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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/97873] Failure to optimize abs optimally (at least one
 completely useless instruction on x86)
Date: Fri, 20 Nov 2020 10:26:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97873

--- Comment #9 from Uro=C5=A1 Bizjak <ubizjak at gmail dot com> ---
Fixed by:

commit fdace7584056de2f63bde2e3087f26beb6b0f97d
Author: Uros Bizjak <ubizjak@gmail.com>
Date:   Fri Nov 20 10:26:34 2020 +0100

    i386: Optimize abs expansion [PR97873]

    The patch introduces absM named pattern to generate optimal insn sequen=
ce
    for CMOVE_TARGET targets.  Currently, the expansion goes through neg+max
    optabs, and the following code is generated:

            movl    %edi, %eax
            negl    %eax
            cmpl    %edi, %eax
            cmovl   %edi, %eax

    This sequence is unoptimal in two ways.  a) The compare instruction is
    not needed, since NEG insn sets the sign flag based on the result.
    The CMOV can use sign flag to select between negated and original value:

            movl    %edi, %eax
            negl    %eax
            cmovs   %edi, %eax

    b) On some targets, CMOV is undesirable due to its performance issues.
    In addition to TARGET_EXPAND_ABS bypass, the patch introduces STV
    conversion of abs RTX to use PABS SSE insn:

            vmovd   %edi, %xmm0
            vpabsd  %xmm0, %xmm0
            vmovd   %xmm0, %eax

    The patch changes compare mode of NEG instruction to CCGOCmode,
    which is the same mode as the mode of SUB instruction. IOW, sign bit
    becomes usable.

    Also, the mode iterator of <maxmin:code><mode>3 pattern is changed
    to SWI48x instead of SWI248. The purpose of maxmin expander is to
    prepare max/min RTX for STV to eventually convert them to SSE PMAX/PMIN
    instructions, in order to *avoid* CMOV insns with general registers.

    2020-11-20  Uro=C5=A1 Bizjak  <ubizjak@gmail.com>

    gcc/
            PR target/97873
            * config/i386/i386.md (*neg<mode>2_2): Rename from
            "*neg<mode>2_cmpz".  Use CCGOCmode instead of CCZmode.
            (*negsi2_zext): Rename from *negsi2_cmpz_zext.
            Use CCGOCmode instead of CCZmode.
            (*neg<mode>_ccc_1): New insn pattern.
            (*neg<dwi>2_doubleword): Use *neg<mode>_ccc_1.

            (abs<mode>2): Add FLAGS_REG clobber.
            Use TARGET_CMOVE insn predicate.
            (*abs<mode>2_1): New insn_and_split pattern.
            (*absdi2_doubleword): Ditto.

            (<maxmin:code><mode>3): Use SWI48x mode iterator.
            (*<maxmin:code><mode>3): Use SWI48 mode iterator.

            * config/i386/i386-features.c
            (general_scalar_chain::compute_convert_gain): Handle ABS code.
            (general_scalar_chain::convert_insn): Ditto.
            (general_scalar_to_vector_candidate_p): Ditto.

    gcc/testsuite/
            PR target/97873
            * gcc.target/i386/pr97873.c: New test.
            * gcc.target/i386/pr97873-1.c: New test.=