* weak alias in gas on arm-elf
@ 2007-07-02 12:57 Brian Sidebotham
2007-07-02 13:23 ` Daniel Jacobowitz
0 siblings, 1 reply; 6+ messages in thread
From: Brian Sidebotham @ 2007-07-02 12:57 UTC (permalink / raw)
To: binutils
Hi guys,
I want to make a weak alias in a startup file for target arm-elf.
I have read the docs with regards to .weak and .weakref, but I just
can't get them to do what I want.
.weak works as I expect, and the symbols it creates are branches to the
reset vector (pc=0). I have tried the .weak sym1 = sym2 alias assignment
but this is not supported on arm-elf-as.
I thought .weakref would work for me, but I can't get it too. Basically,
I want to create a list of weak symbols for handlers that may or may not
be present in the C code. If the handler isn't present, then the code
should infinitely loop at a dummy label.
At the moment I have:
.extern handler1
...
ldr pc, __handler1
...
__handler1:
bl handler1
.weak handler1
__dummy:
b .
which creates:
...
ldr pc, [pc, #20] ; 20 <__handler1>
...
__handler1:
bl 0 <startup>
Does anyone have any idea how I could effectively achieve .weak __dummy
= handler1 ? Thanks in advance.
Brian Sidebotham.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: weak alias in gas on arm-elf
2007-07-02 12:57 weak alias in gas on arm-elf Brian Sidebotham
@ 2007-07-02 13:23 ` Daniel Jacobowitz
2007-07-02 14:18 ` Brian Sidebotham
2007-07-02 16:04 ` Brian Sidebotham
0 siblings, 2 replies; 6+ messages in thread
From: Daniel Jacobowitz @ 2007-07-02 13:23 UTC (permalink / raw)
To: Brian Sidebotham; +Cc: binutils
On Mon, Jul 02, 2007 at 01:36:34PM +0100, Brian Sidebotham wrote:
> .weak works as I expect, and the symbols it creates are branches to the reset
> vector (pc=0).
That's your mistake. .weak does not create symbols; like .globl, it
changes the visibility of a label.
__handler1:
bl handler1
.weak handler1
handler1:
bl startup
--
Daniel Jacobowitz
CodeSourcery
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: weak alias in gas on arm-elf
2007-07-02 13:23 ` Daniel Jacobowitz
@ 2007-07-02 14:18 ` Brian Sidebotham
2007-07-02 16:04 ` Brian Sidebotham
1 sibling, 0 replies; 6+ messages in thread
From: Brian Sidebotham @ 2007-07-02 14:18 UTC (permalink / raw)
To: binutils
Thankyou for your response Daniel, that sorted me out.
Brian Sidebotham.
Daniel Jacobowitz wrote:
> On Mon, Jul 02, 2007 at 01:36:34PM +0100, Brian Sidebotham wrote:
>> .weak works as I expect, and the symbols it creates are branches to the reset
>> vector (pc=0).
>
> That's your mistake. .weak does not create symbols; like .globl, it
> changes the visibility of a label.
>
> __handler1:
> bl handler1
>
> .weak handler1
> handler1:
> bl startup
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: weak alias in gas on arm-elf
2007-07-02 13:23 ` Daniel Jacobowitz
2007-07-02 14:18 ` Brian Sidebotham
@ 2007-07-02 16:04 ` Brian Sidebotham
2007-07-02 22:00 ` Daniel Jacobowitz
1 sibling, 1 reply; 6+ messages in thread
From: Brian Sidebotham @ 2007-07-02 16:04 UTC (permalink / raw)
To: binutils
Hi Daniel,
I have just tested a few things and realised why I was getting so
confused to begin with.
The original code doesn't branch, because these are entries in a vector
table - so the PC was loaded with the address of the handler instead of
having a branch instruction to the handler.
This leads to:
b _start /* MODE: Supervisor */
ldr pc, srUndef /* MODE: Undefined */
ldr pc, srSWI /* MODE: Supervisor */
ldr pc, srPAbort /* MODE: Abort */
ldr pc, srDAbort /* MODE: Abort */
nop /* reserved (Checksum) */
ldr pc, [pc,#-0x120] /* MODE: IRQ */
ldr pc, srFIQ /* MODE: FIQ */
.weak srUndef
srUndef:
b .
.weak srSWI
srSWI:
b .
...
Emitting the following output:
00000000 <startup>:
0: ea000039 b ec <start>
4: e59ff014 ldr pc, [pc, #20] ; 20 <srUndef>
8: e59ff014 ldr pc, [pc, #20] ; 24 <srUndef+0x4>
c: e59ff014 ldr pc, [pc, #20] ; 28 <srPAbort>
10: e59ff014 ldr pc, [pc, #20] ; 2c <srDAbort>
14: e1a00000 nop (mov r0,r0)
18: e51ff120 ldr pc, [pc, #-288] ; ffffff00 <__ethram_beg__+0x801fff00>
1c: e59ff00c ldr pc, [pc, #12] ; 30 <srFIQ>
20: eafffffe b 20 <srUndef>
24: eafffffe b 24 <srUndef+0x4>
28: eafffffe b 28 <srPAbort>
2c: eafffffe b 2c <srDAbort>
30: eafffffe b 30 <srFIQ>
Disassembly of section prog:
00000034 <srSWI>:
/*
==========================================================================
*/
int __attribute__((interrupt("SWI"))) srSWI(int reason, void* args)
{
34: e52dc004 str ip, [sp, #-4]!
