[ubsan] Skip null checks on pointers to the start of an alloca

Pointers to the start of an alloca are non-null, so we don't need to
emit runtime null checks for them.

Testing: check-clang, check-ubsan.

This significantly reduces the amount of null checks we emit when
compiling X86ISelLowering.cpp. Here are the numbers from patched /
unpatched clangs based on r300371.

  -------------------------------------
  | Setup          | # of null checks |
  -------------------------------------
  | unpatched, -O0 |            45439 |
  | patched, -O0   |            25251 | (-44.4%)
  -------------------------------------

llvm-svn: 300508

1 files changed, 11 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 265ef27a46b..b5d6c659e9e 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -533,6 +533,15 @@ 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,
@@ -554,7 +563,8 @@ void CodeGenFunction::EmitTypeCheck(TypeCheckKind TCK, SourceLocation Loc,
   bool AllowNullPointers = TCK == TCK_DowncastPointer || TCK == TCK_Upcast ||
                            TCK == TCK_UpcastToVirtualBase;
   if ((SanOpts.has(SanitizerKind::Null) || AllowNullPointers) &&
-      !SkippedChecks.has(SanitizerKind::Null)) {
+      !SkippedChecks.has(SanitizerKind::Null) &&
+      !canOmitPointerNullCheck(Ptr)) {
     // The glvalue must not be an empty glvalue.
     llvm::Value *IsNonNull = Builder.CreateIsNotNull(Ptr);