On 11/13/20 1:20 AM, Richard Sandiford via Gcc-patches wrote:
> This patch adds yet another way of propagating into an instruction and
> simplifying the result. (The net effect of the series is to keep the
> total number of propagation approaches the same though, since a later
> patch removes the fwprop.c routines.)
>
> One of the drawbacks of the validate_replace_* routines is that
> they only do simple simplifications, mostly canonicalisations:
>
> /* Do changes needed to keep rtx consistent. Don't do any other
> simplifications, as it is not our job. */
> if (simplify)
> simplify_while_replacing (loc, to, object, op0_mode);
>
> But substituting can often lead to real simplification opportunities.
> simplify-rtx.c:simplify_replace_rtx does fully simplify the result,
> but it only operates on specific rvalues rather than full instruction
> patterns. It is also nondestructive, which means that it returns a
> new rtx whenever a substitution or simplification was possible.
> This can create quite a bit of garbage rtl in the context of a
> speculative recog, where changing the contents of a pointer is
> often enough.
>
> The new routines are therefore supposed to provide simplify_replace_rtx-
> style substitution in recog. They go to some effort to prevent garbage
> rtl from being created.
>
> At the moment, the new routines fail if the pattern would still refer
> to the old "from" value in some way. That might be unnecessary in
> some contexts; if so, it could be put behind a configuration parameter.
>
> gcc/
> * recog.h (insn_propagation): New class.
> * recog.c (insn_propagation::apply_to_mem_1): New function.
> (insn_propagation::apply_to_rvalue_1): Likewise.
> (insn_propagation::apply_to_lvalue_1): Likewise.
> (insn_propagation::apply_to_pattern_1): Likewise.
> (insn_propagation::apply_to_pattern): Likewise.
> (insn_propagation::apply_to_rvalue): Likewise.
>
OK
jeff
