From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-return-154293-listarch-gcc=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 15342 invoked by alias); 28 May 2009 17:48:40 -0000
Received: (qmail 15332 invoked by uid 22791); 28 May 2009 17:48:38 -0000
X-SWARE-Spam-Status: No, hits=-1.3 required=5.0 	tests=AWL,BAYES_00,J_CHICKENPOX_22,J_CHICKENPOX_31,J_CHICKENPOX_43
X-Spam-Check-By: sourceware.org
Received: from web111606.mail.gq1.yahoo.com (HELO web111606.mail.gq1.yahoo.com) (67.195.23.48)     by sourceware.org (qpsmtpd/0.43rc1) with SMTP; Thu, 28 May 2009 17:48:33 +0000
Received: (qmail 38594 invoked by uid 60001); 28 May 2009 17:48:31 -0000
Message-ID: <516867.32690.qm@web111606.mail.gq1.yahoo.com>
Received: from [70.98.240.34] by web111606.mail.gq1.yahoo.com via HTTP; Thu, 28 May 2009 10:48:31 PDT
Date: Thu, 28 May 2009 18:55:00 -0000
From: Jamie Prescott <jpresss@yahoo.com>
Subject: Forgetting return values
To: gcc@gcc.gnu.org
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
X-IsSubscribed: yes
Mailing-List: contact gcc-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc/>
List-Post: <mailto:gcc@gcc.gnu.org>
List-Help: <http://gcc.gnu.org/ml/>
Sender: gcc-owner@gcc.gnu.org
X-SW-Source: 2009-05/txt/msg00660.txt.bz2


What am I missing?
I have a simple:

static inline int set_prop(char const *path, char const *name,
                                          void const *data, int size)
{
        int error;

        asm volatile ("int\t11\n\t"
                      : "=a0" (error): "a0" (path), "a1" (name), "a2" (data),
                        "a3" (size));

        return error;
}

extern int calc(int);

int proc(int i)
{
        int j = calc(i);

        return set_prop(0, 0, &j, sizeof(int));
}

The aX classes maps to the rX registers, no problem for GCC in there.
The code above, compiled with GCC 4.4.0 and -O3 produces:


 _proc:
        push    FP
        mov     SP,FP
        sub     SP,4,SP
        call   _calc
        mov     0,r0
        mov     4,r3
        add     FP,-4,r2
        mov     r0,r1
;       APP
; 69 "xxxx.c" 1
        int     11
        
;       NO_APP
        add     SP,4,SP
        pop     FP
        ret

GCC forgets about the calc() return value, and passes an address (FP-4) that
has never been written into.
buf if I add a "memory" clobber to the asm inline, everything comes back to
normal:

_proc:
        push    FP
        mov     SP,FP
        sub     SP,4,SP
        call   _calc
        str.w    r0,FP[-4]
        mov     0,r0
        mov     4,r3
        add     FP,-4,r2
        mov     r0,r1
;       APP
; 69 "xxxx.c" 1
        int     11
        
;       NO_APP
        add     SP,4,SP
        pop     FP
        ret

Why is the memory clobber required, and why GCC does not understand to
sync the value to memory when passing the address to a function?
Thanks,


- Jamie