[llvm] [hwasan] Omit tag check for null pointers (PR #122206)

[Vitaly Buka via llvm-commits]

================
@@ -1163,12 +1163,23 @@ void HWAddressSanitizer::instrumentMemIntrinsic(MemIntrinsic *MI) {
 }
 
 bool HWAddressSanitizer::instrumentMemAccess(InterestingMemoryOperand &O,
-                                             DomTreeUpdater &DTU,
-                                             LoopInfo *LI) {
+                                             DomTreeUpdater &DTU, LoopInfo *LI,
+                                             const DataLayout &DL) {
   Value *Addr = O.getPtr();
 
   LLVM_DEBUG(dbgs() << "Instrumenting: " << O.getInsn() << "\n");
 
+  // If the pointer is statically known to be zero, the tag check will pass
+  // since:
+  // 1) it has a zero tag
+  // 2) the shadow memory corresponding to address 0 is initialized to zero and
+  // never updated.
+  // We can therefore elide the tag check.
+  llvm::KnownBits Known(DL.getPointerTypeSizeInBits(Addr->getType()));
+  llvm::computeKnownBits(Addr, Known, DL);
+  if (Known.getMinValue() == 0 && Known.getMaxValue() == 0)
----------------
vitalybuka wrote:

can this be just 

if (Known.isZero())

https://github.com/llvm/llvm-project/pull/122206