[llvm-bugs] [Bug 123456] missed optimization to simple vptest when using libstdc++ std::experimental::simd to detect all-zero pattern on x86-64 avx

[LLVM Bugs via llvm-bugs]

Issue	123456
Summary	missed optimization to simple vptest when using libstdc++ std::experimental::simd to detect all-zero pattern on x86-64 avx
Labels	new issue
Assignees
Reporter	ImpleLee

    The following code uses libstdc++ experimental simd, and wants to detect several all-zero patterns that can be easily done with the vptest instructions. All the code is available at https://godbolt.org/z/Kx68E1T6v .

```c++
#include <experimental/simd>
#include <cstdint>
namespace stdx = std::experimental;

template <class T, std::size_t N>
using simd_of = stdx::simd<T, stdx::simd_abi::deduce_t<T, N>>;

using data_t = simd_of<std::int32_t, 4>;

bool simple_ptest(data_t x) {
    return all_of(x == 0);
}

bool ptest_and(data_t a, data_t b) {
    return all_of((a & b) == 0);
}

bool ptest_andn(data_t a, data_t b) {
    return all_of((a & ~b) == 0);
}
```

Equivalent assembly (hand-written):

```asm
simple_ptest:
        vptest  %xmm0, %xmm0
 sete    %al
        ret
ptest_and:
        vptest  %xmm0, %xmm1
 sete    %al
        ret
ptest_andn:
        vptest  %xmm0, %xmm1
 setc    %al
        ret
```

But clang++ generates the following code at `-O3 -march=x86-64-v3`.

```asm
simple_ptest(std::experimental::parallelism_v2::simd<int, std::experimental::parallelism_v2::simd_abi::_VecBuiltin<16>>):
 vpxor   xmm1, xmm1, xmm1
        vpcmpeqd        xmm0, xmm0, xmm1
 vpcmpeqd        xmm1, xmm1, xmm1
        vptest  xmm0, xmm1
        setb al
        ret

ptest_and(std::experimental::parallelism_v2::simd<int, std::experimental::parallelism_v2::simd_abi::_VecBuiltin<16>>, std::experimental::parallelism_v2::simd<int, std::experimental::parallelism_v2::simd_abi::_VecBuiltin<16>>):
 vpand   xmm0, xmm1, xmm0
        vpxor   xmm1, xmm1, xmm1
        vpcmpeqd xmm0, xmm0, xmm1
        vpcmpeqd        xmm1, xmm1, xmm1
 vptest  xmm0, xmm1
        setb    al
 ret

ptest_andn(std::experimental::parallelism_v2::simd<int, std::experimental::parallelism_v2::simd_abi::_VecBuiltin<16>>, std::experimental::parallelism_v2::simd<int, std::experimental::parallelism_v2::simd_abi::_VecBuiltin<16>>):
 vpandn  xmm0, xmm1, xmm0
        vpxor   xmm1, xmm1, xmm1
        vpcmpeqd xmm0, xmm0, xmm1
        vpcmpeqd        xmm1, xmm1, xmm1
 vptest  xmm0, xmm1
        setb    al
        ret
```

reference: [the same issue at gcc bugzilla](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118416) (identified as a duplicate to another missed optimization).

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