[Bug target/105354] New: __builtin_shuffle for alignr generates suboptimal code unless SSSE3 is enabled

["john_platts at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105354

            Bug ID: 105354
           Summary: __builtin_shuffle for alignr generates suboptimal code
                    unless SSSE3 is enabled
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: john_platts at hotmail dot com
  Target Milestone: ---

The below code generates suboptimal code if SSE2 is enabled but SSSE3 is not
enabled:
#include <cstdint>

typedef std::uint8_t Simd128U8VectT __attribute__((__vector_size__(16)));

template<int RotateAmt>
static inline Simd128U8VectT RotateRightByByteAmt(Simd128U8VectT vect) noexcept
{
    constexpr int NormalizedRotateAmt = RotateAmt & 15;

    if constexpr(NormalizedRotateAmt == 0)
        return vect;
    else
        return __builtin_shuffle(vect, vect, (Simd128U8VectT){
            NormalizedRotateAmt, NormalizedRotateAmt + 1,
            NormalizedRotateAmt + 2, NormalizedRotateAmt + 3,
            NormalizedRotateAmt + 4, NormalizedRotateAmt + 5,
            NormalizedRotateAmt + 6, NormalizedRotateAmt + 7,
            NormalizedRotateAmt + 8, NormalizedRotateAmt + 9,
            NormalizedRotateAmt + 10, NormalizedRotateAmt + 11,
            NormalizedRotateAmt + 12, NormalizedRotateAmt + 13,
            NormalizedRotateAmt + 14, NormalizedRotateAmt + 15 });
}

auto func1(Simd128U8VectT vect) noexcept {
    return RotateRightByByteAmt<5>(vect);
}

Here is the code that is generated on GCC 11 if the -O2 -mssse3 options are
specified:
func1(unsigned char __vector(16)):
        palignr xmm0, xmm0, 5
        ret

Here is the code that is generated on GCC 11 if the -O2 option is specified but
the -mssse3 option is not specified on 64-bit x86 platforms:
func1(unsigned char __vector(16)):
        sub     rsp, 144
        movd    ecx, xmm0
        movaps  XMMWORD PTR [rsp+8], xmm0
        movzx   edx, BYTE PTR [rsp+20]
        movzx   ecx, cl
        movaps  XMMWORD PTR [rsp+24], xmm0
        movzx   eax, BYTE PTR [rsp+35]
        sal     rdx, 8
        movaps  XMMWORD PTR [rsp+40], xmm0
        or      rdx, rax
        movzx   eax, BYTE PTR [rsp+50]
        movaps  XMMWORD PTR [rsp+56], xmm0
        sal     rdx, 8
        movaps  XMMWORD PTR [rsp+72], xmm0
        or      rdx, rax
        movzx   eax, BYTE PTR [rsp+65]
        movaps  XMMWORD PTR [rsp+88], xmm0
        sal     rdx, 8
        movaps  XMMWORD PTR [rsp+104], xmm0
        or      rdx, rax
        movzx   eax, BYTE PTR [rsp+80]
        movaps  XMMWORD PTR [rsp-104], xmm0
        sal     rdx, 8
        movaps  XMMWORD PTR [rsp-88], xmm0
        movzx   edi, BYTE PTR [rsp-85]
        or      rdx, rax
        movzx   eax, BYTE PTR [rsp+95]
        movaps  XMMWORD PTR [rsp-72], xmm0
        sal     rdx, 8
        movaps  XMMWORD PTR [rsp-56], xmm0
        or      rdx, rax
        movzx   eax, BYTE PTR [rsp+110]
        movaps  XMMWORD PTR [rsp-40], xmm0
        sal     rdx, 8
        movaps  XMMWORD PTR [rsp-24], xmm0
        or      rdx, rax
        movzx   eax, BYTE PTR [rsp-100]
        movaps  XMMWORD PTR [rsp+120], xmm0
        movzx   esi, BYTE PTR [rsp+125]
        movaps  XMMWORD PTR [rsp-8], xmm0
        sal     rdx, 8
        sal     rax, 8
        or      rdx, rsi
        or      rax, rdi
        movzx   edi, BYTE PTR [rsp-70]
        sal     rax, 8
        or      rax, rdi
        movzx   edi, BYTE PTR [rsp-55]
        sal     rax, 8
        or      rax, rdi
        sal     rax, 8
        or      rax, rcx
        movzx   ecx, BYTE PTR [rsp-25]
        sal     rax, 8
        or      rax, rcx
        movzx   ecx, BYTE PTR [rsp-10]
        sal     rax, 8
        or      rax, rcx
        movzx   ecx, BYTE PTR [rsp+5]
        mov     QWORD PTR [rsp-120], rdx
        sal     rax, 8
        or      rax, rcx
        mov     QWORD PTR [rsp-112], rax
        movdqa  xmm0, XMMWORD PTR [rsp-120]
        add     rsp, 144
        ret

Here is a more optimal implementation of the above code on 64-bit x86 platforms
when SSE2 is enabled but SSSE3 is not enabled:
func1(unsigned char __vector(16)):
        movdqa  xmm1, xmm0
        psrldq  xmm1, 5
        pslldq  xmm0, 11
        por     xmm0, xmm1
        ret