[LLVM][CHERI] Introduce the llvm.launder.alignment intrinsic and use it to lower __builtin_cheri_address_get on sealed pointers

Author: xdoardo

clang/lib/CodeGen/CGBuiltin.cpp

@@ -6026,10 +6026,15 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
       return RValue::get(Builder.CreateIntToPtr(Ptr, ResultTy));
   }
 
-  case Builtin::BI__builtin_cheri_address_get:
-    return RValue::get(
-        Builder.CreateIntrinsic(llvm::Intrinsic::cheri_cap_address_get,
-                                {IntPtrTy}, {EmitScalarExpr(E->getArg(0))}));
+  case Builtin::BI__builtin_cheri_address_get: {
+    const auto *Arg = E->getArg(0);
+    auto *RArg = EmitScalarExpr(Arg);
+    if (Arg->getType()->isCHERISealedCapabilityType(getContext()))
+      RArg = Builder.CreateIntrinsic(llvm::Intrinsic::launder_alignment,
+                                     {UnqualPtrTy}, {RArg});
+    return RValue::get(Builder.CreateIntrinsic(
+        llvm::Intrinsic::cheri_cap_address_get, {IntPtrTy}, {RArg}));
+  }
   case Builtin::BI__builtin_cheri_address_set: {
     Value *Cap = EmitScalarExpr(E->getArg(0));
     Value *Address = EmitScalarExpr(E->getArg(1));

clang/test/CodeGen/cheri/riscv/cheriot-static-sealed-value-attr.c

@@ -25,18 +25,32 @@ SealedInt Obj2 = 10;
 // CHECK: @llvm.compiler.used = appending addrspace(200) global [2 x ptr addrspace(200)] [ptr addrspace(200) @Obj1, ptr addrspace(200) @Obj2], section "llvm.metadata"
 
 void doSomething(struct SealedStructObj *__sealed_capability obj);
+void doSomethingWithAddr(int addr);
 void doSomething2(SealedInt *__sealed_capability obj);
 
 // CHECK: ; Function Attrs: minsize nounwind optsize
 // CHECK: define dso_local void @func() local_unnamed_addr addrspace(200) #1 {
 void func() {
 // CHECK: entry:
-// CHECK: 	tail call void @doSomething(ptr addrspace(200) noundef nonnull @Obj1) #3
+// CHECK: 	tail call void @doSomething(ptr addrspace(200) noundef nonnull @Obj1) #5
   doSomething(&Obj1);
 
-// CHECK: 	tail call void @doSomething2(ptr addrspace(200) noundef nonnull @Obj2) #3
+// Verify that observing the address of a sealed global value is done through a call to the launder built-in, so that 
+// the KnownBits optimisation pass can't assume anything about the value of the pointer, specifically nothing about the alignment of the pointer.
+// This is because the CHERIoT RTOS uses the lower bits of the address to store the permissions of the sealed capability, and KnownBits can in turn 
+// optimise away logical computations on lower parts of the address.
+
+// CHECK:  %0 = tail call ptr addrspace(200) @llvm.launder.alignment.p200(ptr addrspace(200) nonnull @Obj1)
+// CHECK:  %1 = tail call i32 @llvm.cheri.cap.address.get.i32(ptr addrspace(200) nonnull %0)
+// CHECK:  tail call void @doSomethingWithAddr(i32 noundef %1) #5
+  doSomethingWithAddr(__builtin_cheri_address_get(&Obj1));
+
+// CHECK: 	tail call void @doSomething2(ptr addrspace(200) noundef nonnull @Obj2) #5
   doSomething2(&Obj2);
 }
 
 // CHECK: declare void @doSomething(ptr addrspace(200) noundef) local_unnamed_addr addrspace(200) #2
+// CHECK: declare void @doSomethingWithAddr(i32 noundef) local_unnamed_addr addrspace(200) #2
+// CHECK: declare ptr addrspace(200) @llvm.launder.alignment.p200(ptr addrspace(200)) addrspace(200) #3
+// CHECK: declare i32 @llvm.cheri.cap.address.get.i32(ptr addrspace(200)) addrspace(200) #4
 // CHECK: declare void @doSomething2(ptr addrspace(200) noundef) local_unnamed_addr addrspace(200) #2

llvm/docs/LangRef.rst

@@ -26826,6 +26826,38 @@ Returns another pointer that aliases its argument but which has no associated
 ``invariant.group`` metadata.
 It does not read any memory and can be speculated.
 
+'``llvm.launder.alignment``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+This is an overloaded intrinsic. The returned pointer must belong to the same
+address space as the argument.
+
+::
+
+      declare ptr @llvm.launder.alignment.p0(ptr <ptr>)
+
+Overview:
+"""""""""
+
+The '``llvm.launder.alignment``' intrinsic can be used when the informations
+regarding the alignment of the type the argument points to must be removed, so
+to block unwanted optimizations.
+
+
+Arguments:
+""""""""""
+
+The ``llvm.launder.alignment`` takes only one argument, which is a pointer
+to the memory.
+
+Semantics:
+""""""""""
+
+Returns another pointer that aliases its argument but which has no associated
+alignment metadata.
+It does not read any memory and can be speculated.
 
 
 .. _constrainedfp:

llvm/include/llvm/IR/Intrinsics.td

@@ -1699,6 +1699,10 @@ def int_strip_invariant_group : DefaultAttrsIntrinsic<[llvm_anyptr_ty],
                                           [LLVMMatchType<0>],
                                           [IntrSpeculatable, IntrNoMem]>;
 
+def int_launder_alignment : DefaultAttrsIntrinsic<[llvm_anyptr_ty],
+                                            [LLVMMatchType<0>],
+                                            [IntrInaccessibleMemOnly, IntrSpeculatable]>;
+
 //===------------------------ Stackmap Intrinsics -------------------------===//
 //
 def int_experimental_stackmap : DefaultAttrsIntrinsic<[],

llvm/lib/Analysis/ValueTracking.cpp

@@ -6757,6 +6757,7 @@ bool llvm::isIntrinsicReturningPointerAliasingArgumentWithoutCapturing(
   // MustPreserveNullness (and, at time of writing, they are not), but we
   // document this fact out of an abundance of caution.
   case Intrinsic::amdgcn_make_buffer_rsrc:
+  case Intrinsic::launder_alignment:
     return true;
   case Intrinsic::ptrmask:
     return !MustPreserveNullness;

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -7463,6 +7463,7 @@ void SelectionDAGBuilder::visitIntrinsicCall(const CallInst &I,
   case Intrinsic::annotation:
   case Intrinsic::ptr_annotation:
   case Intrinsic::launder_invariant_group:
+  case Intrinsic::launder_alignment:
   case Intrinsic::strip_invariant_group:
     // Drop the intrinsic, but forward the value
     setValue(&I, getValue(I.getOperand(0)));