i686 architecture behaviour in gcc

[David Fernandez]

        Hi there,

        Nobody seems to know about this in gcc-help, so, there I go:

-------- Forwarded Message --------
From: David Fernandez <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
Subject: i686 architecture behaviour in gcc
Date: Tue, 21 Feb 2006 17:08:32 +0000

        Hi there,

        Can anyone explain why has been chosen that -march=i686 makes the
compiler change the normal behaviour, and zero-expand unsigned short
parameters into 32-bit registers by all means?

        E.g.

        void __attribute(( regparm(2) )) myoutl( d, p )
        {
            asm("    outl    %0,(%w1)    \n"
                :
                : "a" (d), "d" (p)           );
        }

assembled with

        gcc -Os -fomit-frame-pointer -march=i586 ...

gives us

        outl    %eax,(%dx)
        ret

but changing "-march=i686" gives us

        movzwl  %dx,%edx
        outl    %eax,(%dx)
        ret

        Is it a bug, or had any developer a good reason to do so?

David