[Bug c/25225] New: code generation -- extra instructions generated

["bcrl at kvack dot org" <gcc-bugzilla at gcc dot gnu dot org>]

The following test program generates extra use of a temporary register for the
return value.  I have narrowed the cause of the poor code generation to the
addition of the pos = file->f_pos; line -- removing it causes the extra
temporary register to disappear (and saves several unnecessary mov
instructions).

/* compile with  gcc -O2 -fomit-frame-pointer -fno-asynchronous-unwind-tables
-S -o fs/test-sys-read.s fs/test-sys-read.c && cat fs/test-sys-read.s
*/
struct file {
        long            f_pos;
};

struct file *fget_light(unsigned int fd);
long vfs_read(struct file *, char *, unsigned long, long *);

static inline int IS_ERR(const void *ptr)
{
        return (unsigned long)ptr > (unsigned long)-1000L;
}

long sys_read(unsigned int fd, char *buf, unsigned long count)
{
        struct file *file;
        long ret, pos;

        file = fget_light(fd);
        if (IS_ERR(file)) {
                pos = file->f_pos;      // comment this line to get better code
                ret = vfs_read(file, buf, count, &pos);
        } else
                ret = (long)file;

        return ret;
}


-- 
           Summary: code generation -- extra instructions generated
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bcrl at kvack dot org
 GCC build triplet: x86_64-unknown-linux
  GCC host triplet: x86_64-unknown-linux
GCC target triplet: x86_64-unknown-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25225