Why can't Clang get __m128's data by index in constexpr function

Regardless of constexpr, a[pos] is only valid as a GNU C extension, not portable to MSVC. Storing to an array, or C++20 std::bit_cast to a struct might work. bit_cast is constexpr-compatible, unlike other type-punning methods. Although I'd be worried about how efficiently that would compile across compilers for runtime-variable pos

bit_cast does compile ok with clang, and works in a constexpr function. But compiles inefficiently for GCC.

Correction: clang compiles this, but rejects it if called in a context that requires it to be constant-evaluated. note: constexpr bit_cast involving type '__attribute__((__vector_size__(4 * sizeof(float)))) float const' (vector of 4 'float' values) is not yet supported.

Other failed attempts with current clang in a constexpr context:

• _mm_store_ps - not supported. Nor is *(__m128*)f = a; because it's a reinterpret_cast.
• f[0] = vec[0] etc. initializers: no, even literal constant indexing of a GNU C native vector isn't supported in clang in constexpr.
• union type punning: reading an inactive member not allowed in a constexpr context
• _mm_cvtss_f32(vec) - non-constexpr function unusable, so no chance of using if constexpr for separate shuffles and returns.

Not-working answer, may work at some point in the future but not with clang trunk pre 15.0

#include <cstddef>
#include <immintrin.h>
#include <bit>

// portable, but inefficient with GCC
constexpr float get_data(__m128 a, std::size_t pos) {
    struct foo { float f[4]; } s;
    s = std::bit_cast<foo>(a);
    return s.f[pos];
}

float test_idx2(__m128 a){
    return get_data(a, 2);
}

float test_idxvar(__m128 a, size_t pos){
    return get_data(a, pos);
}

These compile to decent asm on Godbolt, the same you'd get from clang with a[pos]. I used -O3 -march=haswell -std=gnu++20

# clang 14 -O3 -march=haswell -std=gnu++20
# get_data has no asm output; constexpr is like inline in that respect

test_idx2(float __vector(4)):
        vpermilpd       xmm0, xmm0, 1           # xmm0 = xmm0[1,0]
        ret
test_idxvar(float __vector(4), unsigned long):
        vmovups xmmword ptr [rsp - 16], xmm0
        vmovss  xmm0, dword ptr [rsp + 4*rdi - 16] # xmm0 = mem[0],zero,zero,zero
        ret

Store/reload is a sensible strategy for a runtime-variable index, although vmovd / vpermilps would be an option since AVX introduced a variable-control shuffle that uses dword indices. An out-of-range index is UB so the compiler doesn't have any requirement to return any specific data in that case.

Using vpermilpd for the constant index 2 is a waste of code-size vs. vmovhlps xmm0, xmm0, xmm0 or vunpckhpd. It costs a longer VEX prefix and an immediate, so 2 bytes of machine-code size, but otherwise same performance on most CPUs.

Unfortunately GCC doesn't do such a good job

We get a store/reload even for the fixed index of 2, and even worse, reload by bouncing through a GP-integer register. This is a missed optimization, but IDK how quickly it would get fixed if reported. So if you're going to do this, perhaps #ifdef __clang__ or #ifdef __llvm__ for bit_cast, and #ifdef __GNUC__ for a[pos]. (Clang defines __GNUC__ so check for that after special-casing clang.)

# gcc12 -O3 -march=haswell -std=gnu++20
test_idx2(float __vector(4)):
        vmovaps XMMWORD PTR [rsp-24], xmm0
        mov     rax, QWORD PTR [rsp-16]
        vmovd   xmm0, eax              # slow: should have loaded directly from mem
        ret

test_idxvar(float __vector(4), unsigned long):
        vmovdqa XMMWORD PTR [rsp-24], xmm0
        vmovss  xmm0, DWORD PTR [rsp-24+rdi*4]   # this is fine, same as clang
        ret

Interestingly the runtime-variable version didn't have the same anti-optimization for GCC.