From mboxrd@z Thu Jan  1 00:00:00 1970
From: alan@spri.levels.unisa.edu.au (Alan Modra)
To: gas2@cygnus.com
Subject: Re: gas fixup problem?
Date: Sat, 24 Jun 1995 03:51:00 -0000
Message-id: <m0sPSnu-0000y0C@mullet>
References: <no.id>
X-SW-Source: 1995/msg00126.html

> 
> /usr/i486-linux/bin/as zzz.s; nm a.out
> 00000000 t L0
> 00000006 t L1
> 00000002 t L2		!!!!!!!!!
> 00000006 t L3
> 00000015 t L4
> 00000094 t L5
> 
>  --------------------zzz.s----------------
> 	.text
> L0:
> 	jz L5
> L1:
> L2:
> L3:
> 	.space -(L2 - L0 + 0x7f) & (16-1), 0x90
> 
> L4:
> 	.space 0x7f, 0x90
> L5:
>  -----------------------------------------
> 
> The idea of the .space here is to align L5, given we know how much
> code is between the .space and L5.  Maybe this bug is just related to
> .space, but if it's something to do with expression evaluation, then
> it is a more serious one.  I'll see if I can fix it myself, but my
> time is rather limited at the moment.  Maybe someone else will get
> curious as to what is wrong here?
> 
A bit more info.  The problem seems to be caused inside S_GET_VALUE()
in write.c around line 1937.  S_GET_VALUE tries to evaluate the
expression, and in so doing resolves L2 to be equal to 2, correct for
the first time round when the "jz" is assumed to be a short
instruction.  When gas decide that the "jz" needs to be a long branch
occupying 6 bytes, the value of L2 is marked as resolved so isn't
adjusted.
I'm not sure I have the time to figure out how all the gas internals
work, so a fix from me will take a while...

.org also exhibits the same sort of behaviour with
-------------------------------
        .text
Z0:
        jz Z9
Z1:
Z2:
        .org (15 & -(Z2 - Z0)) + .
Z5:
        .space 0x7f, 0x90
Z9:
-------------------------------
similarly giving wrong values for Z2

The slightly more contrived
-------------------------------
        .text
Z0:
        jz Z9
Z1:
Z2:
        .org (Z2 - Z0 + 1) + .
Z5:
        .space 0x7f, 0x90
Z9:
-------------------------------
bombs with
z1.s: Assembler messages:
z1.s:9: Error: attempt to .org/.space backwards? (-3)
z1.s:9: Internal error!
Assertion failure in write_contents at write.c line 924.
Please report this bug.