Re: [PATCH 1/3] PHIOPT: Add diamond bb form to factor_out_conditional_conversion

[Richard Biener <richard.guenther@gmail.com>]

On Sun, May 7, 2023 at 7:05 AM Andrew Pinski via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> So the function factor_out_conditional_conversion already supports
> diamond shaped bb forms, just need to be called for such a thing.
>
> harden-cond-comp.c needed to be changed as we would optimize out the
> conversion now and that causes the compare hardening not needing to
> split the block which it was testing. So change it such that there
> would be no chance of optimization.
>
> Also add two testcases that showed the improvement. PR 103771 is
> solved in ifconvert also for the vectorizer but now it is solved
> in a general sense.
>
> OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

So what does it do for the diamond case?  Factor out _common_ operations
only or also promote conditionally executed operations to unconditionally
executed?  Apart from cost considerations (with the looping patch any number
might end up promoted) there's correctness issues for negation (signed overflow)
and for non-call exceptions there's possible external throws.

If it only factors out common operations the patch is OK (but the testcases
suggest that's not the case).  Otherwise we should add a limit as to how many
ops we hoist (maybe one for the diamond case?).  Thinking over it the same
issue of course exists for the non-diamond case?

Richard.

>         PR tree-optimization/49959
>         PR tree-optimization/103771
>
> gcc/ChangeLog:
>
>         * tree-ssa-phiopt.cc (pass_phiopt::execute): Support
>         Diamond shapped bb form for factor_out_conditional_conversion.
>
> gcc/testsuite/ChangeLog:
>
>         * c-c++-common/torture/harden-cond-comp.c: Change testcase
>         slightly to avoid the new phiopt optimization.
>         * gcc.dg/tree-ssa/abs-2.c: New test.
>         * gcc.dg/tree-ssa/pr103771.c: New test.
> ---
>  .../c-c++-common/torture/harden-cond-comp.c   |  6 +++---
>  gcc/testsuite/gcc.dg/tree-ssa/abs-2.c         | 20 +++++++++++++++++++
>  gcc/testsuite/gcc.dg/tree-ssa/pr103771.c      | 18 +++++++++++++++++
>  gcc/tree-ssa-phiopt.cc                        |  2 +-
>  4 files changed, 42 insertions(+), 4 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/abs-2.c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr103771.c
>
> diff --git a/gcc/testsuite/c-c++-common/torture/harden-cond-comp.c b/gcc/testsuite/c-c++-common/torture/harden-cond-comp.c
> index 5aad890a1d3..dcf364ee993 100644
> --- a/gcc/testsuite/c-c++-common/torture/harden-cond-comp.c
> +++ b/gcc/testsuite/c-c++-common/torture/harden-cond-comp.c
> @@ -1,11 +1,11 @@
>  /* { dg-do compile } */
>  /* { dg-options "-fharden-conditional-branches -fharden-compares -fdump-tree-hardcbr -fdump-tree-hardcmp -ffat-lto-objects" } */
>
> -int f(int i, int j) {
> +int f(int i, int j, int k, int l) {
>    if (i == 0)
> -    return j != 0;
> +    return (j != 0) + l;
>    else
> -    return i * j != 0;
> +    return (i * j != 0) * k;
>  }
>
>  /* { dg-final { scan-tree-dump-times "Splitting edge" 2 "hardcbr" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/abs-2.c b/gcc/testsuite/gcc.dg/tree-ssa/abs-2.c
> new file mode 100644
> index 00000000000..328b1802541
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/abs-2.c
> @@ -0,0 +1,20 @@
> +/* PR tree-optimization/49959 */
> +/* { dg-do compile } */
> +/* { dg-options "-O -fdump-tree-phiopt1-details" } */
> +
> +#define ABS(X)    (((X)>0)?(X):-(X))
> +unsigned long
> +test_abs(int *cur)
> +{
> +  unsigned long sad = 0;
> +  if (cur[0] > 0)
> +    sad = cur[0];
> +  else
> +    sad = -cur[0];
> +  return sad;
> +}
> +
> +/* We should figure out that test_abs has an ABS_EXPR in it. */
> +/* { dg-final { scan-tree-dump " = ABS_EXPR" "phiopt1"} } */
> +/* { dg-final { scan-tree-dump-times "changed to factor conversion out from" 1 "phiopt1"} } */
> +
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr103771.c b/gcc/testsuite/gcc.dg/tree-ssa/pr103771.c
> new file mode 100644
> index 00000000000..97c9db846cb
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/pr103771.c
> @@ -0,0 +1,18 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O3 -fdump-tree-phiopt1-details" } */
> +/* { dg-final { scan-tree-dump-times "changed to factor conversion out from COND_EXPR." 1 "phiopt1" } } */
> +
> +typedef unsigned char uint8_t;
> +
> +static uint8_t x264_clip_uint8 (int x)
> +{
> +  return x & (~255) ? (-x) >> 31 : x;
> +}
> +
> +void
> +mc_weight (uint8_t* __restrict dst, uint8_t* __restrict src,
> +          int i_width,int i_scale)
> +{
> +  for(int x = 0; x < i_width; x++)
> +    dst[x] = x264_clip_uint8 (src[x] * i_scale);
> +}
> diff --git a/gcc/tree-ssa-phiopt.cc b/gcc/tree-ssa-phiopt.cc
> index f14b7e8b7e6..41fea78dc8d 100644
> --- a/gcc/tree-ssa-phiopt.cc
> +++ b/gcc/tree-ssa-phiopt.cc
> @@ -4087,7 +4087,7 @@ pass_phiopt::execute (function *)
>
>        gphi *newphi;
>        if (single_pred_p (bb1)
> -         && !diamond_p
> +         && EDGE_COUNT (merge->preds) == 2
>           && (newphi = factor_out_conditional_conversion (e1, e2, phi,
>                                                           arg0, arg1,
>                                                           cond_stmt)))
> --
> 2.31.1
>