Re: RFA: [buildsym.c] Turn off unused addr bits in linetable

[Fernando Nasser <fnasser@redhat.com>]

Andrew Cagney wrote:
> 
> Fernando Nasser wrote:
> >
> > Some systems like ARM may have extra bits set in the address to indicate
> > a mode or something else (the LSB=1 means thumb mode for ARM).
> >
> > So, before storing line number addresses we may need to turn this bits
> > off as they may be passed to gdb with the bits on as result of
> > relocation (ld has no way of knowing it is for a line table entry).
> >
> > The following patch passes the line addresses through the target
> > dependent macro ADDR_BITS_REMOVE() before storing it on the table.
> >
> > OK to commit?
> >
> > ChangeLog:
> >
> >         * buildsym.c (record_line): Turn off unused addr bits.
> 
> I'm not sure about this.  The ARM has two ways of identifying THUMB code
> - the LSB and a tacky symbol lookup.
> 

Yes, but the loader relocation produces _some_ thumb addresses with the
LSB bit on.  The bit in the symbol table is only used to detect thumb
addresses that had the bit turned off (I have a second patch that
optimizes the ARM implementation of ADDR_BITS_REMOVE() but it is not
necessary for correctness).


> Others, such as MIPS/MIPS16 don't.  I'm not sure how this will affect
> that target.  Does the code that uses this address of line also ``turn
> off unused addr bits''?
> 

Well, they should.  BRAKPOINT_FROM_PC() is supposed to make the
necessary address adjustments.  In the ARM case, it does turn off the
thumb bit (the others will never be set where it is used).

I don't have a MIPS target to test.  I will depend on someone doing it
for me.  I will take a look at the macros implementation though.

Thanks for the heads up.


-- 
Fernando Nasser
Red Hat Canada Ltd.                     E-Mail:  fnasser@redhat.com
2323 Yonge Street, Suite #300
Toronto, Ontario   M4P 2C9