Memory copies from string literals do not need to preserve tags

Author: arichardson

clang/lib/CodeGen/CodeGenTypes.cpp

@@ -1013,6 +1013,11 @@ CodeGenTypes::copyShouldPreserveTags(const Expr *E, Optional<CharUnits> Size) {
   QualType Ty = E->IgnoreParenImpCasts()->getType();
   if (Ty->isAnyPointerType())
     Ty = Ty->getPointeeType();
+  const Expr *UnderlyingExpr = E->IgnoreParenCasts();
+  if (const auto *SL = dyn_cast<StringLiteral>(UnderlyingExpr)) {
+    // String literals can never contain tag bits.
+    return llvm::PreserveCheriTags::Unnecessary;
+  }
   // TODO: Find the underlying VarDecl to improve diagnostics
   const VarDecl *UnderlyingVar = nullptr;
   // TODO: this assertion may be overly aggressive.

clang/test/CodeGen/cheri/no-tag-copy-attribute-with-caps.c

@@ -110,12 +110,16 @@ void test_string_constant(struct OneCap *cap) {
   // CHECK-LABEL: void @test_string_constant(
   // Same for string -> char*
   __builtin_memmove(cap, "abcdefghijklmnopqrstuvwxyz", sizeof(*cap));
-  // expected-warning@-1{{memcpy operation with capability argument <unknown type> and underaligned destination (aligned to 1 bytes) may be inefficient or result in CHERI tags bits being stripped}} expected-note@-1{{For more information}}
   // CHECK: call void @llvm.memmove.p200i8.p200i8.i64(i8 addrspace(200)* align 16 {{%[a-z0-9]+}}, i8 addrspace(200)* align 1 getelementptr inbounds ([27 x i8], [27 x i8] addrspace(200)* @.str
   // CHECK-SAME: , i64 0
   // CHECK-SAME: , i64 0)
-  // CHECK-SAME: , i64 16, i1 false) [[MUST_PRESERVE_ATTR]]{{$}}
-  // FIXME-SAME: , i64 16, i1 false) [[NO_PRESERVE_ATTR]]{{$}}
+  // CHECK-SAME: , i64 16, i1 false) [[NO_PRESERVE_ATTR]]{{$}}
+  /// Check that the explicit cast does not affect the analysis.
+  __builtin_memmove(cap, (void*)"abcdefghijklmnopqrstuvwxyz", sizeof(*cap));
+  // CHECK: call void @llvm.memmove.p200i8.p200i8.i64(i8 addrspace(200)* align 16 {{%[a-z0-9]+}}, i8 addrspace(200)* align 1 getelementptr inbounds ([27 x i8], [27 x i8] addrspace(200)* @.str
+  // CHECK-SAME: , i64 0
+  // CHECK-SAME: , i64 0)
+  // CHECK-SAME: , i64 16, i1 false) [[NO_PRESERVE_ATTR]]{{$}}
 }
 
 void test_void_buffer(struct OneCap *cap, void *buf) {