Hi!

On 2022-10-11T10:31:37+0200, Aldy Hernandez via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> When solving 0 = _15 & 1, we calculate _15 as:
>
>       [irange] int [-INF, -2][0, +INF] NONZERO 0xfffffffe
>
> The known value of _15 is [0, 1] NONZERO 0x1 which is intersected with
> the above, yielding:
>
>       [0, 1] NONZERO 0x0
>
> This eventually gets copied to a _Bool [0, 1] NONZERO 0x0.
>
> This is problematic because here we have a bool which is zero, but
> returns false for irange::zero_p, since the latter does not look at
> nonzero bits.  This causes logical_combine to assume the range is
> not-zero, and all hell breaks loose.
>
> I think we should just normalize a nonzero mask of 0 to [0, 0] at
> creation, thus avoiding all this.

1. This commit r13-3217-gc4d15dddf6b9eacb36f535807ad2ee364af46e04
"[PR107195] Set range to zero when nonzero mask is 0" broke a GCC/nvptx
offloading test case:

    UNSUPPORTED: libgomp.oacc-c/../libgomp.oacc-c-c++-common/nvptx-sese-1.c -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none  -O0
    PASS: libgomp.oacc-c/../libgomp.oacc-c-c++-common/nvptx-sese-1.c -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none  -O2  (test for excess errors)
    PASS: libgomp.oacc-c/../libgomp.oacc-c-c++-common/nvptx-sese-1.c -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none  -O2  execution test
    [-PASS:-]{+FAIL:+} libgomp.oacc-c/../libgomp.oacc-c-c++-common/nvptx-sese-1.c -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none  -O2   scan-nvptx-none-offload-rtl-dump mach "SESE regions:.* [0-9]+{[0-9]+->[0-9]+(\\.[0-9]+)+}"

Same for C++.

I'll later send a patch (for the test case!) to fix that up.

2. Looking into this, I found that this
commit r13-3217-gc4d15dddf6b9eacb36f535807ad2ee364af46e04
"[PR107195] Set range to zero when nonzero mask is 0" actually enables a
code transformation/optimization that GCC apparently has not been doing
before!  I've tried to capture that in the attached
"Add 'c-c++-common/torture/pr107195-1.c' [PR107195]".

Will you please verify that one?  In its current '#if 1' configuration,
it's all-PASS after commit
r13-3217-gc4d15dddf6b9eacb36f535807ad2ee364af46e04
"[PR107195] Set range to zero when nonzero mask is 0", whereas before, we
get two calls to 'foo', because GCC apparently didnn't understand the
relation (optimization opportunity) between 'r *= 2;' and the subsequent
'if (r & 1)'.

I've left in the other '#if' variants in case you'd like to experiment
with these, but would otherwise clean that up before pushing.

Where does one put such a test case?

Should the file be named 'pr107195' or something else?

Do we scan 'optimized', or an earlier dump?

At '-O1', the actual code transformation is visible already in the 'dom2'
dump:

       <bb 3> [local count: 536870913]:
       gimple_assign <mult_expr, r_7, r_6(D), 2, NULL>
    +  gimple_assign <bit_and_expr, _11, r_7, 1, NULL>
    +  goto <bb 6>; [100.00%]

    -  <bb 4> [local count: 1073741824]:
    -  # gimple_phi <r_4, r_6(D)(2), r_7(3)>
    +  <bb 4> [local count: 536870912]:
    +  # gimple_phi <r_4, r_6(D)(2)>
       gimple_assign <bit_and_expr, _2, r_4, 1, NULL>
       gimple_cond <ne_expr, _2, 0, NULL, NULL>
    -    goto <bb 5>; [50.00%]
    +    goto <bb 5>; [100.00%]
       else
    -    goto <bb 6>; [50.00%]
    +    goto <bb 6>; [0.00%]

       <bb 5> [local count: 536870913]:
       gimple_call <foo, _3, r_4>
       gimple_assign <plus_expr, r_8, _3, r_4, NULL>

       <bb 6> [local count: 1073741824]:
    -  # gimple_phi <r_5, r_4(4), r_8(5)>
    +  # gimple_phi <r_5, r_4(4), r_8(5), r_7(3)>
       gimple_return <r_5>

