public inbox for gcc-bugs@sourceware.org help / color / mirror / Atom feed
From: "enkovich.gnu at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/64691] New: Suboptimal register allocation for bytes comparison on i386 Date: Tue, 20 Jan 2015 15:26:00 -0000 [thread overview] Message-ID: <bug-64691-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64691 Bug ID: 64691 Summary: Suboptimal register allocation for bytes comparison on i386 Product: gcc Version: 5.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: enkovich.gnu at gmail dot com This problem was actually found in 256.bzip2 benchmark codes compiled by GCC 5.0 on -O2. There is a small loop with bytes comparison which appeared to be ineffective because compared values were not allocated on registers allowing byte access. That caused additional copies and as a result significant loop slow down. Situation may be simulated on a small test if we restrict registers usage. >cat test.c void test (unsigned char *p, unsigned char val) { unsigned char tmp1, tmp2; int i; i = 0; tmp1 = p[0]; while (val != tmp1) { i++; tmp2 = tmp1; tmp1 = p[i]; p[i] = tmp2; } p[0]= tmp1; } >gcc -O2 -m32 -ffixed-ebx test.c -S Here is a loop: .L3: movzbl (%eax), %ebp movl %esi, %ecx movb %dl, (%eax) addl $1, %eax movl %ebp, %edx cmpb %dl, %cl jne .L3 We have an extra register copy esi->ecx to perform comparison. Suppose the easiest way to get better register allocation here would be to transform QI comparison into SI one to relax register constraints.
next reply other threads:[~2015-01-20 15:26 UTC|newest] Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top 2015-01-20 15:26 enkovich.gnu at gmail dot com [this message] 2015-05-12 9:58 ` [Bug target/64691] " ysrumyan at gmail dot com 2015-05-12 10:01 ` ysrumyan at gmail dot com 2021-12-25 7:59 ` pinskia at gcc dot gnu.org
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=bug-64691-4@http.gcc.gnu.org/bugzilla/ \ --to=gcc-bugzilla@gcc.gnu.org \ --cc=gcc-bugs@gcc.gnu.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).