ira: Consider save/restore costs of callee-save registers [PR110071]

ira: Consider save/restore costs of callee-save registers [PR110071]

In improve_allocation() routine, IRA checks for each allocno if spilling
any conflicting allocnos can improve the allocation of this allocno.
This routine computes the cost improvement for usage of each profitable
hard register for a given allocno. The existing code in
improve_allocation() does not consider the save/restore costs of callee
save registers while computing the cost improvement.

This can result in a callee save register being assigned to a pseudo
that is live in the entire function and across a call, overriding a
non-callee save register assigned to the pseudo by graph coloring. So
the entry basic block requires a prolog, thereby causing shrink wrap to
fail.

2023-09-14  Surya Kumari Jangala  <jskumari@linux.ibm.com>

gcc/
	PR rtl-optimization/110071
	* ira-color.cc (improve_allocation): Consider cost of callee
	save registers.

gcc/testsuite/
	PR rtl-optimization/110071
	* gcc.target/powerpc/pr110071.c: New test.