[Runtime] Have MetadataCacheKey::operator== try memcmp before deeper comparison.

Experiment shows that the vast majority (~99.99%) of keys with equal hashes have equal bytes. Fast-path that case by doing a memcmp before doing the deeper comparison.

rdar://110478978

Author: mikeash

stdlib/public/runtime/MetadataCache.h

@@ -606,12 +606,28 @@ class MetadataCacheKey {
     // Compare the hashes.
     if (hash() != rhs.hash()) return false;
 
+    // Fast path the case where they're bytewise identical. That's nearly always
+    // the case if the hashes are the same, and we can skip the slower deep
+    // comparison.
+    auto *adata = begin();
+    auto *bdata = rhs.begin();
+
+    auto asize = (uintptr_t)end() - (uintptr_t)adata;
+    auto bsize = (uintptr_t)rhs.end() - (uintptr_t)bdata;
+
+    // If sizes don't match, they can never be equal.
+    if (asize != bsize)
+      return false;
+
+    // If sizes match, see if the bytes match. If they do, then the contents
+    // must necessarily match. Otherwise do a deep comparison.
+    if (memcmp(adata, bdata, asize) == 0)
+      return true;
+
     // Compare the layouts.
     if (Layout != rhs.Layout) return false;
 
     // Compare the content.
-    auto *adata = begin();
-    auto *bdata = rhs.begin();
     const uintptr_t *packCounts = reinterpret_cast<const uintptr_t *>(adata);
 
     unsigned argIdx = 0;