On Mon, May 25, 2020 at 02:59:30AM +0000, Yangfei (Felix) wrote:
> > It creates better code on all targets :-) A quite small improvement, but
> > not
> > entirely trivial.
>
> Thanks for the effort. It's great to hear that :- )
Yes :-)
> > > > p.s. Please use a correct mime type? application/octet-stream
> > > > isn't something I can reply to. Just text/plain is fine :-)
> > >
> > > I have using plain text now, hope that works for you. :-)
> >
> > Nope:
> >
> > [-- Attachment #2: pr94026-v2.diff --]
> > [-- Type: application/octet-stream, Encoding: base64, Size: 5.9K --]
>
> This time I switched to use UUEncode type for the attachment. Does it work?
No:
[-- Attachment #2: pr94026-v3.diff --]
[-- Type: application/octet-stream, Encoding: base64, Size: 5.8K --]
> I am using Outlook and I didn't find the place to change the MIME type : - (
The simplest option is to use a different email client, one that plays
nicely with others. You use git, maybe you could even use git-send-email?
I'll paste things manually...
> From a444419238c02c1e6ab9593a14a13e1e3dff90ed Mon Sep 17 00:00:00 2001
> From: Fei Yang <felix.y...@huawei.com>
> Date: Mon, 25 May 2020 10:19:30 +0800
> Subject: [PATCH] combine: missed opportunity to simplify comparisons with zero
> [PR94026]
(Capital "M" on "Missed" please)
But, the subject should say what the patch *does*. So maybe
combine: Simplify more comparisons with zero (PR94026)
> If we have (and (lshiftrt X C) M) and M is a constant that would select
> a field of bits within an item, but not the entire word, fold this into
> a simple AND if we are in an equality comparison against zero.
But that subject doesn't really describe what the patch does, anyway?
> gcc/
> PR rtl-optimization/94026
> * combine.c (make_compound_operation_int): If we have (and
> (lshiftrt X C) M) and M is a constant that would select a field
> of bits within an item, but not the entire word, fold this into
> a simple AND if we are in an equality comparison.
>
> gcc/testsuite/
> PR rtl-optimization/94026
> * gcc.dg/pr94026.c: New test.
> --- a/gcc/ChangeLog
> +++ b/gcc/ChangeLog
> @@ -1,3 +1,11 @@
> +2020-05-25 Felix Yang <felix.y...@huawei.com>
> +
> + PR rtl-optimization/94026
> + * combine.c (make_compound_operation_int): If we have (and
> + (lshiftrt X C) M) and M is a constant that would select a field
> + of bits within an item, but not the entire word, fold this into
> + a simple AND if we are in an equality comparison.
Don't put the changelog in the patch.
> diff --git a/gcc/combine.c b/gcc/combine.c
> index b044f29fd36..76d62b0bd17 100644
> --- a/gcc/combine.c
> +++ b/gcc/combine.c
> @@ -8178,6 +8178,10 @@ make_compound_operation_int (scalar_int_mode mode, rtx
> *x_ptr,
> if (!CONST_INT_P (XEXP (x, 1)))
> break;
>
> + HOST_WIDE_INT pos;
> + unsigned HOST_WIDE_INT len;
> + pos = get_pos_from_mask (UINTVAL (XEXP (x, 1)), &len);
unsigned HOST_WIDE_INT len;
HOST_WIDE_INT pos = get_pos_from_mask (UINTVAL (XEXP (x, 1)), &len);
> @@ -8231,6 +8235,22 @@ make_compound_operation_int (scalar_int_mode mode, rtx
> *x_ptr,
> new_rtx = make_compound_operation (new_rtx, in_code);
> }
>
> + /* If we have (and (lshiftrt X C) M) and M is a constant that would
> select
> + a field of bits within an item, but not the entire word, this might be
> + representable by a simple AND if we are in an equality comparison. */
> + else if (pos > 0 && equality_comparison
That "&& equality_comparison" should be on a separate line as well.
> + && GET_CODE (XEXP (x, 0)) == LSHIFTRT
> + && CONST_INT_P (XEXP (XEXP (x, 0), 1))
> + && pos + UINTVAL (XEXP (XEXP (x, 0), 1))
> + <= GET_MODE_BITSIZE (mode))
> + {
> + new_rtx = make_compound_operation (XEXP (XEXP (x, 0), 0), next_code);
> + HOST_WIDE_INT real_pos = pos + UINTVAL (XEXP (XEXP (x, 0), 1));
> + unsigned HOST_WIDE_INT mask = ((unsigned HOST_WIDE_INT)1 << len) - 1;
Space after cast.
> + new_rtx = gen_rtx_AND (mode, new_rtx,
> + gen_int_mode (mask << real_pos, mode));
> + }
So this changes
((X >> C) & M) == ...
to
(X & (M << C)) == ...
?
Where then does it check what ... is? This is only valid like this if
that is zero.
Why should this go in combine and not in simplify-rtx instead?
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pr94026.c
> @@ -0,0 +1,21 @@
> +/* { dg-do compile { target aarch64*-*-* i?86-*-* x86_64-*-* } } */
Why restrict this to only some targets?
> +/* { dg-options "-O2 -fdump-rtl-combine" } */
> +
> +int
> +foo (int c)
> +{
> + int a = (c >> 8) & 7;
> +
> + if (a >= 2) {
> + return 1;
> + }
> +
> + return 0;
> +}
> +
> +/* The combine phase should transform (compare (and (lshiftrt x 8) 6) 0)
> + to (compare (and (x 1536)) 0). We look for the *attempt* to match this
> + RTL pattern, regardless of whether an actual insn may be found on the
> + platform. */
> +
> +/* { dg-final { scan-rtl-dump "\\(const_int 1536" "combine" } } */
That is a very fragile test.
Segher
