[PATCH] D147021: [Builtins] Add __builtin_implicit_object_fence.

[David Goldblatt via Phabricator via cfe-commits]

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davidtgoldblatt updated this revision to Diff 509069.
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Update


davidtgoldblatt added a comment.

Adding @zygoloid as a p2590 author for review


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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D147021

Files:
  clang/include/clang/Basic/Builtins.def
  clang/lib/CodeGen/CGBuiltin.cpp
  clang/test/CodeGen/builtin-implicit-object-fence.c


Index: clang/test/CodeGen/builtin-implicit-object-fence.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/builtin-implicit-object-fence.c
@@ -0,0 +1,19 @@
+// 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;
+}
Index: clang/lib/CodeGen/CGBuiltin.cpp
===================================================================
--- clang/lib/CodeGen/CGBuiltin.cpp
+++ clang/lib/CodeGen/CGBuiltin.cpp
@@ -2472,7 +2472,7 @@
       return RValue::get(emitUnaryMaybeConstrainedFPBuiltin(*this, E,
                                    Intrinsic::roundeven,
                                    Intrinsic::experimental_constrained_roundeven));
-                                   
+
     case Builtin::BIsin:
     case Builtin::BIsinf:
     case Builtin::BIsinl:
@@ -3904,6 +3904,14 @@
 
     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:
Index: clang/include/clang/Basic/Builtins.def
===================================================================
--- clang/include/clang/Basic/Builtins.def
+++ clang/include/clang/Basic/Builtins.def
@@ -662,6 +662,7 @@
 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")


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