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[avr-gcc-list] It is more than: .byte SYMA - SYMB

From: Erik Christiansen
Subject: [avr-gcc-list] It is more than: .byte SYMA - SYMB
Date: Thu, 11 Sep 2003 12:23:06 +1000
User-agent: Mutt/1.3.28i

On Tue, May 27, 2003 at 11:19:48AM +0200, Joerg Wunsch wrote:
> I tried to define a byte in avr-as to the difference of two
> (relocatable) symbols.  I pulled my hair out, but failed so far.
> So something like:
> .text
> SYMA: mov     rthis, rthat
> SYMB: ldi     rthat, 42
> ...
>       .byte   SYMA-SYMB

The problem appears to be more widespread. While implementing a simple
assembly language struct:

.section .bss,"a"
.org 0x800060                <- To show the absolute case also fails.
q_length = 32
log_queue:   .space q_length  ; Queue of log data to be transmitted.
log_wr_ptr:  .space 1         ; Write pointer
log_rd_ptr:  .space 1         ; Read pointer
log_bytes:   .space 1         ; Count of bytes queued.

.section .text,"ax",@progbits
ldd   r16,Y+(log_bytes - log_queue)    ; Byte count
ldd   r17,Y+(log_rd_ptr - log_queue)   ; Read pointer
cpse  r16,zero                         ; Queue empty?
rjmp  1f  

we are hit with: "Error: constant value required" on both the ldd lines.

As Jörg already explained, these expressions are an assembly-time
constant, fully resolvable, in the absolute case, without resort to even
the linker. And, as Jörg found with .word:

lbytes = (log_bytes - log_queue)     ; Assembles without complaint.

but then:

ldd   r16,Y+(lbytes)                 ; Shows it's still confused:
Error: constant value required

We can see in avr_get_constant (str, max) in
binutils-030512/gas/config/tc-avr.c, the failure to handle the constant

  expression (&ex);
  if (ex.X_op != O_constant)
    as_bad (_("constant value required"));

Tracking the "#define expression(result) expr (0, result)"   to
"segT expr (rankarg, resultP)", 242 lines of laborious hand parsing are
found. The really good news is that we have left tc-avr.c and are now in
the gas-generic expr.c file.

Maybe, instead of "expression (&ex);", the following (from
config/tc-avr.h) should be used. (Jörg mentions that .word works):

/* You may define this macro to parse an expression used in a data
   allocation pseudo-op such as `.word'.  You can use this to
   recognize relocation directives that may appear in such directives.  */
#define TC_PARSE_CONS_EXPRESSION(EXPR,N) avr_parse_cons_expression (EXPR,N)
void avr_parse_cons_expression (expressionS *exp, int nbytes);

Without getting seriously into it at this stage, it's clear that it uses
"expression()", so ex.X_op should be set. (With a bit of luck :-)

If the implementation had been lex/yacc, I'd just have made a quick
grammar tweak, but wading into somone else's hand beaten parser is
several orders of magnitude more murky. If a maintainer could offer some
pointers on whether I'm headed in the right direction, I'll still give
it a try.

In the interim, any suggested workaround would be welcome indeed!


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