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

If the pointer to be checked is statically known to be zero, the tag
check will always pass since:
1) the tag is zero
2) shadow memory for address 0 is initialized to 0 and never updated.
We can therefore elide the tag check.

We perform the elision in two places:
1) the HWASan pass
2) when lowering the CHECK_MEMACCESS intrinsic. Conceivably, the HWASan
pass may encounter a "cannot currently statically prove to be null"
pointer (and is therefore unable to omit the intrinsic) that later
optimization passes convert into a statically known-null pointer. As a
last line of defense, we perform elision here too.

This also updates the tests from
llvm#122186

Author: thurstond

llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp

@@ -605,6 +605,15 @@ void AArch64AsmPrinter::LowerKCFI_CHECK(const MachineInstr &MI) {
 
 void AArch64AsmPrinter::LowerHWASAN_CHECK_MEMACCESS(const MachineInstr &MI) {
   Register Reg = MI.getOperand(0).getReg();
+
+  // The HWASan pass won't emit a CHECK_MEMACCESS intrinsic with a pointer
+  // statically known to be zero. However, conceivably, the HWASan pass may
+  // encounter a "cannot currently statically prove to be null" pointer (and is
+  // therefore unable to omit the intrinsic) that later optimization passes
+  // convert into a statically known-null pointer.
+  if (Reg == AArch64::XZR)
+    return;
+
   bool IsShort =
       ((MI.getOpcode() == AArch64::HWASAN_CHECK_MEMACCESS_SHORTGRANULES) ||
        (MI.getOpcode() ==

llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp

@@ -348,7 +348,7 @@ class HWAddressSanitizer {
   bool ignoreMemIntrinsic(OptimizationRemarkEmitter &ORE, MemIntrinsic *MI);
   void instrumentMemIntrinsic(MemIntrinsic *MI);
   bool instrumentMemAccess(InterestingMemoryOperand &O, DomTreeUpdater &DTU,
-                           LoopInfo *LI);
+                           LoopInfo *LI, const DataLayout &DL);
   bool ignoreAccessWithoutRemark(Instruction *Inst, Value *Ptr);
   bool ignoreAccess(OptimizationRemarkEmitter &ORE, Instruction *Inst,
                     Value *Ptr);
@@ -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.isZero())
+    return false;
+
   if (O.MaybeMask)
     return false; // FIXME
 
@@ -1701,8 +1712,9 @@ void HWAddressSanitizer::sanitizeFunction(Function &F,
   PostDominatorTree *PDT = FAM.getCachedResult<PostDominatorTreeAnalysis>(F);
   LoopInfo *LI = FAM.getCachedResult<LoopAnalysis>(F);
   DomTreeUpdater DTU(DT, PDT, DomTreeUpdater::UpdateStrategy::Lazy);
+  const DataLayout &DL = F.getDataLayout();
   for (auto &Operand : OperandsToInstrument)
-    instrumentMemAccess(Operand, DTU, LI);
+    instrumentMemAccess(Operand, DTU, LI, DL);
   DTU.flush();
 
   if (ClInstrumentMemIntrinsics && !IntrinToInstrument.empty()) {

llvm/test/CodeGen/AArch64/hwasan-zero-ptr.ll

@@ -1,11 +1,14 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
 ; RUN: llc -filetype asm -o - %s | FileCheck %s
 
-; This shows that when dereferencing a null pointer, HWASan will call
-; __hwasan_check_x4294967071_19_fixed_0_short_v2
-; (N.B. 4294967071 == 2**32 - 239 + 14 == 2**32 - X0 + XZR
+; This shows that CodeGen for AArch64 will elide the tag check when lowering
+; the HWASan memaccess intrinsic for null pointers.
 ;
-; The source was generated from llvm/test/Instrumentation/HWAddressSanitizer/zero-ptr.ll.
+; N.B. The HWASan pass will normally omit the memaccess intrinsic if the
+;      pointer is already statically known to be null.
+;
+; The source was generated from llvm/test/Instrumentation/HWAddressSanitizer/zero-ptr.ll
+; with the memaccess deliberately retained.
 
 ; ModuleID = '<stdin>'
 source_filename = "<stdin>"
@@ -25,7 +28,6 @@ define void @test_store_to_zeroptr() #0 {
 ; CHECK-NEXT:    str x30, [sp, #-16]! // 8-byte Folded Spill
 ; CHECK-NEXT:    .cfi_def_cfa_offset 16
 ; CHECK-NEXT:    .cfi_offset w30, -16
-; CHECK-NEXT:    bl __hwasan_check_x4294967071_19_fixed_0_short_v2
 ; CHECK-NEXT:    mov x8, xzr
 ; CHECK-NEXT:    mov w9, #42 // =0x2a
 ; CHECK-NEXT:    str x9, [x8]

llvm/test/Instrumentation/HWAddressSanitizer/zero-ptr.ll

@@ -2,7 +2,7 @@
 ; RUN: opt < %s -passes=hwasan -S | FileCheck %s
 ; RUN: opt < %s -passes=hwasan -hwasan-recover=0 -hwasan-mapping-offset=0 -S | FileCheck %s --check-prefixes=ABORT-ZERO-BASED-SHADOW
 
-; This shows that HWASan will emit a memaccess check when dereferencing a null
+; This shows that HWASan omits the memaccess check when dereferencing a null
 ; pointer.
 ; The output is used as the source for llvm/test/CodeGen/AArch64/hwasan-zero-ptr.ll.
 
@@ -15,7 +15,6 @@ define void @test_store_to_zeroptr() sanitize_hwaddress {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[DOTHWASAN_SHADOW:%.*]] = call ptr asm "", "=r,0"(ptr @__hwasan_shadow)
 ; CHECK-NEXT:    [[B:%.*]] = inttoptr i64 0 to ptr
-; CHECK-NEXT:    call void @llvm.hwasan.check.memaccess.shortgranules(ptr [[DOTHWASAN_SHADOW]], ptr [[B]], i32 19)
 ; CHECK-NEXT:    store i64 42, ptr [[B]], align 8
 ; CHECK-NEXT:    ret void
 ;
@@ -24,7 +23,6 @@ define void @test_store_to_zeroptr() sanitize_hwaddress {
 ; ABORT-ZERO-BASED-SHADOW-NEXT:  entry:
 ; ABORT-ZERO-BASED-SHADOW-NEXT:    [[DOTHWASAN_SHADOW:%.*]] = call ptr asm "", "=r,0"(ptr null)
 ; ABORT-ZERO-BASED-SHADOW-NEXT:    [[B:%.*]] = inttoptr i64 0 to ptr
-; ABORT-ZERO-BASED-SHADOW-NEXT:    call void @llvm.hwasan.check.memaccess.shortgranules.fixedshadow(ptr [[B]], i32 19, i64 0)
 ; ABORT-ZERO-BASED-SHADOW-NEXT:    store i64 42, ptr [[B]], align 8
 ; ABORT-ZERO-BASED-SHADOW-NEXT:    ret void
 ;