[PATCH] [Aarch64] Optimize subtract in shift counts

[Michael Collison <Michael dot Collison at arm dot com>]

This patch improves code generation for shifts with subtract instructions where the first operand to the subtract is equal to the bit-size of the operation.


long f1(long x, int i)
{
  return x >> (64 - i);
}

int f2(int x, int i)
{
  return x << (32 - i);
}


With trunk at -O2 we generate:

f1:
	mov	w2, 64
	sub	w1, w2, w1
	asr	x0, x0, x1
	ret

f2:
	mov	w2, 32
	sub	w1, w2, w1
	lsl	w0, w0, w1
	ret

with the patch we generate:

f1:
	neg	w2, w1
	asr	x0, x0, x2
	ret
	.size	f1, .-f1
	.align	2
	.p2align 3,,7
	.global	f2
	.type	f2, %function
f2:
	neg	w2, w1
	lsl	w0, w0, w2
	ret

Okay for trunk?

2017-08-07  Michael Collison <michael.collison@arm.com>

	* config/aarch64/aarch64.md (*aarch64_reg_<optab>_minus<mode>3):
	New pattern.

2016-08-07  Michael Collison <michael.collison@arm.com>

	* gcc.target/aarch64/var_shift_mask_2.c: New test.

Attachment: pr7313v6.patch
Description: pr7313v6.patch