Re: PUSH and POP next to first/last register use

[James E Wilson <wilson at specifixinc dot com>]

Sami Khawam wrote:
> I am using a custom CPU architecture where I would like to place the 
> PUSH and POP instuctions of a register just before the first time it is 
> written to and just after the last time it is read, and not in the 
> prologue / epilogue of a function.

This is called "shrink-wrapping".  I would think this works best if you 
emit the saves/restores into the prologue/epilogue, and then run a 
shrink-wrapping optimization pass that tries to move them inward as far 
as you can.

This should be feasible for store/load instructions, but for push/pop 
this will be complicated.  This will change stack offsets, so it will 
interfere with frame pointer elimination.  You need to worry about other 
things that change the stack, like alloca and Variable Length Array 
allocations and deallocations.  You will need to worry about the 
-fdefer-pop optimization, as that may leave the stack unbalanced for 
some sections of code.  And of course if you have overlapping register 
lifetimes, then the push/pops for those registers will conflict.  You 
either have to use the union of the register lifetimes for both 
registers, or else only shrink-wrap one of them.  This all sounds like 
more trouble than it is worth for a target that uses push/pop for 
register saves and restores.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com