[sanitizer] MmapAlignedOrDie changes to reduce fragmentation

Summary:
The reasoning behind this change is explained in D33454, which unfortunately
broke the Windows version (due to the platform not supporting partial unmapping
of a memory region).

This new approach changes `MmapAlignedOrDie` to allow for the specification of
a `padding_chunk`. If non-null, and the initial allocation is aligned, this
padding chunk will hold the address of the extra memory (of `alignment` bytes).
This allows `AllocateRegion` to get 2 regions if the memory is aligned
properly, and thus help reduce fragmentation (and saves on unmapping
operations). As with the initial D33454, we use a stash in the 32-bit Primary
to hold those extra regions and return them on the fast-path.

The Windows version of `MmapAlignedOrDie` will always return a 0
`padding_chunk` if one was requested.

Reviewers: alekseyshl, dvyukov, kcc

Reviewed By: alekseyshl

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D34152

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@305391 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 5 insertions, 1 deletions

diff --git a/lib/sanitizer_common/sanitizer_win.cc b/lib/sanitizer_common/sanitizer_win.cc
index afc3bb0ac..493167a19 100644
--- a/lib/sanitizer_common/sanitizer_win.cc
+++ b/lib/sanitizer_common/sanitizer_win.cc
@@ -132,10 +132,14 @@ void UnmapOrDie(void *addr, uptr size) {
 }
 
 // We want to map a chunk of address space aligned to 'alignment'.
-void *MmapAlignedOrDie(uptr size, uptr alignment, const char *mem_type) {
+void *MmapAlignedOrDie(uptr size, uptr alignment, const char *mem_type,
+                       uptr *padding_chunk) {
   CHECK(IsPowerOfTwo(size));
   CHECK(IsPowerOfTwo(alignment));
 
+  if (padding_chunk)
+    *padding_chunk = 0;
+
   // Windows will align our allocations to at least 64K.
   alignment = Max(alignment, GetMmapGranularity());