Bug 1038 - resolution of weak symbol not left for the linker if value is already known
Summary: resolution of weak symbol not left for the linker if value is already known
Status: NEW
Alias: None
Product: binutils
Classification: Unclassified
Component: gas (show other bugs)
Version: 2.15
: P2 normal
Target Milestone: ---
Assignee: unassigned
Depends on:
Reported: 2005-06-28 22:02 UTC by Joerg Wunsch
Modified: 2010-09-20 05:16 UTC (History)
3 users (show)

See Also:
Target: avr-*-*
Last reconfirmed:

Testcase for bug 1038 (119 bytes, text/plain)
2010-09-20 05:13 UTC, Joerg Wunsch

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Description Joerg Wunsch 2005-06-28 22:02:32 UTC
avr-libc's C run-time startup code uses something like the following to
initialize the stack:

        .weak   __stack
        .set    __stack, RAMEND
        ldi     r28,lo8(__stack)
#ifdef SPH
        ldi     r29,hi8(__stack)
        out     _SFR_IO_ADDR(SPH), r29
        out     _SFR_IO_ADDR(SPL), r28

Here, RAMEND as a C preprocessor macro that depends on the actual processor
type this code is being compiled for, and which evaluates to a 16-bit
number by the time this code is being assembled.

The idea behind that is that while the stack initially starts at top of
RAM (growing downwards), the user can tune the actual stack location at
link time by supplying a different value for the weak symbol __stack.

However, as the value of __stack is already known to the assembler at
that point, the assembler actually inserts the fixed value RAMEND instead
of a reference to the weak symbol __stack, thereby entirely defeating the
idea behind marking __stack as `weak' in the first place.

By reordering the code, and placing the definition of __stack behind its
use in the assembly code, the problem can be worked around, but I believe
the assembler must not resolve symbols marked as `weak' in the first place
but always leave that to the linker.
Comment 1 Bjoern Haase 2006-04-22 07:29:53 UTC
The method to resolve the issue would probably be to disallow gas to do any 
fixup work on weak lables. 
Probably one could use the TC_VALIDATE_FIX hook in order to realize this. E.g. 
by adapting an expression like 
#define TC_VALIDATE_FIX(FIXP,SEG,SKIP)                      \ 
if (FIXP->fx_r_type == BFD_RELOC_AVR_7_PCREL                \ 
    || FIXP->fx_r_type == BFD_RELOC_AVR_13_PCREL            \ 
    || FIXP->fx_r_type == BFD_RELOC_AVR_LO8_LDI_PM          \ 
    || FIXP->fx_r_type == BFD_RELOC_AVR_HI8_LDI_PM          \ 
    || FIXP->fx_r_type == BFD_RELOC_AVR_HH8_LDI_PM          \ 
    || FIXP->fx_r_type == BFD_RELOC_AVR_16_PM)              \     
  {                                                         \ 
     goto SKIP;                                             \ 
to handle the case that FIXP refers to weak lables. The only question would be, 
how to find out whether or not FIXP refers to a weak label.?  
This probably would solve the issue for the avr target. I think, however, that 
the issue should be addressed for all of the targets since I believe it to be 
present also elsewhere. 
Comment 2 Eric Weddington 2010-09-20 02:57:11 UTC
This is an old bug. Do you still happen to have a test case for this?

Comment 3 Joerg Wunsch 2010-09-20 05:13:44 UTC
Created attachment 4990 [details]
Testcase for bug 1038
Comment 4 Joerg Wunsch 2010-09-20 05:16:49 UTC
In reply to comment #2:

> This is an old bug. Do you still happen to have a test case for this?

The testcase from the original posting is still valid.  I've put it into
a compilable file, see attachment.  Compile with:

avr-gcc -mmcu=atmega8 -c bug1038.S

Disassemble the result:

% avr-objdump -d bug1038.o

bug1038.o:     file format elf32-avr

Disassembly of section .text:

00000000 <.text>:
   0:   cf e5           ldi     r28, 0x5F       ; 95
   2:   d4 e0           ldi     r29, 0x04       ; 4
   4:   de bf           out     0x3e, r29       ; 62
   6:   cd bf           out     0x3d, r28  

As can be seen, the assembler already resolved the weak symbol __stack
(despite as marking it as weak in the symbol table), and inserted the
(compile-time) constant RAMEND.