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

[Jiong Wang <jiwang@tilera.com>]

>> 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.

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