Question about DRAP register and reserving hard registers

[Steve Ellcey]

On Fri, 2015-06-19 at 09:09 -0400, Richard Henderson wrote:
> On 06/16/2015 07:05 PM, Steve Ellcey  wrote:
> >
> > I have a question about the DRAP register (used for dynamic stack alignment)
> > and about reserving/using hard registers in general.  I am trying to understand
> > where, if a drap register is allocated, GCC is told not to use it during
> > general register allocation.  There must be some code somewhere for this
> > but I cannot find it.
> 
> There isn't.  Because the vDRAP register is a pseudo.  The DRAP register is 
> only live from somewhere in the middle of the prologue to the end of the prologue.
> 
> See ix86_get_drap_rtx, wherein we coordinate with the to-be-generated prologue 
> (crtl->drap_reg), allocate the pseudo, and emit the hard-reg-to-pseudo copy at 
> entry_of_function.
> 
> 
> r~

OK, that makes more sense now.  In my work on MIPS I was trying to cut
out some of the complexity of the x86 implementation and just use a hard
register as my DRAP register.  One of the issues I ran into, and perhaps
the one that caused x86 to use a virtual register, was saving and
restoring the register during setjmp/longjmp and C++ exception handling
usage.  I will trying switching to a virtual register and see if that
works better.

Other than exceptions, the main complexity in dynamic stack alignment
seems to involve the debug information.  I am still trying to understand
the handling of the drap register and dynamic stack alignment in
dwarf2out.c and dwarf2cfi.c.

Steve Ellcey
sellcey@imgtec.com