Unity Shader - if 和 keyword 的指令比较

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环境

Unity : 2020.3.37f1
Pipeline : BRP

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TestingIf4Sample

下面是 shaderlab 和 arm mobile studio 中的 graphics analyzer 的 glsl 代码

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unity shaderlab 中的 TestingIf4Sample.shader

// Unity built-in shader source. Copyright (c) 2016 Unity Technologies. MIT license (see license.txt)

Shader "TestingIf4Sample"
{
    Properties
    {
        _A_Tex ("A Texture", 2D) = "white" {}
        _B_Tex ("B Texture", 2D) = "white" {}
        _USE_TEX_A ("Using Tex A", Float) = 1.0
    }
    SubShader
    {
        Tags
        {
            "Queue"="Geometry"
            "RenderType"="Opaque"
        }
        Pass
        {
            Name "Default"
        CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma target 2.0
            #include "UnityCG.cginc"
            struct appdata_t
            {
                float4 vertex   : POSITION;
                float2 texcoord : TEXCOORD0;
            };
            struct v2f
            {
                float4 vertex   : SV_POSITION;
                float2 texcoord  : TEXCOORD0;
            };
            sampler2D _A_Tex;
            sampler2D _B_Tex;
            fixed _USE_TEX_A;
            v2f vert(appdata_t v)
            {
                v2f OUT;
                float4 vPosition = UnityObjectToClipPos(v.vertex);
                OUT.vertex = vPosition;
                OUT.texcoord = v.texcoord.xy;
                return OUT;
            }
            fixed4 frag(v2f IN) : SV_Target
            {
                return _USE_TEX_A == 1.0 ? tex2D(_A_Tex, IN.texcoord) : tex2D(_B_Tex, IN.texcoord);
            }
        ENDCG
        }
    }
}

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graphics analyzer 中的 TestingIf4Sample.glsl

VS

#version 320 es

precision highp float;
precision highp int;
#define HLSLCC_ENABLE_UNIFORM_BUFFERS 1
#if HLSLCC_ENABLE_UNIFORM_BUFFERS
#define UNITY_UNIFORM
#else
#define UNITY_UNIFORM uniform
#endif
#define UNITY_SUPPORTS_UNIFORM_LOCATION 1
#if UNITY_SUPPORTS_UNIFORM_LOCATION
#define UNITY_LOCATION(x) layout(location = x)
#define UNITY_BINDING(x) layout(binding = x, std140)
#else
#define UNITY_LOCATION(x)
#define UNITY_BINDING(x) layout(std140)
#endif
uniform 	mediump float _USE_TEX_A;
UNITY_LOCATION(0) uniform mediump sampler2D _A_Tex;
UNITY_LOCATION(1) uniform mediump sampler2D _B_Tex;
in highp vec2 vs_TEXCOORD0;
layout(location = 0) out mediump vec4 SV_Target0;
vec4 u_xlat0;
bool u_xlatb0;
vec4 u_xlat1;
vec4 u_xlat2;
void main()
{
#ifdef UNITY_ADRENO_ES3
    u_xlatb0 = !!(_USE_TEX_A==1.0);
#else
    u_xlatb0 = _USE_TEX_A==1.0;
#endif
    u_xlat1 = texture(_A_Tex, vs_TEXCOORD0.xy);
    u_xlat2 = texture(_B_Tex, vs_TEXCOORD0.xy);
    u_xlat0 = (bool(u_xlatb0)) ? u_xlat1 : u_xlat2;
    SV_Target0 = u_xlat0;
    return;
}

