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

[Florian Mayer via llvm-commits]

https://github.com/fmayer updated https://github.com/llvm/llvm-project/pull/87029

>From 278d49fed3ee1227e358aec29dbd3d7fd1916108 Mon Sep 17 00:00:00 2001
From: Florian Mayer <fmayer at google.com>
Date: Thu, 28 Mar 2024 18:17:24 -0700
Subject: [PATCH 1/2] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20in?=
 =?UTF-8?q?itial=20version?=
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Created using spr 1.3.4
---
 .../Instrumentation/HWAddressSanitizer.cpp     | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp b/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp
index f7d4803ded155a..0bc866164540ea 100644
--- a/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp
+++ b/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 NN = 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));

>From d1cc6b8fb1b3753dc8e443747148d42602f7784c Mon Sep 17 00:00:00 2001
From: Florian Mayer <fmayer at google.com>
Date: Thu, 28 Mar 2024 18:19:43 -0700
Subject: [PATCH 2/2] nit

Created using spr 1.3.4
---
 llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp b/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp
index 0bc866164540ea..d0d349c891a37b 100644
--- a/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp
@@ -1314,7 +1314,7 @@ void HWAddressSanitizer::emitPrologue(IRBuilder<> &IRB, bool WithFrameRecord) {
       // 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 NN = 1
+      // Example of the wrap case for N = 1
       // Pointer:   0x01AAAAAAAAAAAFF8
       //                     +
       //            0x0000000000000008