Re: [RFC/TileGX 2/6] simplify the handling of skip prologue for plt stub

[Pedro Alves <palves@redhat.com>]

On 01/18/2013 03:12 PM, Jiong Wang wrote:
> 
>>> for tilegx,  when skip prologue, if the start_pc is a plt stub address, then
>>> stop to go further, just return the start_pc.
>>>
>>> gdb/ChangeLog:
>>>
>>>         * tilegx-tdep.c (tilegx_skip_prologue): simplify the handling for
>>>         plt stub.
>> Can you provide an example where this becomes necessary? I don't
>> see a problem with the patch per se, but I don't remember seeing
>> other ports doing this...
>>
> yes, there are some tricky thing.
> 
>   for the simple hello.c
> 
> #include <stdio.h>
> int main(int argc, char **argv)
> {
> printf("hello, %d\n", argc);
> return 0
> }
> 
> without this patch
> ===
> -bash-4.1$ gcc -o hello.tile hello.c
> -bash-4.1$ ./gdb hello.tile
> (gdb) b printf
> Error accessing memory address 0x10a40: Success.
> 
> after this patch
> ===
> (gdb) b printf
> Breakpoint 1 at 0x10a40
> 
> 
> this is because tilegx skip_prologue will invoke tilegx_analyze_prologue, which
> will prefetch 32*8 bytes.
> 
> while for when the address is in plt stub,  you can see it near the eh_frame_hdr section
> 
>   [14] .plt                         0000000000010a00 000a00 0000a0 28  AX  0   0 64
>   ...
>   [16] .eh_frame_hdr     0000000000010ac0 000ac0 000024 00   A  0 0  4
>   [17] .eh_frame             0000000000010ae8 000ae8 0000b4 00   A 0   0  8
> 
> the .eh_frame_hdr aligns to 4, there is a hole between .eh_frame_hdr and .eh_frame, and this
> will cause trouble for section_table_xfer_memory_partial.
> 
> after fetch memory starting from 0x10ac0 to 0x10ae4, then the memaddr will be 0x10ae4 in section_table_xfer_memory_partial,
> while this function did not consider this hole situation, so goes to line 666, error occured.

Right.  It's this issue that needs solving.  Otherwise you're just
papering over the problem.

> 
> for other targets, like x86, I have done a brief exploration,  seems they do not have large prefetch window



-- 
Pedro Alves