[IRMemoryMap] Fix the alignment adjustment in Malloc

This prevents Malloc from allocating the same chunk of memory twice, as
a byproduct of an alignment adjustment which gave the client access to
unallocated memory.

Prior to this patch, the newly-added test failed with:

$ lldb-test ir-memory-map ... ir-memory-map-overlap1.test
...
Command: malloc(size=64, alignment=32)
Malloc: address = 0x1000cd080
Command: malloc(size=64, alignment=8)
Malloc: address = 0x1000cd0b0
Malloc error: overlapping allocation detected, previous allocation at [0x1000cd080, 0x1000cd0c0)

Differential Revision: https://reviews.llvm.org/D47551

llvm-svn: 333697

1 files changed, 12 insertions, 6 deletions

diff --git a/lldb/source/Expression/IRMemoryMap.cpp b/lldb/source/Expression/IRMemoryMap.cpp
index 8fc2b4a4520..1953e80852f 100644
--- a/lldb/source/Expression/IRMemoryMap.cpp
+++ b/lldb/source/Expression/IRMemoryMap.cpp
@@ -301,15 +301,21 @@ lldb::addr_t IRMemoryMap::Malloc(size_t size, uint8_t alignment,
   lldb::addr_t allocation_address = LLDB_INVALID_ADDRESS;
   lldb::addr_t aligned_address = LLDB_INVALID_ADDRESS;
 
-  size_t alignment_mask = alignment - 1;
   size_t allocation_size;
 
-  if (size == 0)
+  if (size == 0) {
+    // FIXME: Malloc(0) should either return an invalid address or assert, in
+    // order to cut down on unnecessary allocations.
     allocation_size = alignment;
-  else
-    allocation_size = (size & alignment_mask)
-                          ? ((size + alignment) & (~alignment_mask))
-                          : size;
+  } else {
+    // Round up the requested size to an aligned value.
+    allocation_size = llvm::alignTo(size, alignment);
+
+    // The process page cache does not see the requested alignment. We can't
+    // assume its result will be any more than 1-byte aligned. To work around
+    // this, request `alignment - 1` additional bytes.
+    allocation_size += alignment - 1;
+  }
 
   switch (policy) {
   default: