Re: [Fastboot] Re: [-mm patch] i386: enable REGPARM by default

[Mark Kettenis <mark.kettenis@xs4all.nl>]

   Date: Tue, 28 Jun 2005 16:54:12 +0530
   From: Vivek Goyal <vgoyal@in.ibm.com>

   > Thanks. Any idea what might be amiss with my case where I am not seeing 
   > proper function parameter values while analyzing kdump generated crash
   > dump with gdb. I am using following gdb and gcc versions.
   > 
   > GNU gdb Red Hat Linux (6.1post-1.20040607.62rh)
   > gcc (GCC) 3.4.3 20041212 (Red Hat 3.4.3-9.EL4)
   > 

   Some more info. I dumped the stack contents and it seems that stack is fine 
   and parameters are intact on stack. So now it seems to be a matter of 
   how gdb is interpreting the stack contents. Any guess, what the problem is?

I'd say the problem is with a user building stuff with non-standard
"optimizations", probably even stripping his executable, and expecting
to be able to debug the result.

   Why func2() and func1() are not showing right parameter values. 

Repeating what Daniel said before, by using "regparm", function
arguments are now passed in registers instead of on the stack.  It's
extremely unlikely that these function arguments will stay in those
registers for ever, especially since you've only got a handfull of
them on the i386.  So eventually they will be moved to some other
register or, more likely, to memory.  If the compiler doesn't tell gdb
about it, gdb will still think the value is in the register, and
display whatever what's there now, which is likely to be the wrong
value.  There are two ways the compiler can tell gdb where things are:

1. By explicitly specifying the new location.  Both DWARF 2 and stabs
   debugging formats can do this, but AFAIK, GCC won't do this if a
   register is spilled to the stack.

2. By specifying where registers are saved.  Only DWARF 2 can do this.

We've seen cases where the information generated by GCC for 1 or 2 is
either incomplete or wrong.  There also have been cases where GDB
didn't interpret that information correctly.  And then some people
tend to remove some of the debug information by stripping their code
or using broken linker scripts. You'll need to find out where the
problem is, but my bet is that its's a problem with GCC since you make
it generate non-standard code.

Oh, by the way, don't expect gdb to be able to call "regparm"
functions either.

Mark
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