Re: [PATCH v2] implement constant-folding in __builtin_bswap*()

[Christopher Li <sparse@chrisli.org>]

On Mon, Nov 21, 2016 at 5:45 PM, Johannes Berg
<johannes@sipsolutions.net> wrote:
> Yes, that'd make some sense. I have no idea how to pull it off though

I take a brief look.

It seems that you need to implement a symbol with ".expand" implement as
"expand_bswap", similar to "expand_constant_p".

Later in in "eval_init_table", just add one entry for __builtin_bswap.
eval_init_table[] = {
{ "__builtin_constant_p", &builtin_fn_type, MOD_TOPLEVEL, &constant_p_op },
{ "__builtin_safe_p", &builtin_fn_type, MOD_TOPLEVEL, &safe_p_op },
{ "__builtin_warning", &builtin_fn_type, MOD_TOPLEVEL, &warning_op },
{ "__builtin_expect", &builtin_fn_type, MOD_TOPLEVEL, &expect_op },
{ "__builtin_choose_expr", &builtin_fn_type, MOD_TOPLEVEL, &choose_op },
{ NULL, NULL, 0 }
};

As you can see in "expand_call", the cost of arguments is zero
if all arguments are constant.

The function "expand_symbol_call" even have the comment about expanding
the builtins there.

In side the "expand_bswap", it should abort if cost is not zero.
It means arguments are not constant, this expression can't
expand into a constant value.

Otherwise, expand_bswap should proceed to calculate the return
value of the bswap call and rewrite the call expression into a constant.
Some what similar to the "__builtin_constant_p".

I will see if I can hack up some thing very quick.

Chris