I mean I now know I cannot compare the type info in this way, but are there any
already implemented functions that let you retrieve the vtable and the address
of the base class vtable from a type that someone could point me to?
________________________________
From: Gcc <gcc-bounces~tdebock=drwuk....@gcc.gnu.org> on behalf of Thomas de
Bock via Gcc <gcc@gcc.gnu.org>
Sent: 06 July 2025 16:46:16
To: gcc@gcc.gnu.org
Subject: [ext] building generic tree to compare base classes at runtime
Hello, currently working on a tiny experimental compiler optimization in the
C++ frontend. Trying to compare the base classes of 2 types at runtime, then if
they are equal, statically cast the second operand's type to the first and the
tree should return the now-casted ptr of the second operand, otherwise the tree
should return a null ptr. I read the docs on the website describing the trees,
but I did not find information on comparing runtime type info. Currently I am
doing:
//context:
build_dynamic_cast_1 (location_t loc, tree type, tree expr,
tsubst_flags_t complain)
...
target_type = TYPE_MAIN_VARIANT (TREE_TYPE (type));
static_type = TYPE_MAIN_VARIANT (TREE_TYPE (exprtype));
td2 = get_tinfo_decl (target_type);
td2 = cp_build_addr_expr (td2, complain);
td3 = get_tinfo_decl (static_type);
td3 = cp_build_addr_expr (td3, complain);
...
// `expr` is what this code attempts to statically cast to the type of `type`,
if their base type matches up
tree cast_expr = cp_convert(type, build_static_cast(loc, type, expr,
complain), complain);
tree cond = build2 (NE_EXPR, boolean_type_node, td2, td3);
tree null_ptr = cp_convert (type, nullptr_node, complain);
tree result = build3 (COND_EXPR, type, cond, null_ptr, cast_expr);
result = cp_convert(type, result, complain);
return build_if_nonnull(expr, result, complain);
the output GIMPLE and assembly don't look anything like what I am trying to
achieve though. Does anyone know how the base classes should be compared from
the runtime type info, should I be doing it manually with MEM_REF instead? I
was unable to find documentation on this, as well as most of the tree building
functions, sorry if stupid/unclear question.
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