Avoid adding impossible copies in ira-conflicts.c:process_reg_shuffles

Avoid adding impossible copies in ira-conflicts.c:process_reg_shuffles

If an insn requires two operands to be tied, and the input operand dies
in the insn, IRA acts as though there were a copy from the input to the
output with the same execution frequency as the insn.  Allocating the
same register to the input and the output then saves the cost of a move.

If there is no such tie, but an input operand nevertheless dies
in the insn, IRA creates a similar move, but with an eighth of the
frequency.  This helps to ensure that chains of instructions reuse
registers in a natural way, rather than using arbitrarily different
registers for no reason.

This heuristic seems to work well in the vast majority of cases.
However, for SVE, the handling of untied operands ends up creating
copies between dying predicate registers and vector outputs, even though
vector and predicate registers are distinct classes and can never be
tied.  This is a particular problem because the dying predicate tends
to be the loop control predicate, which is used by most instructions
in a vector loop and so (rightly) has a very high allocation priority.
Any copies involving the loop predicate therefore tend to get processed
before copies involving only vector registers.  The end result is that
we tend to allocate the output of the last vector instruction in a loop
ahead of its natural place in the allocation order and don't benefit
from chains created between vector registers.

This patch tries to avoid the problem by not adding register shuffle
copies if there appears to be no chance that the two operands could be
allocated to the same register.

2019-09-21  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* ira-conflicts.c (can_use_same_reg_p): New function.
	(process_reg_shuffles): Take an insn parameter.  Ignore cases
	in which input operand op_num could seemingly never be allocated
	to the same register as the destination.
	(add_insn_allocno_copies): Update call to process_reg_shuffles.

gcc/testsuite/
	* gcc.target/aarch64/sve/cond_convert_1.c: Remove XFAILs.
	* gcc.target/aarch64/sve/cond_convert_4.c: Likewise.
	* gcc.target/aarch64/sve/cond_unary_2.c: Likewise.

From-SVN: r276018