Re: [patch] Perform anonymous constant propagation during inlining

[Richard Biener <richard dot guenther at gmail dot com>]

On Fri, May 1, 2015 at 8:09 PM, Eric Botcazou <ebotcazou@adacore.com> wrote:
>> OK, how aggressive then?  We could as well do the substitution for all
>> copies:
>>
>>       /* For EXPAND_INITIALIZER try harder to get something simpler.
>>        Otherwise, substitute copies on the RHS, this can propagate
>>        constants at -O0 and thus simplify arithmetic operations.  */
>>       if (g == NULL
>>         && !SSA_NAME_IS_DEFAULT_DEF (exp)
>>         && (optimize || DECL_IGNORED_P (SSA_NAME_VAR (exp)))
>>         && (modifier == EXPAND_INITIALIZER
>>
>>             || (modifier != EXPAND_WRITE
>>
>>                 && gimple_assign_copy_p (SSA_NAME_DEF_STMT (exp))))
>>         && stmt_is_replaceable_p (SSA_NAME_DEF_STMT (exp)))
>>       g = SSA_NAME_DEF_STMT (exp);
>
> This doesn't work (this generates wrong code because this creates overlapping
> live ranges for SSA_NAMEs with the same base variable).  Here's the latest
> working version, all the predicates and accessors used are inlined.

Hum, the fact that your earlier version created wrong code
(get_gimple_for_ssa_name
already returned false here) points at some issues with
EXPAND_INITIALIZER as well, no...?

That said, the path you add is certainly safe (though maybe we want to change
get_gimple_for_ssa_name to return tcc_constant single-use defs even if
TER is disabled
(thus at -O0 - and only at -O0, otherwise it shouldn't happen).  That
would cover
more cases of get_gimple_for_ssa_name uses (I can see
optimize_bitfield_expansion
for example...)

So, your patch is ok for trunk unless you want to explore the
get_gimple_for_ssa_name
improvement suggestion.

I also wonder about EXPAND_INITIALIZER creating overlapping
life-ranges (or moving
loads across stores).

Thanks,
Richard.

> Tested on x86_64-suse-linux, OK for the mainline?
>
>
> 2015-05-01  Eric Botcazou  <ebotcazou@adacore.com>
>
>         * expr.c (expand_expr_real_1) <SSA_NAME>: Try to substitute constants
>         on the RHS of expressions.
>         * gimple-expr.h (is_gimple_constant): Reorder.
>
>
> --
> Eric Botcazou