And, the actual "avoid second call 'foo'" optimization is visiable
starting 'dom3':

       <bb 3> [local count: 536870913]:
       gimple_assign <mult_expr, r_7, r_6(D), 2, NULL>
    +  goto <bb 6>; [100.00%]

    -  <bb 4> [local count: 1073741824]:
    -  # gimple_phi <r_4, r_6(D)(2), r_7(3)>
    -  gimple_assign <bit_and_expr, _2, r_4, 1, NULL>
    +  <bb 4> [local count: 536870912]:
    +  gimple_assign <bit_and_expr, _2, r_6(D), 1, NULL>
       gimple_cond <ne_expr, _2, 0, NULL, NULL>
    -    goto <bb 5>; [50.00%]
    +    goto <bb 5>; [100.00%]
       else
    -    goto <bb 6>; [50.00%]
    +    goto <bb 6>; [0.00%]

       <bb 5> [local count: 536870913]:
    -  gimple_call <foo, _3, r_4>
    -  gimple_assign <plus_expr, r_8, _3, r_4, NULL>
    +  gimple_assign <integer_cst, _3, 0, NULL, NULL>
    +  gimple_assign <ssa_name, r_8, r_6(D), NULL, NULL>

       <bb 6> [local count: 1073741824]:
    -  # gimple_phi <r_5, r_4(4), r_8(5)>
    +  # gimple_phi <r_5, r_6(D)(4), r_6(D)(5), r_7(3)>
       gimple_return <r_5>

..., but I don't know if either of those would be stable/appropriate to
scan instead of 'optimized'?


Grüße
 Thomas

>       PR tree-optimization/107195
>
> gcc/ChangeLog:
>
>       * value-range.cc (irange::set_range_from_nonzero_bits): Set range
>       to [0,0] when nonzero mask is 0.
>
> gcc/testsuite/ChangeLog:
>
>       * gcc.dg/tree-ssa/pr107195-1.c: New test.
>       * gcc.dg/tree-ssa/pr107195-2.c: New test.
> ---
>  gcc/testsuite/gcc.dg/tree-ssa/pr107195-1.c | 15 +++++++++++++++
>  gcc/testsuite/gcc.dg/tree-ssa/pr107195-2.c | 16 ++++++++++++++++
>  gcc/value-range.cc                         |  5 +++++
>  3 files changed, 36 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr107195-1.c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr107195-2.c
>
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr107195-1.c b/gcc/testsuite/gcc.dg/tree-ssa/pr107195-1.c
> new file mode 100644
> index 00000000000..a0c20dbd4b1
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/pr107195-1.c
> @@ -0,0 +1,15 @@
> +// { dg-do run }
> +// { dg-options "-O1 -fno-tree-ccp" }
> +
> +int a, b;
> +int main() {
> +  int c = 0;
> +  if (a)
> +    c = 1;
> +  c = 1 & (a && c) && b;
> +  if (a) {
> +    b = c;
> +    __builtin_abort ();
> +  }
> +  return 0;
> +}
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr107195-2.c b/gcc/testsuite/gcc.dg/tree-ssa/pr107195-2.c
> new file mode 100644
> index 00000000000..d447c78bdd3
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/pr107195-2.c
> @@ -0,0 +1,16 @@
> +// { dg-do run }
> +// { dg-options "-O1" }
> +
> +int a, b;
> +int main() {
> +  int c = 0;
> +  long d;
> +  for (; b < 1; b++) {
> +    (c && d) & 3 || a;
> +    d = c;
> +    c = -1;
> +    if (d)
> +      __builtin_abort();
> +  }
> +  return 0;
> +}
> diff --git a/gcc/value-range.cc b/gcc/value-range.cc
> index a14f9bc4394..e07d2aa9a5b 100644
> --- a/gcc/value-range.cc
> +++ b/gcc/value-range.cc
> @@ -2903,6 +2903,11 @@ irange::set_range_from_nonzero_bits ()
>       }
>        return true;
>      }
> +  else if (popcount == 0)
> +    {
> +      set_zero (type ());
> +      return true;
> +    }
>    return false;
>  }
>
> --
> 2.37.3


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