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

[Vivek Goyal <vgoyal@in.ibm.com>]

> > 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

I built another kernel with -fno-omit-frame-pointer and output seems to
have worsen a lot now. I am not able to build a kernel without -02. There
seems to be some dependencies which I am sorting out.

With frame pointer support, following is the command line.

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     -fno-omit-frame-pointer -fno-optimize-sibling-calls -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

And the gdb trace has worsened. Trace is not even showing all the calls
as it was showing when kernel was built with -fomit-frame-pointer.


#0  crash_get_current_regs (regs=0xec3b5e34) at arch/i386/kernel/crash.c:103
#1  0xc0114077 in crash_save_self (saved_regs=0xec3b5e34)
    at arch/i386/kernel/crash.c:134
#2  0xec3b5f04 in ?? ()
#3  0x00000014 in ?? ()
#4  0xec3b5e98 in ?? ()
#5  0xc013d7e6 in crash_kexec (regs=0x2) at kernel/kexec.c:1059
#6  0xecb29c80 in ?? ()
#7  0xecb29c80 in ?? ()
#8  0x00000000 in ?? ()

Thanks
Vivek