[Piglit] [PATCH] glsl-1.30: Reproduce a bug in the i965 backend optimizer

[Ilia Mirkin]

On Wed, Sep 12, 2018 at 7:59 PM, Ian Romanick <idr at freedesktop.org> wrote:
> On 09/12/2018 04:33 PM, Ilia Mirkin wrote:
>> On Wed, Sep 12, 2018 at 7:29 PM, Ian Romanick <idr at freedesktop.org> wrote:
>>> From: Ian Romanick <ian.d.romanick at intel.com>
>>>
>>> The optimizer recently added the ability to replace a compare with a
>>> subtraction under certain circumstances.  This can fail for integers.
>>> For inputs a = 0x80000000, b = 4, int(0x80000000) < 4, but
>>> int(0x80000000) - 4 overflows and results in 0x7ffffffc.  That's not
>>> less than zero, so the flags get set differently than for (a < b).
>>>
>>> This really only affected the signed comparisons because the subtract
>>> would always have a signed source types, so it wouldn't be seen as a
>>> match for the compare with unsigned source types.
>>>
>>> Signed-off-by: Ian Romanick <ian.d.romanick at intel.com>
>>> Cc: Matt Turner <mattst88 at gmail.com>
>>> ---
>>>  tests/spec/CMakeLists.txt                          |  1 +
>>>  .../fs-absoluteDifference-int.shader_test          | 85 +++++++++++++++++++
>>>  .../fs-absoluteDifference-uint.shader_test         | 85 +++++++++++++++++++
>>>  .../vs-absoluteDifference-int.shader_test          | 96 ++++++++++++++++++++++
>>>  .../vs-absoluteDifference-uint.shader_test         | 96 ++++++++++++++++++++++
>>>  5 files changed, 363 insertions(+)
>>>  create mode 100644 tests/spec/glsl-1.30/execution/fs-absoluteDifference-int.shader_test
>>>  create mode 100644 tests/spec/glsl-1.30/execution/fs-absoluteDifference-uint.shader_test
>>>  create mode 100644 tests/spec/glsl-1.30/execution/vs-absoluteDifference-int.shader_test
>>>  create mode 100644 tests/spec/glsl-1.30/execution/vs-absoluteDifference-uint.shader_test
>>>
>>> diff --git a/tests/spec/CMakeLists.txt b/tests/spec/CMakeLists.txt
>>> index 4df9d331d..28abf3634 100644
>>> --- a/tests/spec/CMakeLists.txt
>>> +++ b/tests/spec/CMakeLists.txt
>>> @@ -2,6 +2,7 @@ add_subdirectory (amd_framebuffer_multisample_advanced)
>>>  add_subdirectory (amd_depth_clamp_separate)
>>>  add_subdirectory (amd_performance_monitor)
>>>  add_subdirectory (amd_pinned_memory)
>>> +add_subdirectory (amd_transform_feedback3_lines_triangles)
>>>  add_subdirectory (arb_arrays_of_arrays)
>>>  add_subdirectory (arb_base_instance)
>>>  add_subdirectory (arb_bindless_texture)
>>> diff --git a/tests/spec/glsl-1.30/execution/fs-absoluteDifference-int.shader_test b/tests/spec/glsl-1.30/execution/fs-absoluteDifference-int.shader_test
>>> new file mode 100644
>>> index 000000000..cdac53cdf
>>> --- /dev/null
>>> +++ b/tests/spec/glsl-1.30/execution/fs-absoluteDifference-int.shader_test
>>> @@ -0,0 +1,85 @@
>>> +[require]
>>> +GL >= 3.0
>>> +GLSL >= 1.30
>>> +
>>> +[vertex shader passthrough]
>>> +
>>> +[fragment shader]
>>> +#extension GL_EXT_shader_integer_mix: enable
>>> +
>>> +// { A, B, absoluteDifference(A, B) }
>>> +uniform ivec3 data[40];
>>> +
>>> +out vec4 color;
>>> +
>>> +uint abs_diff(int a, int b)
>>> +{
>>> +    /* This can fail if the compiler replaces the (a < b) with the result of
>>> +     * one of the subtractions.  For inputs a = 0x80000000, b = 4,
>>> +     * int(0x80000000) < 4, but int(0x80000000)-4 overflows and results in
>>> +     * 0x7ffffffc.  That's not less than zero, so the flags get set
>>> +     * differently than for (a < b).
>>> +     */
>>> +#ifndef GL_EXT_shader_integer_mix
>>> +    return (a < b) ? uint(b - a) : uint(a - b);
>>> +#else
>>> +    return mix(uint(a - b), uint(b - a), a < b);
>>> +#endif
>>
>> Why bother with this? It'll end up testing different code depending on
>> the extensions that the driver supports, which seems like a really bad
>> idea.
>
> We only hit the bug in the mix() case because how how we lower flow
> control to bcsel.  I thought about having it require
> GL_EXT_shader_integer_mix.  That means that when a driver enabled that
> extension the test could transition SKIP->FAIL which seems more likely
> to be ignored than PASS->FAIL.  I guess now there's a possible
> FAIL->PASS transition.  Meh.
>
> I'm not married to it being either way.  I just want to reproduce the
> bug that I added to the i965 driver a few months ago. :)

Hmm. Meh indeed. I dunno though -- skip -> fail when enabling an ext
seems pretty bad to me. I'd just require EXT_shader_integer_mix. In
practice, it's enabled everywhere GL 3.0 is (in mesa), so it's all a
bit academic.

  -ilia