(size);
     if (sc <= 0 || sc >= kNumClasses) {
       Log(kCrash, __FILE__, __LINE__,
@@ -203,6 +203,11 @@ void SizeMap::Init() {
       Log(kCrash, __FILE__, __LINE__,
           "Bad (class, size, requested)", sc, s, size);
     }
+    if (size <= kMaxSmallSize) {
+      size += 8;
+    } else {
+      size += 128;
+    }
   }
 
   // Initialize the num_objects_to_move array.