In the code below, gcc fails to optimize a hand-crafted memcpy as a call to the
memcpy function. (or perhaps as "rep;movsb" when compiling with "-Os") I've
tried "-O2", "-O3", and "-Os". There are probably a great number of similar
optimizations that could be done.
This optimization is not unheard of; it is done by Visual Studio 2005. IMHO it
is not really any more insane than autovectorization.
junk 0 $ cat foo.c
void *foo(void *restrict, const void *restrict, unsigned long);
void *foo(void *restrict dst, const void *restrict src, unsigned long n)
{
const char *p = src;
char *q = dst;
while (n--) {
*q++ = *p++;
}
return dst;
}
junk 0 $ gcc -m32 -std=gnu99 -fomit-frame-pointer -Os -S foo.c
junk 0 $ cat foo.s
.file "foo.c"
.text
.globl foo
.type foo, @function
foo:
pushl %ebx
movl 16(%esp), %ebx
movl 8(%esp), %ecx
movl 12(%esp), %edx
jmp .L2
.L3:
movb -1(%edx), %al
movb %al, -1(%ecx)
.L2:
decl %ebx
incl %ecx
incl %edx
cmpl $-1, %ebx
jne .L3
movl 8(%esp), %eax
popl %ebx
ret
.size foo, .-foo
.ident "GCC: (GNU) 4.1.1 20060828 (Red Hat 4.1.1-20)"
.section .note.GNU-stack,"",@progbits
junk 0 $ gcc -v
Using built-in specs.
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-libgcj-multifile
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk
--disable-dssi --enable-plugin
--with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre --with-cpu=generic
--host=x86_64-redhat-linux
Thread model: posix
gcc version 4.1.1 20060828 (Red Hat 4.1.1-20)
junk 0 $
--
Summary: does not recognize memcpy and optimize it
Product: gcc
Version: 4.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: acahalan at gmail dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29174
