[PATCH] [Aarch64] Optimize subtract in shift counts

[Michael Collison]

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.colli...@arm.com>

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

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

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

pr7313v6.patch
Description: pr7313v6.patch