Cap SkVarAlloc's desired block at 64K.

This means we can store fLgMinSize in 4 bits (TBD).

Local perf comparison calls this harmless-to-slightly-helpful.  Nothing to get
excited about, but seems to certainly not harm perf.

BUG=skia:

Review URL: https://codereview.chromium.org/722293003

2 files changed, 12 insertions, 3 deletions

diff --git a/src/core/SkVarAlloc.cpp b/src/core/SkVarAlloc.cpp
index cc1fc1c734..6f5b5a18d3 100644
--- a/src/core/SkVarAlloc.cpp
+++ b/src/core/SkVarAlloc.cpp
@@ -7,6 +7,11 @@
     #include <malloc.h>
 #endif
 
+enum {
+    kMinLgSize =  4,  // The smallest block we'd ever want to allocate is 16B,
+    kMaxLgSize = 16,  // and we see no benefit allocating blocks larger than 64K.
+};
+
 struct SkVarAlloc::Block {
     Block* prev;
     char* data() { return (char*)(this + 1); }
@@ -22,7 +27,7 @@ struct SkVarAlloc::Block {
 SkVarAlloc::SkVarAlloc()
     : fByte(NULL)
     , fRemaining(0)
-    , fLgMinSize(4)
+    , fLgSize(kMinLgSize)
     , fBlock(NULL) {}
 
 SkVarAlloc::~SkVarAlloc() {
@@ -37,7 +42,7 @@ SkVarAlloc::~SkVarAlloc() {
 void SkVarAlloc::makeSpace(size_t bytes, unsigned flags) {
     SkASSERT(SkIsAlignPtr(bytes));
 
-    size_t alloc = 1<<(fLgMinSize++);
+    size_t alloc = 1<<fLgSize;
     while (alloc < bytes + sizeof(Block)) {
         alloc *= 2;
     }
@@ -45,6 +50,10 @@ void SkVarAlloc::makeSpace(size_t bytes, unsigned flags) {
     fByte = fBlock->data();
     fRemaining = alloc - sizeof(Block);
 
+    if (fLgSize < kMaxLgSize) {
+        fLgSize++;
+    }
+
 #if defined(SK_BUILD_FOR_MAC)
     SkASSERT(alloc == malloc_good_size(alloc));
 #elif defined(SK_BUILD_FOR_LINUX)
diff --git a/src/core/SkVarAlloc.h b/src/core/SkVarAlloc.h
index 19b2d75e76..2b5401b7f0 100644
--- a/src/core/SkVarAlloc.h
+++ b/src/core/SkVarAlloc.h
@@ -28,7 +28,7 @@ private:
 
     char* fByte;
     unsigned fRemaining;
-    unsigned fLgMinSize;
+    unsigned fLgSize;  // This is always in the range [4, 16], so it really only needs 4 bits.
 
     struct Block;
     Block* fBlock;