From: Denis Vlasenko <vda@ilport.com.ua>
To: Gerold Jury <gerold.ml@inode.at>, jakub@redhat.com
Subject: Re: memcpy(a,b,CONST) is not inlined by gcc 3.4.1 in Linux kernel
Date: Wed, 30 Mar 2005 09:15:59 +0300
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On Wednesday 30 March 2005 05:27, Gerold Jury wrote:
> 
> >> On Tue, Mar 29, 2005 at 05:37:06PM +0300, Denis Vlasenko wrote:
> >> > /*
> >> >  * This looks horribly ugly, but the compiler can optimize it totally,
> >> >  * as the count is constant.
> >> >  */
> >> > static inline void * __constant_memcpy(void * to, const void * from,
> >> > size_t n) {
> >> >         if (n <= 128)
> >> >                 return __builtin_memcpy(to, from, n);
> >>
> >> The problem is that in GCC < 4.0 there is no constant propagation
> >> pass before expanding builtin functions, so the __builtin_memcpy
> >> call above sees a variable rather than a constant.
> >
> >or change "size_t n" to "const size_t n" will also fix the issue.
> >As we do some (well very little and with inlining and const values)
> >const progation before 4.0.0 on the trees before expanding the builtin.
> >
> >-- Pinski
> >-
> I used the following "const size_t n" change on x86_64
> and it reduced the memcpy count from 1088 to 609 with my setup and gcc 3.4.3.
> (kernel 2.6.12-rc1, running now)

What do you mean, 'reduced'?

(/me is checking....)

Oh shit... It still emits half of memcpys, to be exact - for
struct copies:

arch/i386/kernel/process.c:

int copy_thread(int nr, unsigned long clone_flags, unsigned long esp,
        unsigned long unused,
        struct task_struct * p, struct pt_regs * regs)
{
        struct pt_regs * childregs;
        struct task_struct *tsk;
        int err;

        childregs = ((struct pt_regs *) (THREAD_SIZE + (unsigned long) p->thread_info)) - 1;
        *childregs = *regs;
        ^^^^^^^^^^^^^^^^^^^
        childregs->eax = 0;
        childregs->esp = esp;

# make arch/i386/kernel/process.s

copy_thread:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %edi
        pushl   %esi
        pushl   %ebx
        subl    $20, %esp
        movl    24(%ebp), %eax
        movl    4(%eax), %esi
        pushl   $60
        leal    8132(%esi), %ebx
        pushl   28(%ebp)
        pushl   %ebx
        call    memcpy  <=================
        movl    $0, 24(%ebx)
        movl    16(%ebp), %eax
        movl    %eax, 52(%ebx)
        movl    24(%ebp), %edx
        addl    $8192, %esi
        movl    %ebx, 516(%edx)
        movl    %esi, -32(%ebp)
        movl    %esi, 504(%edx)
        movl    $ret_from_fork, 512(%edx)

Jakub, is there a way to instruct gcc to inine this copy, or better yet,
to use user-supplied inline version of memcpy?
--
vda

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