[lld] [lld-macho,BalancedPartition] Simplify relocation hash and avoid xxHash (PR #121729)

[Fangrui Song via llvm-commits]

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@@ -90,23 +93,24 @@ class BPSectionMacho : public BPSectionBase {
                             &sectionToIdx) const override {
     constexpr unsigned windowSize = 4;
 
-    // Calculate content hashes
-    size_t dataSize = isec->data.size();
-    for (size_t i = 0; i < dataSize; i++) {
-      auto window = isec->data.drop_front(i).take_front(windowSize);
-      hashes.push_back(xxHash64(window));
-    }
+    // Calculate content hashes: k-mers and the last k-1 bytes.
+    ArrayRef<uint8_t> data = isec->data;
+    if (data.size() >= windowSize)
+      for (size_t i = 0; i <= data.size() - windowSize; ++i)
+        hashes.push_back(llvm::support::endian::read32le(data.data() + i));
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MaskRay wrote:

I agree that this is weird. `xxh3_64bits(window)` for relocation hashing is just because it's easy: no need to handle the shorter-than-4-bytes case.

Hmmm. `Reloc::length` is actually a logarithm field. For Mach-O arm64, the relocation offsets are aligned to start of the instruction. Shall we compute one single hash for a relocation? I guess the sliding window doesn't help.



https://github.com/llvm/llvm-project/pull/121729