Messages in this thread | | | Date | Wed, 29 Jun 2005 12:06:59 +0200 (CEST) | Subject | Re: [Fastboot] Re: [-mm patch] i386: enable REGPARM by default | From | "Mark Kettenis" <> |
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> 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
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