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

["Mark Kettenis" <mark dot kettenis at xs4all dot nl>]

> On Tue, Jun 28, 2005 at 09:59:36PM +0200, Mark Kettenis wrote:
>>    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.
>>
>
>
> In this case I am building linux kernel with debug info (-g) and -mregparm
> is not specified. So parameters should be passed on stack. Following
> is the effective command line to build kernel/sysfs.c. I am not sure if
> any of the below mentioned options are going to affect the gdb results.
>
>   gcc -m32 -Wp,-MD,kernel/.ksysfs.o.d  -nostdinc -isystem
> /usr/lib/gcc/i386-redhat-linux/3.4.3/include -D__KERNEL__ -Iinclude
> -Wall -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing
> -fno-common -ffreestanding -O2     -fomit-frame-pointer -g -pipe
> -msoft-float -mpreferred-stack-boundary=2 -fno-unit-at-a-time
> -march=i686 -mtune=pentium4 -Iinclude/asm-i386/mach-default
> -Wdeclaration-after-statement     -DKBUILD_BASENAME=ksysfs
> -DKBUILD_MODNAME=ksysfs -c -o kernel/ksysfs.o kernel/ksysfs.c

-O2 will have some effect.  The compiler might optimize away variables
(including function arguments) and doesn't always record that fact in
the debug information.

But the real killer here is probably -fomit-frame-pointer.  Last time I
looked GCC didn't generate the correct debug information in that case.
I didn't really look into this, but it seemed as if GCC blindly produces
location descriptions relative to the frame pointer even though there no
longer is a frame pointer.  GCC 4.0 or 4.1 might have this fixed.

>
>> 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.
>
> Sorry for the confusion. In the last mail all the results were reported
> with REGPARM disabled. I wanted to make sure that first normal case works
> fine and then discuss the REGPARM case later.

If you're prepared to do some more tests, you might want to check out
what happens if you leave out -O2 and -fomit-frame-pointer, and then add
back only -O2

Mark