[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;
+
+static zx_status_t GetSanitizerHeapVmar(zx_handle_t *vmar) {
+  zx_status_t status = ZX_OK;
+  if (!gSanitizerHeapVmar) {
+    CHECK_EQ(kSanitizerHeapVmarSize % GetPageSizeCached(), 0);
+    uintptr_t base;
+    status = _zx_vmar_allocate(
+        _zx_vmar_root_self(),
+        ZX_VM_CAN_MAP_READ | ZX_VM_CAN_MAP_WRITE | ZX_VM_CAN_MAP_SPECIFIC, 0,
+        kSanitizerHeapVmarSize, &gSanitizerHeapVmar, &base);
+  }
+  *vmar = gSanitizerHeapVmar;
+  if (status == ZX_OK)
+    CHECK_NE(gSanitizerHeapVmar, 0);
+  return status;
+}
+
+static zx_status_t TryVmoMapSanitizerVmar(zx_vm_option_t options,
+                                          size_t vmar_offset, zx_handle_t vmo,
+                                          size_t size, uintptr_t *addr) {
+  zx_status_t status;
+  zx_handle_t vmar;
+  if ((status = GetSanitizerHeapVmar(&vmar)) != ZX_OK)
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frobtech wrote:

Prefer initializing in the declaration to setting the variable inside the `if`.


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