...
As can be seen at address 24: the label srSWI which was given the weak
attribute in the assembler file has gone because there is an srSWI in
the main C code which takes precedence. That makes sense, except that
the weak label code is still present, AND the instruction at address 8
is still loading the PC with the weak symbol address.
This is what originally threw me. I presume this output is intended in
this situation?
If I change the assembler file to branches:
b _start /* MODE: Supervisor */
bl srUndef /* MODE: Undefined */
bl srSWI /* MODE: Supervisor */
bl srPAbort /* MODE: Abort */
bl srDAbort /* MODE: Abort */
nop /* reserved (Checksum) */
ldr pc, [pc,#-0x120] /* MODE: IRQ */
bl srFIQ /* MODE: FIQ */
.weak srUndef
srUndef:
b .
.weak srSWI
srSWI:
b .
...
the weak attribute acts exactly as needed. The following is emitted:
00000000 <startup>:
0: ea000039 b ec <start>
4: eb000005 bl 20 <srUndef>
8: eb000009 bl 34 <srSWI>
c: eb000005 bl 28 <srPAbort>
10: eb000005 bl 2c <srDAbort>
14: e1a00000 nop (mov r0,r0)
18: e51ff120 ldr pc, [pc, #-288] ; ffffff00 <__ethram_beg__+0x801fff00>
1c: eb000003 bl 30 <srFIQ>
20: eafffffe b 20 <srUndef>
24: eafffffe b 24 <srUndef+0x4>
28: eafffffe b 28 <srPAbort>
2c: eafffffe b 2c <srDAbort>
30: eafffffe b 30 <srFIQ>
Disassembly of section prog:
00000034 <srSWI>:
/*
==========================================================================
*/
int __attribute__((interrupt("SWI"))) srSWI(int reason, void* args)
{
...
The reason the ldr is used is because it can address 4Gb memory space,
whereas a branch instruction can only address 32Mb memory space. In my
case this is more than enough and I'm okay to use the branch instructions.
Sorry for the long email!
Best Regards,
Brian Sidebotham.
Daniel Jacobowitz wrote:
> On Mon, Jul 02, 2007 at 01:36:34PM +0100, Brian Sidebotham wrote:
>> .weak works as I expect, and the symbols it creates are branches to the reset
>> vector (pc=0).
>
> That's your mistake. .weak does not create symbols; like .globl, it
> changes the visibility of a label.
>
> __handler1:
> bl handler1
>
> .weak handler1
> handler1:
> bl startup
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: weak alias in gas on arm-elf
2007-07-02 16:04 ` Brian Sidebotham
@ 2007-07-02 22:00 ` Daniel Jacobowitz
2007-07-03 11:01 ` Brian Sidebotham
0 siblings, 1 reply; 6+ messages in thread
From: Daniel Jacobowitz @ 2007-07-02 22:00 UTC (permalink / raw)
To: Brian Sidebotham; +Cc: binutils
On Mon, Jul 02, 2007 at 03:19:39PM +0100, Brian Sidebotham wrote:
> As can be seen at address 24: the label srSWI which was given the weak
> attribute in the assembler file has gone because there is an srSWI in the main
> C code which takes precedence. That makes sense, except that the weak label
> code is still present, AND the instruction at address 8 is still loading the PC
> with the weak symbol address.
>
> This is what originally threw me. I presume this output is intended in this
> situation?
The symbol is weak, not the code following it. Weakness only affects
the definitions of symbols.
Also, using objdump -d on unlinked object files is a good way to get
confused. Try adding -r to see what's really happening.
The ldr version you posted does not make any sense, by the way.
You want the address of the symbol moved to pc, not the contents of
the instruction there! I suspect the original version was supposed to
load the symbol value from a constant pool.
--
Daniel Jacobowitz
CodeSourcery
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: weak alias in gas on arm-elf
2007-07-02 22:00 ` Daniel Jacobowitz
@ 2007-07-03 11:01 ` Brian Sidebotham
0 siblings, 0 replies; 6+ messages in thread
From: Brian Sidebotham @ 2007-07-03 11:01 UTC (permalink / raw)
To: binutils
Daniel Jacobowitz wrote:
> The symbol is weak, not the code following it. Weakness only affects
> the definitions of symbols.
>
> Also, using objdump -d on unlinked object files is a good way to get
> confused. Try adding -r to see what's really happening.
>
> The ldr version you posted does not make any sense, by the way.
> You want the address of the symbol moved to pc, not the contents of
> the instruction there! I suspect the original version was supposed to
> load the symbol value from a constant pool.
>
Daniel,
the ldr version should have been:
...
ldr pc, __hander1
__handler1: .word _hander1
_hander1:
b .
...
Sorry to have posted up misleading information, and thank you for your
explanations.
Best Regards,
Brian Sidebotham.
^ permalink raw reply [flat|nested] 6+ messages in thread
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2007-07-02 12:57 weak alias in gas on arm-elf Brian Sidebotham
2007-07-02 13:23 ` Daniel Jacobowitz
2007-07-02 14:18 ` Brian Sidebotham
2007-07-02 16:04 ` Brian Sidebotham
2007-07-02 22:00 ` Daniel Jacobowitz
2007-07-03 11:01 ` Brian Sidebotham
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