Re: new ppc32 GOT/PLT support

[Steve Munroe <sjmunroe at us dot ibm dot com>]

Alan Modra <amodra@bigpond.net.au> wrote on 05/17/2005 09:33:53 AM:

> ...One problem remains: ppc32 calls
> _mcount before the function prologue, so it's not possible to set up
> registers to call _mcount via the plt in pic code.  So for now, if you
> profile pic code using a dynamic _mcount, the call sequence used will
> force the old exec GOT/PLT.  I'm thinking the best thing is to make
> _mcount available in libc_nonshared, and only keep _mcount in libc.so
> to satisfy explicit versions, but I'm open to suggestions.
> 

Looking at the details, gmon/mcount.c is aliased to __mcount_internal and 
the exported _mcount symbol is defined by the stub in ppc-mcount.S. 
ppc-mount.S; stacks a frame, saves the parameter register (r3-r10), setup 
up the frompc and seflpc parameters in r3/r4, before calling 
__mcount_internal.

Also gcc is still generating the -pg prologue as:

        .data
        .align  2
   0:   .long   0
        .previous
        mflr    r0
        lis     r11,0b@ha
        stw     r0,4(r1)
        addi    r0,r11,0b@l
        bl      _mcount

The .data word and its address in r0 are neaver used (by mcount.c or any 
other code that I can find). Also this is a non-PIC sequence that we 
should elliminate for the new ABI.

One posibililty is to move the ppc-mcount.S stub code into libc_nonshared 
and have it establish the got (using the new ABI). Then we could call 
gmon/mcount.c version of _mcount via the new PLT. So we need two prologues 
for the profiling case. One for the leaf routine case where the mcount 
stub establishes its own got. And one for the none leave case where the bl 
_local_mcount established the lr for the got sequence.

So the leaf case looks like:

func:
     mflr    r0
     stw     r0,4(r1)
     bl      _leaf_mcount
#    we can assume that on return r0/lr are restored by _leaf_mcount
#    to the entry value.
     ...

And the non-leaf case could look like:

func2:
     mflr    r0
     stw     r0,4(r1)
     bl      _local_mcount
1:
#    we can assume that on return r0 are restored by _local_mcount
#    to the lr on entry.
     stwu    r1,-32(r1)
     stw     r31,28(r1)
     stw     r30,24(r1)
     mflr    r30
     stw     r0,36(r1) # this is probably redundent
     addis   r30,r30,(got-1b)@ha
     addi    r30,r30,(got-1b)@l

This will require versioning the _mcount symbol for the new ABI.



Steven J. Munroe
Linux on Power Toolchain Architect
IBM Corporation, Linux Technology Center