Re: optimization/3511: Inlined strlen() could be smarter

Re: optimization/3511: Inlined strlen() could be smarter

[rth]

Synopsis: Inlined strlen() could be smarter

State-Changed-From-To: open->suspended
State-Changed-By: rth
State-Changed-When: Sun Oct  6 20:05:04 2002
State-Changed-Why:
    I doubt this will ever be handled.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=3511

optimization/3511: Inlined strlen() could be smarter

[Matthias Klose]

>Number:         3511
>Category:       optimization
>Synopsis:       Inlined strlen() could be smarter
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Jul 01 09:36:00 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     herbert@gondor.apana.org.au
>Release:        3.0 (Debian GNU/Linux) and HEAD 20010701
>Organization:
The Debian project
>Environment:
System: Debian GNU/Linux (testing/unstable)
Architecture: i686
	
host: i386-linux
build: i386-linux
target: i386-linux
configured with: ../src/configure -v --enable-languages=c,c++,java,f77,proto,objc --prefix=/usr --infodir=/share/info --mandir=/share/man --enable-shared --with-gnu-as --with-gnu-ld --with-system-zlib --enable-long-long --enable-nls --without-included-gettext --disable-checking --enable-threads=posix --enable-java-gc=boehm --with-cpp-install-dir=bin --enable-objc-gc i386-linux
>Description:
[ Reported to the Debian BTS as report #86251.
  Please CC 86251-quiet@bugs.debian.org on replies.
  Log of report can be found at http://bugs.debian.org/86251 ]
 	

gcc -S -O2 on the file

#include <string.h>

size_t mystrlen(const char *s) {
	if (!s) {
		s = "";
	}

	return strlen(s);
}

generates (on i386)

        .file   "bug-86251.c"
        .section        .rodata
.LC0:
        .string ""
        .text
        .align 2
        .p2align 2,,3
.globl mystrlen
        .type   mystrlen,@function
mystrlen:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %edi
        movl    8(%ebp), %edi
        testl   %edi, %edi
        je      .L114
.L113:
        xorl    %eax, %eax
        cld
        movl    $-1, %ecx
        repnz
        scasb
        notl    %ecx
        leal    -1(%ecx), %eax
        movl    (%esp), %edi
        leave
        ret
        .p2align 2,,3
.L114:
        movl    $.LC0, %edi
        jmp     .L113
.Lfe1:
        .size   mystrlen,.Lfe1-mystrlen
        .ident  "GCC: (GNU) 3.1 20010701 (experimental)"

As you can see, strlen() is not optimised out in the !pat case.

>How-To-Repeat:
	
>Fix:
	
>Release-Note:
>Audit-Trail:
>Unformatted: