public inbox for gcc-bugs@sourceware.org help / color / mirror / Atom feed
From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug rtl-optimization/31248] char adding (in loops) gives an extra move or two Date: Tue, 28 Aug 2007 12:02:00 -0000 [thread overview] Message-ID: <20070828120213.11249.qmail@sourceware.org> (raw) In-Reply-To: <bug-31248-1649@http.gcc.gnu.org/bugzilla/> ------- Comment #3 from ubizjak at gmail dot com 2007-08-28 12:02 ------- Current mainline [GCC: (GNU) 4.3.0 20070828] generates: test: .LFB2: xorl %eax, %eax xorl %edx, %edx .align 16 .L2: addb table(%rdx), %al addq $1, %rdx cmpq $10, %rdx jne .L2 movsbl %al,%eax ret Now, there is an optimization problem in the intialization code in loop header. To omit one iteration, it should start with: .LFB2: movzbl table(%rip), %ecx movl $1, %edx .L2: ... BTW: By reversing the loop: for (i = 9; i; i--) val += table[i]; we could remove comparison from the loop, but instead we produce (x86_64): test: .LFB2: xorl %eax, %eax xorl %edx, %edx .align 16 .L2: addb table+9(%rdx), %al subq $1, %rdx cmpq $-9, %rdx jne .L2 movsbl %al,%eax ret However, using -m32 we get: test: xorl %eax, %eax movl $9, %edx .align 16 .L2: addb table(%edx), %al subl $1, %edx jne .L2 movsbl %al,%eax ret In reversed-loop case, we could generate: test: movl $9, %edx movzbl table(%edx), %eax .align 16 .L2: addb table(%edx), %al subl $1, %edx jne .L2 movsbl %al,%eax ret -- ubizjak at gmail dot com changed: What |Removed |Added ---------------------------------------------------------------------------- Summary|char adding gives an extra |char adding (in loops) gives |move or two |an extra move or two http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31248
next prev parent reply other threads:[~2007-08-28 12:02 UTC|newest] Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top 2007-03-17 19:09 [Bug tree-optimization/31248] New: Too much casting for char operands on tree level ubizjak at gmail dot com 2007-03-18 5:30 ` [Bug tree-optimization/31248] " pinskia at gcc dot gnu dot org 2007-03-29 6:16 ` [Bug rtl-optimization/31248] char adding gives an extra move or two pinskia at gcc dot gnu dot org 2007-08-28 12:02 ` ubizjak at gmail dot com [this message] 2008-01-09 9:29 ` [Bug rtl-optimization/31248] char adding (in loops) " ubizjak at gmail dot com 2008-01-09 9:46 ` ubizjak at gmail dot com
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=20070828120213.11249.qmail@sourceware.org \ --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).