[Bug target/126328] x86 costing rejects forming of *sse3_haddv2df3_low from vec_select of sse3_haddv2df

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126328

--- Comment #10 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Richard Biener <[email protected]>:

https://gcc.gnu.org/g:d20adcae36e31cb96a9869566a243e3d6f21ab8d

commit r17-2684-gd20adcae36e31cb96a9869566a243e3d6f21ab8d
Author: Richard Biener <[email protected]>
Date:   Tue Jul 21 15:10:24 2026 +0200

    target/126328 - [x86] insn_cost of *sse3_h{add,sub}v2df3[_low]

    I run into issues with gcc.target/i386/pr54400.c when SLP vectorizing
    a horizontal reduction of a V2DF vector.  The following plugs the
    remaining hole in costing which prevents combine from doing its work
    to recover the single hadd instruction for { p[0] + p[1], q[0] + q[1] }
    The patch avoids the pattern cost special-casing of vec_concat
    by costing the insn based on a new cost_special attribute which I placed
    on all of the haddsub patterns.

            PR target/126328
            PR target/126383
            * config/i386/i386.md (cost_special): New attribute.
            * config/i386/sse.md (avx_h<insn>v4df3): Add cost_special
            attribute with value haddsub.
            (*sse3_haddv2df3): Likewise.
            (sse3_hsubv2df3): Likewise.
            (*sse3_haddv2df3_low): Likewise.
            (*sse3_hsubv2df3_low): Likewise.
            (avx_h<insn>v8sf3): Likewise.
            (sse3_h<insn>v4sf3): Likewise.
            * config/i386/i386.cc (ix86_insn_cost): Cost insns with
            the cost_special attribute value of haddsub like an add.