ARC: Table-driven ashlsi implementation for better code/rtx_costs.

One of the cool features of the H8 backend is its use of tables to select
optimal shift implementations for different CPU variants.  This patch
borrows (plagiarizes) that idiom for SImode left shifts in the ARC backend
(for CPUs without a barrel-shifter).  This provides a convenient mechanism
for both selecting the best implementation strategy (for speed vs. size),
and providing accurate rtx_costs [without duplicating a lot of logic].
Left shift RTX costs are especially important for use in synth_mult.

An example improvement is:

int foo(int x) { return 32768*x; }

which is now generated with -O2 -mcpu=em -mswap as:

foo:	bmsk_s  r0,r0,16
        swap    r0,r0
        j_s.d   [blink]
        ror     r0,r0

where previously the ARC backend would generate a loop:

foo:	mov     lp_count,15
        lp      2f
        add     r0,r0,r0
        nop
2:      # end single insn loop
        j_s     [blink]

2024-01-09  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/arc/arc.cc (arc_shift_alg): New enumerated type for
	left shift implementation strategies.
	(arc_shift_info): Type for each entry of the shift strategy table.
	(arc_shift_context_idx): Return a integer value for each code
	generation context, used as an index
	(arc_ashl_alg): Table indexed by context and shifted bit count.
	(arc_split_ashl): Use the arc_ashl_alg table to select SImode
	left shift implementation.
	(arc_rtx_costs) <case ASHIFT>: Use the arc_ashl_alg table to
	provide accurate costs, when optimizing for speed or size.

0 files changed, 0 insertions, 0 deletions