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[avr-gcc-list] ldd Y+const not working, bug?

From: Klaus Rudolph
Subject: [avr-gcc-list] ldd Y+const not working, bug?
Date: Mon, 08 Nov 2004 00:16:18 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686; de-AT; rv:1.6) Gecko/20040114

Hi all,

why the following code did not compile:

#include <avr/io.h>
.lcomm table1 10
.lcomm table2 10

   ldi YL, lo8(table1)
   ldi YH, hi8(table1)

   ldi r16, 0x10

   ld r17, Y+
   std Y+10-1, r17                 ;1
   std Y+(table2-table1)-1, r17    ;2
   dec r16
   brne lloop


   ldi r16, (table2-table1)-1      ;3
Compile give the following error messages:

avr-gcc -mmcu=at90s8515 -Wa,--gstabs,-D -xassembler-with-cpp -Wa,-alh test.s -nostdlib -o test.o > test.lst
test.s: Assembler messages:
test.s:16: Error: constant value required
test.s:16: Error: number must be less than 64
test.s:23: Warning: expression possibly out of 8-bit range

the expression (2) should result in the same like (1) but it is not allowed to
build a difference of to constants here :-(
in (3) the difference will be calculated correct and the code is ok.00000010 <is_constant>:
 10:   09 e0           ldi     r16, 0x09       ; 9

What this means:
The compiler/assembler/linker is able to build link time constants from differences of
link time constants. But why this will not work for ldd/std???

For me it seams to be a bug!

To add a line like (1) to the code there must be a test in #if ..... #error #endif so that a handcrafted constant like the 10-1 in (1) could be checked in compile or link.
Any idea for such a test? Otherwice the ldd is not usable at all :-(


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