Ping.
https://gcc.gnu.org/ml/gcc-patches/2017-04/msg00931.html
Thanks,
Kyrill
On 21/04/17 09:20, Kyrill Tkachov wrote:
Hi all,
A pattern that sometimes occurs in the wild is to subtract two operands and
separately
compare them. This can be implemented as a single SUBS instruction and we
actually have
a define_insn for this: sub<mode>3_compare1.
However, I'm not sure if that's enough by itself to match these constructs.
Adding a peephole
that will actually bring the subtraction and comparison SETs together into a
PARALLEL helps
a lot in matching these (note that there is no dependency between the subtract
and the comparison).
This patch adds such a peephole. It's really simple and straightforward. The
only thing to look out for
is the case when the output of the subtract is a register that is also one of
the operands:
SUB W0, W0, W1
CMP W0, W1
should not be transformed into:
SUBS W0, W0, W1.
The testcase in the patch provides a motivating example where we now generate a
single SUBS instead of a SUB
followed by a CMP.
This transformation triggers a few times in SPEC2006. Not enough to actually
move the needle,
but it's the Right Thing to Do (tm).
I've seen it catch cases that compute an absolute difference, for example:
int
foo (int a, int b)
{
if (a < b)
return b - a;
else
return a - b;
}
will now generate:
foo:
sub w2, w1, w0
subs w3, w0, w1
csel w0, w3, w2, ge
ret
instead of:
foo:
sub w2, w1, w0
sub w3, w0, w1
cmp w0, w1
csel w0, w3, w2, ge
ret
Bootstrapped and tested on aarch64-none-linux-gnu.
Ok for GCC 8?
Thanks,
Kyrill
2017-04-21 Kyrylo Tkachov <kyrylo.tkac...@arm.com>
* config/aarch64/aarch64.c (define_peephole2 above
*sub_<shift>_<mode>): New peephole.
2017-04-21 Kyrylo Tkachov <kyrylo.tkac...@arm.com>
* gcc.target/aarch64/subs_compare_1.c: New test.
