Hi all,

In this PR I'm trying to increase the use of the aarch64 instruction TST that performs a
bitwise AND with a bitmask and compares the result with zero.
GCC has many ways of representing these operations in RTL. Depending on the mask, the target
and the context it might be an AND-immediate, a ZERO_EXTRACT or a ZERO_EXTEND of a subreg.

aarch64.md already contains a pattern for the compare with and-immediate case, which is the most
general form of this, but it doesn't match in many common cases

The documentation on canonicalization in md.texi says:
"Equality comparisons of a group of bits (usually a single bit) with zero
  will be written using @code{zero_extract} rather than the equivalent
  @code{and} or @code{sign_extract} operations. "

This means that we should define a compare with a zero-extract pattern in aarch64,
which is what this patch does. It's fairly simple: it constructs the TST mask from
the operands of the zero_extract and updates the SELECT_CC_MODE implementation to
assign the correct CC_NZ mode to such comparisons.  Note that this is valid only
for equality comparisons against zero.

So for the testcase:
int
f1 (int x)
{
   if (x & 1)
     return 1;
   return x;
}

we now generate:
f1:
         tst     x0, 1
         csinc   w0, w0, wzr, eq
         ret

instead of the previous:
f1:
         and     w1, w0, 1
         cmp     w1, wzr
         csinc   w0, w0, wzr, eq
         ret


and for the testcase:
int
f2 (long x)
{
    return ((short) x >= 0) ? x : 0;
}

we now generate:
f2:
         tst     x0, 32768
         csel    x0, x0, xzr, eq
         ret

instead of:
f2:
         sxth    w1, w0
         cmp     w1, wzr
         csel    x0, x0, xzr, ge
         ret

i.e. we test the sign bit rather than perform the full comparison with zero.

Bootstrapped and tested on aarch64-none-linux-gnu.

Ok for trunk?

Thanks,
Kyrill

2015-12-17  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     PR rtl-optimization/68796
     * config/aarch64/aarch64.md (*and<mode>3nr_compare0_zextract):
     New pattern.
     * config/aarch64/aarch64.c (aarch64_select_cc_mode): Handle
     ZERO_EXTRACT comparison with zero.
     (aarch64_mask_from_zextract_ops): New function.
     * config/aarch64/aarch64-protos.h (aarch64_mask_from_zextract_ops):
     New prototype.

2015-12-17  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     PR rtl-optimization/68796
     * gcc.target/aarch64/tst_3.c: New test.
     * gcc.target/aarch64/tst_4.c: Likewise.