On 10/27/2017 09:56 AM, Richard Sandiford wrote: > Jeff Law <l...@redhat.com> writes: >> On 10/23/2017 05:21 AM, Richard Sandiford wrote: >>> This patch adds a way of treating certain kinds of CONST as unique, >>> so that pointer equality is equivalent to value equality. For now it >>> is restricted to VEC_DUPLICATE and VEC_SERIES, although the code to >>> generate them remains in the else arm of an "if (1)" until a later >>> patch. >>> >>> This is needed so that (const (vec_duplicate xx)) can used as the >>> CONSTxx_RTX of a variable-length vector. >> You're brave :-) I know we looked at making CONST_INTs behave in this >> manner eons ago in an effort to reduce memory consumption and it was >> just plain painful. There may still be comments from that project >> littering the source code. >> >> I do wonder if we might want to revisit this again as we have better >> infrastructure in place. > > For vectors it isn't so bad, since we already do the same thing > for CONST_VECTOR. Fortunately CONST_VECTOR and CONST always have > a mode, so there's no awkward sharing between modes... > >>> 2017-10-23 Richard Sandiford <richard.sandif...@linaro.org> >>> Alan Hayward <alan.hayw...@arm.com> >>> David Sherwood <david.sherw...@arm.com> >>> >>> gcc/ >>> * rtl.h (unique_const_p): New function. >>> (gen_rtx_CONST): Declare. >>> * emit-rtl.c (const_hasher): New struct. >>> (const_htab): New variable. >>> (init_emit_once): Initialize it. >>> (const_hasher::hash, const_hasher::equal): New functions. >>> (gen_rtx_CONST): New function. >>> (spare_vec_duplicate, spare_vec_series): New variables. >>> (gen_const_vec_duplicate_1): Add code for use (const (vec_duplicate)), >>> but disable it for now. >>> (gen_const_vec_series): Likewise (const (vec_series)). >>> * gengenrtl.c (special_rtx): Return true for CONST. >>> * rtl.c (shared_const_p): Return true if unique_const_p. >> ISTM that you need an update the rtl.texi's structure sharing >> assumptions section to describe the new rules around CONSTs. > > Oops, yeah. How about the attached? OK.
> >> So what's the purpose of the sparc_vec_* stuff that you're going to use >> in the future? It looks like a single element cache to me. Am I >> missing something? > > No, that's right. When looking up the const for (vec_duplicate x), say, > it's easier to create the vec_duplicate rtx first. But if the lookup > succeeds (and so we already have an rtx with that value), we keep the > discarded vec_duplicate around so that we can reuse it for the next > lookup. OK. Jeff
