[compiler-rt] [compiler-rt][fuchsia] Preallocate a vmar for sanitizer internals (PR #75256)

[Roland McGrath via llvm-commits]

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@@ -129,6 +129,55 @@ uptr GetMaxVirtualAddress() { return GetMaxUserVirtualAddress(); }
 
 bool ErrorIsOOM(error_t err) { return err == ZX_ERR_NO_MEMORY; }
 
+// For any sanitizer internal that needs to map something which can be unmmaped
+// later, first attempt to map to a pre-allocated vmar. This helps reduce
+// fragmentation from many small anonymous mmap calls. A good value for this
+// vmar size would be the total size of your typical sanitizer internal objects
+// allocated in an "average" process lifetime. Examples of this include:
+// FakeStack, LowLevelAllocator mappings, TwoLevelMap, InternalMmapVector,
+// StackStore, CreateAsanThread, etc.
+//
+// 0xc52000 is ~12.32MB. This is roughly equal to the total sum of sanitizer
+// internal mappings in a run of boot-libc-unittests.
+constexpr size_t kSanitizerHeapVmarSize = 0xc52000;
+static zx_handle_t gSanitizerHeapVmar = 0;
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frobtech wrote:

It's fine to just assume that default zero initialization is right here, but if you're going to be explicit then use `ZX_HANDLE_INVALID`.


https://github.com/llvm/llvm-project/pull/75256