[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
diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 719147a..d0aacf6 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -533,15 +533,6 @@
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 @@
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 @@
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));