[Builtins] Add __builtin_implicit_object_fence.

[davidtgoldblatt]

Summary

This is a builtin that allows the easy implementation of C++23's
start_lifetime_as. It takes a pointer, and returns a pointer allowing
implicit object creation at the given region of storage that will keep
the same object representation.

For now this just means forwarding llvm.tbaa.fence, which I believe is the only
place that C++-level type information makes it into LLVM IR in a
semantics-affecting way.

[davidtgoldblatt]

Update

[davidtgoldblatt]

Adding @zygoloid as a p2590 author for review

clang/include/clang/Basic/Builtins.def

 BUILTIN(__builtin_shufflevector, "v."   , "nct")
 BUILTIN(__builtin_convertvector, "v."   , "nct")
 BUILTIN(__builtin_alloca, "v*z"   , "Fn")
 BUILTIN(__builtin_alloca_uninitialized, "v*z", "Fn")
 BUILTIN(__builtin_alloca_with_align, "v*zIz", "Fn")
 BUILTIN(__builtin_alloca_with_align_uninitialized, "v*zIz", "Fn")
 BUILTIN(__builtin_call_with_static_chain, "v.", "nt")
 BUILTIN(__builtin_nondeterministic_value, "v.", "nt")
+BUILTIN(__builtin_implicit_object_fence, "v*v*", "n")
 
 BUILTIN(__builtin_elementwise_abs, "v.", "nct")
 BUILTIN(__builtin_elementwise_max, "v.", "nct")
 BUILTIN(__builtin_elementwise_min, "v.", "nct")
 BUILTIN(__builtin_elementwise_ceil, "v.", "nct")
 BUILTIN(__builtin_elementwise_cos, "v.", "nct")
 BUILTIN(__builtin_elementwise_exp, "v.", "nct")
 BUILTIN(__builtin_elementwise_exp2, "v.", "nct")

clang/lib/CodeGen/CGBuiltin.cpp

if (FD->hasAttr<ConstAttr>() ||
     case Builtin::BI__builtin_roundeven:
     case Builtin::BI__builtin_roundevenf:
     case Builtin::BI__builtin_roundevenf16:
     case Builtin::BI__builtin_roundevenl:
     case Builtin::BI__builtin_roundevenf128:
       return RValue::get(emitUnaryMaybeConstrainedFPBuiltin(*this, E,
                                    Intrinsic::roundeven,
                                    Intrinsic::experimental_constrained_roundeven));
-                                   
     case Builtin::BIsin:
     case Builtin::BIsinf:
     case Builtin::BIsinl:
     case Builtin::BI__builtin_sin:
     case Builtin::BI__builtin_sinf:
     case Builtin::BI__builtin_sinf16:
     case Builtin::BI__builtin_sinl:
     case Builtin::BI__builtin_sinf128:
case Builtin::BI__builtin_launder: {
     const Expr *Arg = E->getArg(0);
     QualType ArgTy = Arg->getType()->getPointeeType();
     Value *Ptr = EmitScalarExpr(Arg);
     if (TypeRequiresBuiltinLaunder(CGM, ArgTy))
       Ptr = Builder.CreateLaunderInvariantGroup(Ptr);
 
     return RValue::get(Ptr);
   }
+  case Builtin::BI__builtin_implicit_object_fence: {
+    const Expr *Arg = E->getArg(0);
+    Value *Ptr = EmitScalarExpr(Arg);
+    Function *F =
+        Intrinsic::getDeclaration(&CGM.getModule(), Intrinsic::tbaa_fence);
+    Value *Result = Builder.CreateCall(F, {Ptr});
+    return RValue::get(Result);
+  }
   case Builtin::BI__sync_fetch_and_add:
   case Builtin::BI__sync_fetch_and_sub:
   case Builtin::BI__sync_fetch_and_or:
   case Builtin::BI__sync_fetch_and_and:
   case Builtin::BI__sync_fetch_and_xor:
   case Builtin::BI__sync_fetch_and_nand:
   case Builtin::BI__sync_add_and_fetch:
   case Builtin::BI__sync_sub_and_fetch:

clang/test/CodeGen/builtin-implicit-object-fence.c

+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -emit-llvm -o - %s | FileCheck %s
+
+// CHECK-LABEL: @test1(
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    [[A_ADDR:%.*]] = alloca ptr, align 8
+// CHECK-NEXT:    [[B:%.*]] = alloca ptr, align 8
+// CHECK-NEXT:    store ptr [[A:%.*]], ptr [[A_ADDR]], align 8
+// CHECK-NEXT:    [[TMP0:%.*]] = load ptr, ptr [[A_ADDR]], align 8
+// CHECK-NEXT:    [[TMP1:%.*]] = call ptr @llvm.tbaa.fence(ptr [[TMP0]])
+// CHECK-NEXT:    store ptr [[TMP1]], ptr [[B]], align 8
+// CHECK-NEXT:    [[TMP2:%.*]] = load ptr, ptr [[B]], align 8
+// CHECK-NEXT:    [[TMP3:%.*]] = load float, ptr [[TMP2]], align 4
+// CHECK-NEXT:    ret float [[TMP3]]
+//
+float test1(int *a) {
+  float *b = __builtin_implicit_object_fence(a);
+  return *b;
+}