[compiler-rt] [compiler-rt] [sanitizer] avoid UB in allocator (PR #126977)

[Vitaly Buka via llvm-commits]

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@@ -166,7 +166,11 @@ struct SizeClassAllocator32LocalCache {
       DCHECK_GT(c->count, 0);
     }
     void *res = c->batch[--c->count];
-    PREFETCH(c->batch[c->count - 1]);
+    // By not doing pointer arithmetic, we avoid the OOB if c->count = 0.
+    // We just prefetch the previous member of the PerClass struct, which
+    // doesn't do harm.
+    PREFETCH(reinterpret_cast<uptr>(c->batch) +
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vitalybuka wrote:

seems `Max(0, c->count - 1)`  is easier to understand

https://github.com/llvm/llvm-project/pull/126977