[Bug target/108229] [13 Regression] unprofitable STV transform since r13-4873-g0b2c1369d035e928

cvs-commit at gcc dot gnu.org gcc-bugzilla@gcc.gnu.org
Tue Jan 3 13:38:42 GMT 2023


--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Roger Sayle <sayle@gcc.gnu.org>:


commit r13-4976-gde59d8bd163a4b2e50ab566441ab49d7212c3356
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Tue Jan 3 13:37:31 2023 +0000

    PR target/108229: A minor STV compute_convert_gain tweak on x86.

    This patch addresses PR target/108229, which is a change in code
    generation during the STV pass, due to the recently approved patch
    to handle vec_select (reductions) in the vector unit.  The recent
    change is innocent, but exposes a latent STV "gain" calculation issue
    that is benign (or closely balanced) on most microarchitectures.

    The issue is when STV considers converting PLUS with a MEM operand.

    On TARGET_64BIT (m=1):
            addq 24(%rdi), %rdx             // 4 bytes
    or with -m32 (m=2)
            addl    24(%esi), %eax          // 3 bytes
            adcl    28(%esi), %edx          // 3 bytes
    is being converted by STV to
            vmovq   24(%rdi), %xmm5         // 5 bytes
            vpaddq  %xmm5, %xmm4, %xmm4     // 4 bytes

    The current code in general_scalar_chain::compute_convert_gain
    considers that scalar unit addition is replaced with a vector
    unit addition (usually about the same cost), but doesn't consider
    anything special about MEM operands, assuming that a scalar load
    gains/costs nothing compared to a vector load.  We can allow the
    backend slightly better fine tuning by including in the gain
    calculation that m scalar loads are being replaced by one vector
    load, and when optimizing for size including that we're increasing
    code size (e.g. an extra vmovq instruction for a MEM operand).

    This patch is a win on the CSiBE benchmark when compiled with -Os.

    2023-01-03  Roger Sayle  <roger@nextmovesoftware.com>

            PR target/108229
            * config/i386/i386-features.cc
            (general_scalar_chain::compute_convert_gain) <case PLUS>: Consider
            the gain/cost of converting a MEM operand.

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