[PATCH] D37544: [ubsan] Skip alignment checks which are folded away

[Vedant Kumar via Phabricator via cfe-commits]

vsk created this revision.

Don't emit alignment checks which the IR constant folder throws away.

I've tested this out on X86FastISel.cpp. While this doesn't decrease
end-to-end compile-time significantly, it results in 122 fewer type
checks (1% reduction) overall, without adding any real complexity.


https://reviews.llvm.org/D37544

Files:
  lib/CodeGen/CGExpr.cpp
  test/CodeGenCXX/ubsan-suppress-checks.cpp


Index: test/CodeGenCXX/ubsan-suppress-checks.cpp
===================================================================
--- test/CodeGenCXX/ubsan-suppress-checks.cpp
+++ test/CodeGenCXX/ubsan-suppress-checks.cpp
@@ -17,6 +17,15 @@
   // CHECK: ret void
 }
 
+// CHECK-LABEL: define void @_Z31use_char_aligned_array_elementsv
+void use_char_aligned_array_elements() {
+  static const char Arr[] = {0, 1, 2};
+  char X = Arr[2];
+
+  // CHECK-NOT: and i64 {{.*}}, !nosanitize
+  // CHECK: ret void
+}
+
 struct A {
   int foo;
 
@@ -229,4 +238,5 @@
   d->load_member_3();
 
   load_non_null_pointers();
+  use_char_aligned_array_elements();
 }
Index: lib/CodeGen/CGExpr.cpp
===================================================================
--- lib/CodeGen/CGExpr.cpp
+++ lib/CodeGen/CGExpr.cpp
@@ -618,6 +618,7 @@
   auto PtrToAlloca =
       dyn_cast<llvm::AllocaInst>(Ptr->stripPointerCastsNoFollowAliases());
 
+  llvm::Value *True = llvm::ConstantInt::getTrue(getLLVMContext());
   llvm::Value *IsNonNull = nullptr;
   bool IsGuaranteedNonNull =
       SkippedChecks.has(SanitizerKind::Null) || PtrToAlloca;
@@ -629,8 +630,7 @@
 
     // The IR builder can constant-fold the null check if the pointer points to
     // a constant.
-    IsGuaranteedNonNull =
-        IsNonNull == llvm::ConstantInt::getTrue(getLLVMContext());
+    IsGuaranteedNonNull = IsNonNull == True;
 
     // Skip the null check if the pointer is known to be non-null.
     if (!IsGuaranteedNonNull) {
@@ -684,7 +684,8 @@
           PtrAsInt, llvm::ConstantInt::get(IntPtrTy, AlignVal - 1));
       llvm::Value *Aligned =
           Builder.CreateICmpEQ(Align, llvm::ConstantInt::get(IntPtrTy, 0));
-      Checks.push_back(std::make_pair(Aligned, SanitizerKind::Alignment));
+      if (Aligned != True)
+        Checks.push_back(std::make_pair(Aligned, SanitizerKind::Alignment));
     }
   }
 


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