Make sure that ScratchBuffer::Allocate() always return 8-byte aligned address.

mojo/public/cpp/bindings/lib/scratch_buffer.cc

Index: mojo/public/cpp/bindings/lib/scratch_buffer.cc
diff --git a/mojo/public/cpp/bindings/lib/scratch_buffer.cc b/mojo/public/cpp/bindings/lib/scratch_buffer.cc
index 9d23dcb13bba25b7b2dde5d1d5d5fc4f56847ca7..c7f507be285e921b81f710510535c9624311ddce 100644
--- a/mojo/public/cpp/bindings/lib/scratch_buffer.cc
+++ b/mojo/public/cpp/bindings/lib/scratch_buffer.cc
@@ -25,7 +25,7 @@ namespace internal {
 ScratchBuffer::ScratchBuffer()
     : overflow_(NULL) {
   fixed_.next = NULL;
-  fixed_.cursor = fixed_data_;
+  fixed_.cursor = internal::AlignPointer(fixed_data_);

[darin (slow to review), 2014/05/06 07:32:55]

maybe we should align fixed_.end too?

doesn't this mean that we waste a few bytes of fixed_data_ on 32-bit systems?
maybe if we moved the overflow_ pointer to be listed before fixed_data_ that
would avoid that minor issue?

[darin (slow to review), 2014/05/06 07:35:29]

nevermind. that would increase fixed_.end in some cases, which would be bad.

[yzshen1, 2014/05/06 08:06:43]

On 32-bit systems, it is possible that pointers are 4-byte aligned, therefore,
this whole object may be 4-byte aligned, right? In that case, no matter how we
reorder the fields, we cannot guarantee that |fixed_data_| is 8-byte aligned. (I
probably need to double-check the differences between platforms/compilers.)

   fixed_.end = fixed_data_ + kMinSegmentSize;
 }
 
@@ -80,11 +80,12 @@ bool ScratchBuffer::AddOverflowSegment(size_t delta) {
     return false;
 
   // Ensure segment buffer is aligned.
-  size_t segment_size = internal::Align(sizeof(Segment)) + delta;
-  Segment* segment = static_cast<Segment*>(malloc(segment_size));
+  size_t padded_segment_size = internal::Align(sizeof(Segment));
+  Segment* segment = static_cast<Segment*>(
+      malloc(padded_segment_size + delta));
   if (segment) {
     segment->next = overflow_;
-    segment->cursor = reinterpret_cast<char*>(segment + 1);
+    segment->cursor = reinterpret_cast<char*>(segment) + padded_segment_size;
     segment->end = segment->cursor + delta;
     overflow_ = segment;
     return true;