skia2/resources/sksl/intrinsics/FaceForward.sksl

uniform half4 N, I, NRef;
uniform half4 colorGreen, colorRed;

half4 main(float2 xy) {

    const half4 constN    = half4(1, 2, 3, 4);
    const half4 constI    = half4(1, 1, -100, 1);
    const half4 constNRef = half4(1);

    half4 expectedPos =  half4(1, 2, 3, 4);
    half4 expectedNeg = -half4(1, 2, 3, 4);

    return (faceforward(N.x,         I.x,         NRef.x        ) == expectedNeg.x    &&
            faceforward(N.xy,        I.xy,        NRef.xy       ) == expectedNeg.xy   &&
            faceforward(N.xyz,       I.xyz,       NRef.xyz      ) == expectedPos.xyz  &&
            faceforward(N.xyzw,      I.xyzw,      NRef.xyzw     ) == expectedPos.xyzw &&
            faceforward(constN.x,    constI.x,    constNRef.x   ) == expectedNeg.x    &&
            faceforward(constN.xy,   constI.xy,   constNRef.xy  ) == expectedNeg.xy   &&
            faceforward(constN.xyz,  constI.xyz,  constNRef.xyz ) == expectedPos.xyz  &&
            faceforward(constN.xyzw, constI.xyzw, constNRef.xyzw) == expectedPos.xyzw) ? colorGreen
                                                                                       : colorRed;
}