Added flow control macros to more platforms. The source initially passes through the hlsl compiler which understands these directives, independent of the platform compiler.

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/API/GLCore.hlsl

 #define CBUFFER_START(name)
 #define CBUFFER_END
 
+// flow control attributes
+#define UNITY_BRANCH        [branch]
+#define UNITY_FLATTEN       [flatten]
+#define UNITY_UNROLL        [unroll]
+#define UNITY_UNROLLX(_x)   [unroll(_x)]
+#define UNITY_LOOP          [loop]
+
 // OpenGL 4.1 SM 5.0 https://docs.unity3d.com/Manual/SL-ShaderCompileTargets.html
 #if (SHADER_TARGET >= 46)
 #define OPENGL4_1_SM5 1

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/API/GLES3.hlsl

 #define CBUFFER_START(name)
 #define CBUFFER_END
 
+// flow control attributes
+#define UNITY_BRANCH        [branch]
+#define UNITY_FLATTEN       [flatten]
+#define UNITY_UNROLL        [unroll]
+#define UNITY_UNROLLX(_x)   [unroll(_x)]
+#define UNITY_LOOP          [loop]
+
 // GLES 3.1 + AEP shader feature https://docs.unity3d.com/Manual/SL-ShaderCompileTargets.html
 #if (SHADER_TARGET >= 40)
 #define GLES3_1_AEP 1

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/API/Metal.hlsl

 #define CBUFFER_START(name) cbuffer name {
 #define CBUFFER_END };
 
+// flow control attributes
+#define UNITY_BRANCH        [branch]
+#define UNITY_FLATTEN       [flatten]
+#define UNITY_UNROLL        [unroll]
+#define UNITY_UNROLLX(_x)   [unroll(_x)]
+#define UNITY_LOOP          [loop]
+
 // Initialize arbitrary structure with zero values.
 // Do not exist on some platform, in this case we need to have a standard name that call a function that will initialize all parameters to 0
 #define ZERO_INITIALIZE(type, name) name = (type)0;

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/API/Vulkan.hlsl

 #define CBUFFER_START(name) cbuffer name {
 #define CBUFFER_END };
 
+// flow control attributes
+#define UNITY_BRANCH        [branch]
+#define UNITY_FLATTEN       [flatten]
+#define UNITY_UNROLL        [unroll]
+#define UNITY_UNROLLX(_x)   [unroll(_x)]
+#define UNITY_LOOP          [loop]
+
 // Initialize arbitrary structure with zero values.
 // Do not exist on some platform, in this case we need to have a standard name that call a function that will initialize all parameters to 0
 #define ZERO_INITIALIZE(type, name) name = (type)0;