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From: "ysrumyan at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/64691] Suboptimal register allocation for bytes comparison on i386 Date: Tue, 12 May 2015 09:58:00 -0000 [thread overview] Message-ID: <bug-64691-4-bd6SjC6gLe@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-64691-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64691 Yuri Rumyantsev <ysrumyan at gmail dot com> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |ysrumyan at gmail dot com --- Comment #1 from Yuri Rumyantsev <ysrumyan at gmail dot com> --- I found another register allocation deficiency which can be exhibited at the attached test-case extracted from important benchmark. If we look at inner loop for(i = 0; i < size; i++) { byte xr, xg, xb, t1; sbyte t2, t3; x1 = read[0]; x2 = read[1]; x3 = read[2]; t1 = (byte) (((C1 * x1) + (C2 * x2) + (C3 * x3) + (1 << (SCALE - 1))) >> SCALE); t2 = (sbyte) (((C4 * x1) + (C5 * x2) + (C6 * x3) + (1 << (SCALE - 1))) >> SCALE); t3 = (sbyte) (((C7 * x1) + (C8 * x2) + (C9 * x3) + (1 << (SCALE - 1))) >> SCALE); write[0] = t1; write[1] = (byte) t2; write[2] = (byte) t3; read += 3; write += 3; } we can see that 7 registers is enough to keep all variable (except for upper loop bound): 3 registers for x1,x2,x3, 2 registers for read and write pointers and 2 registers for computation one for t1,t2,t3 computations and one scratch register for multiplications (but since consumers of t1,t2,t3 is byte store this register must belong also to Q_REQS subset, i.e. AREG,BREG,CREG or DREG). But LRA does not perform such allocation and this leads to redundant spill/fills and results in performance degradation. Assembly file produced 6.0 compiler with "-O2 -m32 -march=slm" options is attached too.
next prev parent reply other threads:[~2015-05-12 9:58 UTC|newest] Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top 2015-01-20 15:26 [Bug target/64691] New: " enkovich.gnu at gmail dot com 2015-05-12 9:58 ` ysrumyan at gmail dot com [this message] 2015-05-12 10:01 ` [Bug target/64691] " ysrumyan at gmail dot com 2021-12-25 7:59 ` pinskia at gcc dot gnu.org
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