Patch Fix PR32871 -- [avr] gcc is pushing too many registers

[Andy H]

The attached patch fixs:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32871

Although the bug is against the AVR target it  reveals a weakness in 
using df_regs_ever_live_p.
to determine register to save and restore that may apply to other targets.

The fix determines which registers are used as arguments to function using
INIT_CUMULATIVE_ARGS, FUNCTION_ARG and FUNCTION_ARG_ADVANCE
(This is patch is thus relatively portable for other targets)

The argument registers  are then omitted from epilog/prolog save and 
restores
(as all argument registers are assumed clobbered.)

Tested against HEAD and had no new regression for torture/execute testsuite.
Please review with the intention of committing this chnage for avr target.


2000-03-02  Andrew Hutchinson <hutchinsonandy@aim.com>

    PR target/32871
    * config/avr/avr.c (avr_args): Add new function to set argument
    registers in bitmap.
    (avr_regs_to_save): Don't mark argument registers for saving.
    (sequent_regs_live): Add bitmap argument. Use bitmap to determine
    sequence of registers.
    (expand_prologue): Pass bitmap of registers for sequence.
    (expand_epilogue): Pass bitmap of registers for sequence.
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