[NFC] [HWASan] add example for ring buffer wrap (#87029)

Also another move comment to correct place.

Author: fmayer

llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp

@@ -1114,8 +1114,6 @@ void HWAddressSanitizer::tagAlloca(IRBuilder<> &IRB, AllocaInst *AI, Value *Tag,
     // FIXME: the interceptor is not as fast as real memset. Consider lowering
     // llvm.memset right here into either a sequence of stores, or a call to
     // hwasan_tag_memory.
-    // Mechanical proof of this address calculation can be found at:
-    // https://github.com/google/sanitizers/blob/master/hwaddress-sanitizer/prove_hwasanwrap.smt2
     if (ShadowSize)
       IRB.CreateMemSet(ShadowPtr, Tag, ShadowSize, Align(1));
     if (Size != AlignedSize) {
@@ -1312,6 +1310,22 @@ void HWAddressSanitizer::emitPrologue(IRBuilder<> &IRB, bool WithFrameRecord) {
       // The use of AShr instead of LShr is due to
       //   https://bugs.llvm.org/show_bug.cgi?id=39030
       // Runtime library makes sure not to use the highest bit.
+      //
+      // Mechanical proof of this address calculation can be found at:
+      // https://github.com/google/sanitizers/blob/master/hwaddress-sanitizer/prove_hwasanwrap.smt2
+      //
+      // Example of the wrap case for N = 1
+      // Pointer:   0x01AAAAAAAAAAAFF8
+      //                     +
+      //            0x0000000000000008
+      //                     =
+      //            0x01AAAAAAAAAAB000
+      //                     &
+      // WrapMask:  0xFFFFFFFFFFFFF000
+      //                     =
+      //            0x01AAAAAAAAAAA000
+      //
+      // Then the WrapMask will be a no-op until the next wrap case.
       Value *WrapMask = IRB.CreateXor(
           IRB.CreateShl(IRB.CreateAShr(ThreadLong, 56), 12, "", true, true),
           ConstantInt::get(IntptrTy, (uint64_t)-1));