From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <ee9e417a05031010495f14704a@mail.gmail.com>
Date: Thu, 10 Mar 2005 13:49:17 -0500
From: Russ Cox <russcox@gmail.com>
To: Fans of the OS Plan 9 from Bell Labs <9fans@cse.psu.edu>
Subject: Re: [9fans] got it
In-Reply-To: <Pine.LNX.4.58.0503101129220.12217@enigma.lanl.gov>
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i have tried a few things and cannot reproduce this.
can you post a short c program and a short sequence
of commands that turns the c program into the assembly?

$y-260(SP) means -260(SP) with a comment that
that's expected to be "y".  -260 is just the offset of y
in the stack frame.

russ


On Thu, 10 Mar 2005 11:34:44 -0700 (MST), Ronald G. Minnich
<rminnich@lanl.gov> wrote:
>
> an optimization is dumping onto another, it seems.
>
> on arm.
>
> ulong *x, y[64];
>
>         x = y;
>         *x++ = 1;
>         *x++ = 2;
>
> assembly gives:
>
>         MOVW    $y-260(SP),R3
>         MOVW    $1, R2
>         MOVW.P  R2, $4(R3)
>
> so the 260 puts you two words before the first element of Y, I'm guessing
> (otherwise it would be y-252, right? 63*4). Then it does a pre-increment
> and offset by 4, which puts you right at the first element of y.
>
> Which all fails badly if you're setting x to a constant, as in constant
> page table value in memory for startup.
>
> That's my reading anyway.
>
> I'm still unsure, all I know is the .s code looks wrong to me for the l.s
> changes I'm making. (I'm too lazy to write assembly, I try to have C
> compilers do it for me. Usually works. Maybe not this time.)
>
> ron
>