Re: [PATCH] Fix an issue with the gdb step-over aka. "n" command

[Simon Marchi <simark@simark.ca>]

On 2019-11-24 7:17 a.m., Bernd Edlinger wrote:
> Hi,
> 
> this fixes an issue with the gdb step-over aka. "n" command.
> 
> Apologies, the motivation for this patch was from sub-optimal
> debug experience using some gcc code involving inlined functions,
> and initially I tried to convince gcc folks that it is in fact a
> gcc bug, but...
> 
> It can be seen when you debug an optimized stage-3 cc1
> it does not affect -O0 code, though.
> 
> Note: you can use "gcc -S hello.c -wrapper gdb,--args" to invoke cc1 with
> debugger attached.
> 
> This example debug session will explain the effect.
> 
> (gdb) b get_alias_set
> Breakpoint 5 at 0xa099f0: file ../../gcc-trunk/gcc/alias.c, line 837.
> (gdb) r
> Breakpoint 5, get_alias_set (t=t@entry=0x7ffff7ff7ab0) at ../../gcc-trunk/gcc/alias.c:837
> 837	  if (t == error_mark_node
> (gdb) n
> 839		  && (TREE_TYPE (t) == 0 || TREE_TYPE (t) == error_mark_node)))
> (gdb) n
> 3382	  return __t;  <-- now we have a problem: wrong line info here
> (gdb) bt
> #0  get_alias_set (t=t@entry=0x7ffff7ff7ab0) at ../../gcc-trunk/gcc/tree.h:3382
> #1  0x0000000000b25dfe in set_mem_attributes_minus_bitpos (ref=0x7ffff746f990, t=0x7ffff7ff7ab0, objectp=1, bitpos=...)
>     at ../../gcc-trunk/gcc/emit-rtl.c:1957
> #2  0x0000000001137a55 in make_decl_rtl (decl=0x7ffff7ff7ab0) at ../../gcc-trunk/gcc/varasm.c:1518
> #3  0x000000000113b6e8 in assemble_variable (decl=0x7ffff7ff7ab0, top_level=<optimized out>, at_end=<optimized out>, 
>     dont_output_data=0) at ../../gcc-trunk/gcc/varasm.c:2246
> #4  0x000000000113f0ea in varpool_node::assemble_decl (this=0x7ffff745b000) at ../../gcc-trunk/gcc/varpool.c:584
> #5  0x000000000113fa17 in varpool_node::assemble_decl (this=0x7ffff745b000) at ../../gcc-trunk/gcc/varpool.c:750

I have a hard time understanding what is going wrong and what we should see
instead.  I think it would help if you showed what's $pc at every place where
you are stopping, as well as the output for readelf --debug-dump=decodedline
for those regions.  It would also help if you provided the same session without
and with your patch applied, so we could see the difference.

Simon