FS

#version 320 es

precision highp float;
precision highp int;
#define HLSLCC_ENABLE_UNIFORM_BUFFERS 1
#if HLSLCC_ENABLE_UNIFORM_BUFFERS
#define UNITY_UNIFORM
#else
#define UNITY_UNIFORM uniform
#endif
#define UNITY_SUPPORTS_UNIFORM_LOCATION 1
#if UNITY_SUPPORTS_UNIFORM_LOCATION
#define UNITY_LOCATION(x) layout(location = x)
#define UNITY_BINDING(x) layout(binding = x, std140)
#else
#define UNITY_LOCATION(x)
#define UNITY_BINDING(x) layout(std140)
#endif
uniform 	mediump float _USE_TEX_A;
UNITY_LOCATION(0) uniform mediump sampler2D _A_Tex;
UNITY_LOCATION(1) uniform mediump sampler2D _B_Tex;
in highp vec2 vs_TEXCOORD0;
layout(location = 0) out mediump vec4 SV_Target0;
vec4 u_xlat0;
bool u_xlatb0;
vec4 u_xlat1;
vec4 u_xlat2;
void main()
{
#ifdef UNITY_ADRENO_ES3
    u_xlatb0 = !!(_USE_TEX_A==1.0);
#else
    u_xlatb0 = _USE_TEX_A==1.0;
#endif
    u_xlat1 = texture(_A_Tex, vs_TEXCOORD0.xy);
    u_xlat2 = texture(_B_Tex, vs_TEXCOORD0.xy);
    u_xlat0 = (bool(u_xlatb0)) ? u_xlat1 : u_xlat2;
    SV_Target0 = u_xlat0;
    return;
}

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TestingKW4Sample

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下面是 shaderlab 和 glsl

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unity shaderlab 中的 TestingKW4Sample.shader

// Unity built-in shader source. Copyright (c) 2016 Unity Technologies. MIT license (see license.txt)

Shader "TestingKW4Sample"
{
    Properties
    {
        _A_Tex ("A Texture", 2D) = "white" {}
        _B_Tex ("B Texture", 2D) = "white" {}
        [Toggle(_USE_TEX_A)]_USE_TEX_A ("Using Tex A", Float) = 1.0
    }
    SubShader
    {
        Tags
        {
            "Queue"="Geometry"
            "RenderType"="Opaque"
        }
        Pass
        {
            Name "Default"
        CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma target 2.0
            #pragma multi_compile _ _USE_TEX_A
            #include "UnityCG.cginc"
            struct appdata_t
            {
                float4 vertex   : POSITION;
                float2 texcoord : TEXCOORD0;
            };
            struct v2f
            {
                float4 vertex   : SV_POSITION;
                float2 texcoord  : TEXCOORD0;
            };
            sampler2D _A_Tex;
            sampler2D _B_Tex;
            v2f vert(appdata_t v)
            {
                v2f OUT;
                float4 vPosition = UnityObjectToClipPos(v.vertex);
                OUT.vertex = vPosition;
                OUT.texcoord = v.texcoord.xy;
                return OUT;
            }
            fixed4 frag(v2f IN) : SV_Target
            {
                #ifdef _USE_TEX_A
                    return tex2D(_A_Tex, IN.texcoord);
                #else
                    return tex2D(_B_Tex, IN.texcoord);
                #endif
            }
        ENDCG
        }
    }
}

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graphics analyzer 中的 TestingKW4Sample.glsl

VS： 和上面的一样，就不复制上来的
主要看 FS：

#version 320 es

precision highp float;
precision highp int;
#define UNITY_SUPPORTS_UNIFORM_LOCATION 1
#if UNITY_SUPPORTS_UNIFORM_LOCATION
#define UNITY_LOCATION(x) layout(location = x)
#define UNITY_BINDING(x) layout(binding = x, std140)
#else
#define UNITY_LOCATION(x)
#define UNITY_BINDING(x) layout(std140)
#endif
UNITY_LOCATION(0) uniform mediump sampler2D _A_Tex;
in highp vec2 vs_TEXCOORD0;
layout(location = 0) out mediump vec4 SV_Target0;
mediump vec4 u_xlat16_0;
void main()
{
    u_xlat16_0 = texture(_A_Tex, vs_TEXCOORD0.xy);
    SV_Target0 = u_xlat16_0;
    return;
}

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比较

if 是指令实打实存在的
kw 原理是 macro 的 define，是在预编辑级别的分支，因此指令少一些也很正常

if 的推荐使用 记得是 if … else … 中 cycles 在 < 4 cycles 的，可以使用 if
否则还是使用 kw 的写法
下面的示例是 texture 的 sample，也就是 T unit 的采样

我印象中记得： （到时我再补回来，或是看到的大佬可以指导一下、分享一下资料）

• register op: 1 cycles
• sfu : 4~6 （忘记了）
• texture sample : 300+ cyles

因此 T unit 优化是非常有必要的