public inbox for gcc-bugs@sourceware.org help / color / mirror / Atom feed
From: "rv at rasmusvillemoes dot dk" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/64753] New: Redundant cmp instruction on x86_64 Date: Fri, 23 Jan 2015 15:06:00 -0000 [thread overview] Message-ID: <bug-64753-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64753 Bug ID: 64753 Summary: Redundant cmp instruction on x86_64 Product: gcc Version: 5.0 Status: UNCONFIRMED Severity: enhancement Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: rv at rasmusvillemoes dot dk Host: x86_64 Target: x86_64 The linux kernel's library strncmp is this: int strncmp(const char *cs, const char *ct, size_t count) { unsigned char c1, c2; while (count) { c1 = *cs++; c2 = *ct++; if (c1 != c2) return c1 < c2 ? -1 : 1; if (!c1) break; count--; } return 0; } Compiling with gcc -O2 -S I get this: strncmp: .LFB0: .cfi_startproc testq %rdx, %rdx je .L10 movzbl (%rdi), %ecx movzbl (%rsi), %r8d # cmpb %r8b, %cl # jne .L3 testb %cl, %cl je .L10 subq $1, %rdx xorl %eax, %eax jmp .L4 .p2align 4,,10 .p2align 3 .L6: movzbl 1(%rdi,%rax), %ecx movzbl 1(%rsi,%rax), %r8d # cmpb %r8b, %cl # jne .L3 addq $1, %rax testb %cl, %cl je .L10 .L4: cmpq %rdx, %rax jne .L6 .L10: xorl %eax, %eax ret .p2align 4,,10 .p2align 3 .L3: cmpb %r8b, %cl sbbl %eax, %eax orl $1, %eax ret .cfi_endproc At the two places marked # we do a cmp and a conditional jump to .L3, where for good measure the same cmp is done again... there's no other path to .L3, so it would seem that simply omitting that extra cmp should be ok. This is with gcc-5.0 (GCC) 5.0.0 20150112 (experimental), but I see the same with gcc (Debian 4.7.2-5) 4.7.2.
next reply other threads:[~2015-01-23 15:06 UTC|newest] Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top 2015-01-23 15:06 rv at rasmusvillemoes dot dk [this message] 2015-01-28 12:58 ` [Bug target/64753] " rv at rasmusvillemoes dot dk 2015-01-28 13:31 ` ubizjak at gmail dot com 2024-03-17 7:39 ` 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-64753-4@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).