On Thu, Jul 27, 2023 at 11:38 PM Jan Hubicka via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hi,
> while looking on profile misupdate on hmmer I noticed that loop splitting
> pass is not
> able to handle the loop it has as an example it should apply on:
>
> One transformation of loops like:
>
> for (i = 0; i < 100; i++)
> {
> if (i < 50)
> A;
> else
> B;
> }
>
> into:
>
> for (i = 0; i < 50; i++)
> {
> A;
> }
> for (; i < 100; i++)
> {
> B;
> }
>
> The problem is that ivcanon turns the test into i != 100 and the pass
> explicitly gives up on any loops ending with != test. It needs to know
> the directoin of the induction variable in order to derive right conditions,
> but that can be done also from step.
>
> It turns out that there are no testcases for basic loop splitting. I will add
> some with the profile update fix.
Thanks for doing this.
Here are bug reports you should look at after all of your loop
splitting patches are done:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=37239
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77689
Thanks,
Andrew
>
> There are other issues, like VRP will turn i < 99 into i == 99 based on
> value range which also makes the pass to give up.
>
> Bootstrapped/regtested x86_64-linux, OK?
>
> gcc/ChangeLog:
>
> * tree-ssa-loop-split.cc (split_loop): Also support NE driven
> loops when IV test is not overflowing.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/tree-ssa/ifc-12.c: Disable loop splitting.
> * gcc.target/i386/avx2-gather-6.c: Likewise.
> * gcc.target/i386/avx2-vect-aggressive.c: Likewise.
>
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ifc-12.c
> b/gcc/testsuite/gcc.dg/tree-ssa/ifc-12.c
> index 9468c070489..bedf29c7dbc 100644
> --- a/gcc/testsuite/gcc.dg/tree-ssa/ifc-12.c
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ifc-12.c
> @@ -1,5 +1,5 @@
> /* { dg-do compile } */
> -/* { dg-options "-Ofast -fdump-tree-ifcvt-stats-blocks-details" } */
> +/* { dg-options "-Ofast -fdump-tree-ifcvt-stats-blocks-details
> -fno-split-loops" } */
> /* { dg-require-visibility "" } */
>
> struct st
> diff --git a/gcc/testsuite/gcc.target/i386/avx2-gather-6.c
> b/gcc/testsuite/gcc.target/i386/avx2-gather-6.c
> index b9119581ae2..47a95dbe989 100644
> --- a/gcc/testsuite/gcc.target/i386/avx2-gather-6.c
> +++ b/gcc/testsuite/gcc.target/i386/avx2-gather-6.c
> @@ -1,5 +1,5 @@
> /* { dg-do compile } */
> -/* { dg-options "-O3 -mavx2 -fno-common -fdump-tree-vect-details
> -mtune=skylake" } */
> +/* { dg-options "-O3 -mavx2 -fno-common -fdump-tree-vect-details
> -mtune=skylake -fno-split-loops" } */
>
> #include "avx2-gather-5.c"
>
> diff --git a/gcc/testsuite/gcc.target/i386/avx2-vect-aggressive.c
> b/gcc/testsuite/gcc.target/i386/avx2-vect-aggressive.c
> index 57192791857..fa336e70e84 100644
> --- a/gcc/testsuite/gcc.target/i386/avx2-vect-aggressive.c
> +++ b/gcc/testsuite/gcc.target/i386/avx2-vect-aggressive.c
> @@ -1,6 +1,6 @@
> /* { dg-do run } */
> /* { dg-require-effective-target avx2 } */
> -/* { dg-options "-mavx2 -O3 -fopenmp-simd -fdump-tree-vect-details
> -fdisable-tree-thread1" } */
> +/* { dg-options "-mavx2 -O3 -fopenmp-simd -fdump-tree-vect-details
> -fdisable-tree-thread1 -fno-split-loops" } */
>
> #include "avx2-check.h"
> #define N 64
> diff --git a/gcc/tree-ssa-loop-split.cc b/gcc/tree-ssa-loop-split.cc
> index b41b5e614c2..27780370d85 100644
> --- a/gcc/tree-ssa-loop-split.cc
> +++ b/gcc/tree-ssa-loop-split.cc
> @@ -540,10 +545,17 @@ split_loop (class loop *loop1)
> || !empty_block_p (loop1->latch)
> || !easy_exit_values (loop1)
> || !number_of_iterations_exit (loop1, exit1, &niter, false, true)
> - || niter.cmp == ERROR_MARK
> - /* We can't yet handle loops controlled by a != predicate. */
> - || niter.cmp == NE_EXPR)
> + || niter.cmp == ERROR_MARK)
> return false;
> + if (niter.cmp == NE_EXPR)
> + {
> + if (!niter.control.no_overflow)
> + return false;
> + if (tree_int_cst_sign_bit (niter.control.step) > 0)
> + niter.cmp = GT_EXPR;
> + else
> + niter.cmp = LT_EXPR;
> + }
>
> bbs = get_loop_body (loop1);
>
