Introduce `LocalAddressSpaceView::LoadWritable(...)` and make the `Load(...)` method return a const pointer.

Summary:
This is a follow-up to r346956 (https://reviews.llvm.org/D53975).

The purpose of this change to allow implementers of the
`AddressSpaceView` to be able to distinguish between when a caller wants
read-only memory and when a caller wants writable memory. Being able
distinguish these cases allows implementations to optimize for the
different cases and also provides a way to workaround possible platform
restrictions (e.g. the low level platform interface for reading
out-of-process memory may place memory in read-only pages).

For allocator enumeration in almost all cases read-only is sufficient so
we make `Load(...)` take on this new requirement and introduce the
`LoadWritable(...)` variants for cases where memory needs to be
writable.

The behaviour of `LoadWritable(...)` documented in comments are
deliberately very restrictive so that it will be possible in the future
to implement a simple write-cache (i.e. just a map from target address
to a writable region of memory). These restrictions can be loosened in
the future if necessary by implementing a more sophisticated
write-cache.

rdar://problem/45284065

Reviewers: kcc, cryptoad, eugenis, kubamracek, george.karpenkov

Subscribers: #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D54879

llvm-svn: 350136

1 files changed, 5 insertions, 5 deletions

diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_allocator_secondary.h b/compiler-rt/lib/sanitizer_common/sanitizer_allocator_secondary.h
index e628a796471..0c8505c34c8 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_allocator_secondary.h
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_allocator_secondary.h
@@ -204,10 +204,10 @@ class LargeMmapAllocator {
 
   void EnsureSortedChunks() {
     if (chunks_sorted_) return;
-    Header **chunks = AddressSpaceView::Load(chunks_, n_chunks_);
+    Header **chunks = AddressSpaceView::LoadWritable(chunks_, n_chunks_);
     Sort(reinterpret_cast<uptr *>(chunks), n_chunks_);
     for (uptr i = 0; i < n_chunks_; i++)
-      AddressSpaceView::Load(chunks[i])->chunk_idx = i;
+      AddressSpaceView::LoadWritable(chunks[i])->chunk_idx = i;
     chunks_sorted_ = true;
   }
 
@@ -275,9 +275,9 @@ class LargeMmapAllocator {
   // The allocator must be locked when calling this function.
   void ForEachChunk(ForEachChunkCallback callback, void *arg) {
     EnsureSortedChunks();  // Avoid doing the sort while iterating.
-    Header **chunks = AddressSpaceView::Load(chunks_, n_chunks_);
+    const Header *const *chunks = AddressSpaceView::Load(chunks_, n_chunks_);
     for (uptr i = 0; i < n_chunks_; i++) {
-      Header *t = chunks[i];
+      const Header *t = chunks[i];
       callback(reinterpret_cast<uptr>(GetUser(t)), arg);
       // Consistency check: verify that the array did not change.
       CHECK_EQ(chunks[i], t);
@@ -301,7 +301,7 @@ class LargeMmapAllocator {
     return GetHeader(reinterpret_cast<uptr>(p));
   }
 
-  void *GetUser(Header *h) {
+  void *GetUser(const Header *h) {
     CHECK(IsAligned((uptr)h, page_size_));
     return reinterpret_cast<void*>(reinterpret_cast<uptr>(h) + page_size_);
   }