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From: "john_platts at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/105353] New: __builtin_shufflevector with template
 parameter fails to compile on GCC 12 but compiles on clang
Date: Fri, 22 Apr 2022 21:07:58 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105353

            Bug ID: 105353
           Summary: __builtin_shufflevector with template parameter fails
                    to compile on GCC 12 but compiles on clang
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: john_platts at hotmail dot com
  Target Milestone: ---

The following code fails to compile with GCC 12 but compiles successfully on
clang (with the -std=3Dc++17 flag):
#include <cstdint>

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

template<int ShuffleIndex>
static inline Simd128U8VectT ShufFunc(Simd128U8VectT vect) noexcept {
    if constexpr(unsigned(ShuffleIndex) >=3D 16)
        return Simd128U8VectT { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0=
, 0
};
    else if constexpr(ShuffleIndex =3D=3D 0)
        return vect;
    else
        return __builtin_shufflevector(vect, vect, ShuffleIndex, ShuffleInd=
ex +
1,
            ShuffleIndex + 2, ShuffleIndex + 3, ShuffleIndex + 4, ShuffleIn=
dex
+ 5,
            ShuffleIndex + 6, ShuffleIndex + 7, ShuffleIndex + 8, ShuffleIn=
dex
+ 9,
            ShuffleIndex + 10, ShuffleIndex + 11, ShuffleIndex + 12,
ShuffleIndex + 13,
            ShuffleIndex + 14, ShuffleIndex + 15);
}

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

Here is the assembly output when the above code is compiled on clang 14.0.0
with the -O2 -std=3Dc++17 flags:
func1(unsigned char __vector(16)):                         # @func1(unsigned
char __vector(16))
        movdqa  xmm1, xmm0
        psrldq  xmm1, 5                         # xmm1 =3D
xmm1[5,6,7,8,9,10,11,12,13,14,15],zero,zero,zero,zero,zero
        pslldq  xmm0, 11                        # xmm0 =3D
zero,zero,zero,zero,zero,zero,zero,zero,zero,zero,zero,xmm0[0,1,2,3,4]
        por     xmm0, xmm1
        ret

The below code compiles successfully with GCC 11 and GCC 12 (which uses
__builtin_shuffle instead of __builtin_shufflevector):
#include <cstdint>

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

template<int ShuffleIndex>
static inline Simd128U8VectT ShufFunc(Simd128U8VectT vect) noexcept {
    if constexpr(unsigned(ShuffleIndex) >=3D 16)
        return Simd128U8VectT { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0=
, 0
};
    else if constexpr(ShuffleIndex =3D=3D 0)
        return vect;
    else
        return __builtin_shuffle(vect, vect, (Simd128U8VectT){
            ShuffleIndex, ShuffleIndex + 1,
            ShuffleIndex + 2, ShuffleIndex + 3, ShuffleIndex + 4, ShuffleIn=
dex
+ 5,
            ShuffleIndex + 6, ShuffleIndex + 7, ShuffleIndex + 8, ShuffleIn=
dex
+ 9,
            ShuffleIndex + 10, ShuffleIndex + 11, ShuffleIndex + 12,
ShuffleIndex + 13,
            ShuffleIndex + 14, ShuffleIndex + 15 });
}

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