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Subject: [Bug target/106322] i386: Wrong code at O2 level (O0 / O1 are working)
Date: Tue, 19 Jul 2022 07:58:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106322

--- Comment #10 from Uro=C5=A1 Bizjak <ubizjak at gmail dot com> ---
(In reply to Mathieu Malaterre from comment #9)

> Technically I can also execute the `uint16` portion of the unit test and
> produce a failure (so this seems to be consistent behavior with signed
> counterpart):
>=20
> ```
> HWY_NOINLINE void TestAllMulHigh() {
>   ForPartialVectors<TestMulHigh> test;
> //  test(int16_t());
>   test(uint16_t());
> }


As this is a runtime failure, you will have to provide a (minimized) runtime
testcase. I took a quick look at the sources and it looks to me that the
following procedure can obtain a testcase:

Use tests/mul_tests.cc and strip out as much lines as possible. Above the p=
art
that you show are several tests. Please find out which test fails.

As can be seen from the test run, the failure is in the 128bit emulation pa=
rt.
These operations are in hwy/ops/emu128-inl.h, specifically:

--cut here--
HWY_API Vec128<uint16_t, N> MulHigh(Vec128<uint16_t, N> a,
                                    const Vec128<uint16_t, N> b) {
  for (size_t i =3D 0; i < N; ++i) {
    // Cast to uint32_t first to prevent overflow. Otherwise the result of
    // uint16_t * uint16_t is in "int" which may overflow. In practice the
    // result is the same but this way it is also defined.
    a.raw[i] =3D static_cast<uint16_t>(
        (static_cast<uint32_t>(a.raw[i]) * static_cast<uint32_t>(b.raw[i]))=
 >>
        16);
  }
  return a;
}
--cut here--

Put everything together in one file, check if it still fails, and you have a
testcase. If it is possible, simplify it as much as possible and if you can
convert it to a plain C, the testcase will be much easier to analyse.

The reason the test fails with gcc-12 is that gcc-12 enabled auto-vectorisa=
tion
for -O2. The failure suggests there are some issues with the vectorisation =
of
the above code, or perhaps with the preparation of test values before the l=
oop.=