[ubsan] Skip alignment checks on allocas with known alignment

It's possible to determine the alignment of an alloca at compile-time.
Use this information to skip emitting some runtime alignment checks.

Testing: check-clang, check-ubsan.

This significantly reduces the amount of alignment checks we emit when
compiling X86ISelLowering.cpp. Here are the numbers from patched/unpatched
clangs based on r301361.

  ------------------------------------------
  | Setup          | # of alignment checks |
  ------------------------------------------
  | unpatched, -O0 |                 47195 |
  | patched, -O0   |                 30876 | (-34.6%)
  ------------------------------------------

llvm-svn: 301377

1 files changed, 9 insertions, 12 deletions

diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 719147a58e0..d0aacf65428 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -533,15 +533,6 @@ bool CodeGenFunction::sanitizePerformTypeCheck() const {
          SanOpts.has(SanitizerKind::Vptr);
 }
 
-/// Check if a runtime null check for \p Ptr can be omitted.
-static bool canOmitPointerNullCheck(llvm::Value *Ptr) {
-  // Note: do not perform any constant-folding in this function. That is best
-  // left to the IR builder.
-
-  // Pointers to alloca'd memory are non-null.
-  return isa<llvm::AllocaInst>(Ptr->stripPointerCastsNoFollowAliases());
-}
-
 void CodeGenFunction::EmitTypeCheck(TypeCheckKind TCK, SourceLocation Loc,
                                     llvm::Value *Ptr, QualType Ty,
                                     CharUnits Alignment,
@@ -560,11 +551,16 @@ void CodeGenFunction::EmitTypeCheck(TypeCheckKind TCK, SourceLocation Loc,
   SmallVector<std::pair<llvm::Value *, SanitizerMask>, 3> Checks;
   llvm::BasicBlock *Done = nullptr;
 
+  // Quickly determine whether we have a pointer to an alloca. It's possible
+  // to skip null checks, and some alignment checks, for these pointers. This
+  // can reduce compile-time significantly.
+  auto PtrToAlloca =
+      dyn_cast<llvm::AllocaInst>(Ptr->stripPointerCastsNoFollowAliases());
+
   bool AllowNullPointers = TCK == TCK_DowncastPointer || TCK == TCK_Upcast ||
                            TCK == TCK_UpcastToVirtualBase;
   if ((SanOpts.has(SanitizerKind::Null) || AllowNullPointers) &&
-      !SkippedChecks.has(SanitizerKind::Null) &&
-      !canOmitPointerNullCheck(Ptr)) {
+      !SkippedChecks.has(SanitizerKind::Null) && !PtrToAlloca) {
     // The glvalue must not be an empty glvalue.
     llvm::Value *IsNonNull = Builder.CreateIsNotNull(Ptr);
 
@@ -617,7 +613,8 @@ void CodeGenFunction::EmitTypeCheck(TypeCheckKind TCK, SourceLocation Loc,
       AlignVal = getContext().getTypeAlignInChars(Ty).getQuantity();
 
     // The glvalue must be suitably aligned.
-    if (AlignVal > 1) {
+    if (AlignVal > 1 &&
+        (!PtrToAlloca || PtrToAlloca->getAlignment() < AlignVal)) {
       llvm::Value *Align =
           Builder.CreateAnd(Builder.CreatePtrToInt(Ptr, IntPtrTy),
                             llvm::ConstantInt::get(IntPtrTy, AlignVal - 1));