[SharedCache] Avoid copying strings on each call to VM::MappingAtAddress

`PageMapping` was storing the path to the file it points within. This
was causing unnecessary work within `VM::MappingAtAddress` as the
`PageMapping`, and thus the path, is copied into the return value. This
copying was more expensive than the map lookup.

The file path is now stored within a `LazyMappedFileAccessor` class that
wraps the `SelfAllocatingWeakPtr`. This means the path is still
available via the `PageMapping`, but it does not need to be copied as
often.

This includes two additional improvements / optimizations while I was
touching the code in question:
1. `MMappedFileAccessor::Open` no longer performs two hash lookups when
   a file accessor already exists.
2. `VM::MapPages` takes the path by const reference to avoid an
   unnecessary copy.

1 files changed, 3 insertions, 2 deletions

diff --git a/view/sharedcache/core/SharedCache.cpp b/view/sharedcache/core/SharedCache.cpp
index 33a4654d..0eb52c6a 100644
--- a/view/sharedcache/core/SharedCache.cpp
+++ b/view/sharedcache/core/SharedCache.cpp
@@ -780,7 +780,7 @@ void SharedCache::PerformInitialLoad()
 				if (imageHeader->linkeditPresent && vm->AddressIsMapped(imageHeader->linkeditSegment.vmaddr))
 				{
 					auto mapping = vm->MappingAtAddress(imageHeader->linkeditSegment.vmaddr);
-					imageHeader->exportTriePath = mapping.first.filePath;
+					imageHeader->exportTriePath = mapping.first.fileAccessor->filePath();
 				}
 				MutableState().headers[start.second] = imageHeader.value();
 				CacheImage image;
@@ -3330,7 +3330,8 @@ extern "C"
 						images[i].mappings[j].vmAddress = sectionStart;
 						images[i].mappings[j].size = header.sections[j].size;
 						images[i].mappings[j].name = BNAllocString(header.sectionNames[j].c_str());
-						images[i].mappings[j].filePath = BNAllocString(vm->MappingAtAddress(sectionStart).first.filePath.c_str());
+						auto fileAccessor = vm->MappingAtAddress(sectionStart).first.fileAccessor;
+						images[i].mappings[j].filePath = BNAllocStringWithLength(fileAccessor->filePath().data(), fileAccessor->filePath().length());
 						images[i].mappings[j].loaded = cache->object->IsMemoryMapped(sectionStart);
 					}
 					i++;