使用 Metal 优化 GPU 渲染器

3:45 - Reduce Branch Performance Cost

// Reduce branch performance cost
fragment FragOut frag_material_main(device Material &material [[buffer(0)]]) {
    if(material.is_glossy) {
        material_glossy(material);
    }

    if(material.has_shadows) {
        light_shadows(material);
    }

    if(material.has_reflections) {
        trace_reflections(material);
    }

    if(material.is_volumetric) {
        output_volume_parameters(material);
    }

    return output_material();
}

3:55 - Function constant declaration per material feature

constant bool IsGlossy        [[function_constant(0)]];
constant bool HasShadows      [[function_constant(1)]];
constant bool HasReflections  [[function_constant(2)]];
constant bool IsVolumetric    [[function_constant(3)]];

3:59 - Dynamic branch for the feature codepath is replaced with function constants

if(material.has_reflections) {
  trace_reflections(material);
}

4:05 - Dynamic branch for the feature codepath is replaced with function constants

/* replaced with function constants*/

if(HasReflections) {
	trace_reflections(material);
}

4:13 - Reduce branch performance cost with function constants

constant bool IsGlossy        [[function_constant(0)]];
constant bool HasShadows      [[function_constant(1)]];
constant bool HasReflections  [[function_constant(2)]];
constant bool IsVolumetric    [[function_constant(3)]];

// Reduce branch performance cost
fragment FragOut frag_material_main(device Material &material [[buffer(0)]]) {
    if(IsGlossy) {
        material_glossy(material);
    }

    if(HasShadows) {
        light_shadows(material);
    }

    if(HasReflections) {
        trace_reflections(material);
    }

    if(IsVolumetric) {
        output_volume_parameters(material);
    }

    return output_material();
}

4:58 - Function constants for material parameters

// Function constants for material parameters

constant float4 MaterialColor  [[function_constant(0)]];
constant float4 MaterialWeight [[function_constant(1)]];
constant float4 SheenColor     [[function_constant(2)]];
constant float4 SheenFactor    [[function_constant(3)]];

struct Material {
    float4 blend_factor;
};

void material_glossy(const constant Material& material) {
    float4 light, sheen;
    light = glossy_eval(MaterialColor, MaterialWeight);
    sheen = sheen_eval(SheenColor, SheenFactor);
    glossy_output_write(light, sheen, material.blend_factor);
}

5:21 - MaterialParameter structure for constant parameters

struct MaterialParameter {
    NSString* name;
    MTLDataType type;
    void* value_ptr;
};

MaterialParameter is_glossy{@"IsGlossy",      MTLDataTypeBool,   &material.is_glossy};
MaterialParameter mat_color{@"MaterialColor", MTLDataTypeFloat4, &material.color};

5:51 - Declare and populate MTLFunctionConstantValues

// Declare and populate MTLFunctionConstantValues
MTLFunctionConstantValues* values = [MTLFunctionConstantValues new];
    
for(const MaterialParameter& parameter : shader_parameters) {
    [values setConstantValue: parameter.value_ptr
                        type: parameter.type
                    withName: parameter.name];
}

5:51 - Create pipeline render state object with function constant declarations

struct Material {
  bool is_glossy;
  float color[4];
};

struct MaterialParameter {
    NSString* name;
    MTLDataType type;
    void* value_ptr;
};

// Declare material
Material material = {true, {1.0f,0.0f,0.0f,1.0f}};

// Declare function constant paramters
MaterialParameter is_glossy{@"IsGlossy",      MTLDataTypeBool,   &material.is_glossy};
MaterialParameter mat_color{@"MaterialColor", MTLDataTypeFloat4, &material.color};

MaterialParameter shader_parameters[2] = {is_glossy, mat_color};

// Declare and populate MTLFunctionConstantValues
MTLFunctionConstantValues* values = [MTLFunctionConstantValues new];
    
for(const MaterialParameter& parameter : shader_parameters) {
    [values setConstantValue: parameter.value_ptr
                        type: parameter.type
                    withName: parameter.name];
}

// Create MTLRenderPipelineDescriptor and create shader function from MTLLibrary
MTLRenderPipelineDescriptor *dsc = [MTLRenderPipelineDescriptor new];
NSError* error = nil;

dsc.fragmentFunction = [shader_library newFunctionWithName:@"frag_material_main"
                                            constantValues:values
                                                     error:&error];

// Create pipeline render state object
id<MTLRenderPipelineState> pso = [device newRenderPipelineStateWithDescriptor:dsc
                                                                        error:&error];

6:14 - Create MTLRenderPipelineDescriptor and create shader function from MTLLibrary

// Create MTLRenderPipelineDescriptor and create shader function from MTLLibrary
MTLRenderPipelineDescriptor *dsc = [MTLRenderPipelineDescriptor new];
NSError* error = nil;

dsc.fragmentFunction = [shader_library newFunctionWithName:@"frag_material_main"
                                            constantValues:values
                                                     error:&error];

8:07 - Shader library creation

- (void)newLibraryWithSource:(NSString *)source 
                     options:(MTLCompileOptions *)options
           completionHandler:(MTLNewLibraryCompletionHandler)completionHandler;

8:09 - Render pipeline state creation

- (void)newRenderPipelineStateWithDescriptor:(MTLRenderPipelineDescriptor *)descriptor
              completionHandler:(MTLNewRenderPipelineStateCompletionHandler)completionHandler;

9:00 - Use as many threads as possible for concurrent compilation

@property (atomic) BOOL shouldMaximizeConcurrentCompilation;

10:58 - Assign symbol visibility to default or hidden

__attribute__((visibility(“default")))
             void matrix_mul();

__attribute__((visibility(“hidden")))
             void matrix_mul_internal();

11:19 - Verify device support

//For render pipelines
@property (readonly) BOOL supportsRenderDynamicLibraries;

//For compute pipelines
@property(readonly) BOOL supportsDynamicLibraries;

11:46 - Compile dynamic libraries

//create a dynamic library from an existing Metal library

- (id<MTLDynamicLibrary>) newDynamicLibrary:(id<MTLLibrary>) library 
                                      error:(NSError **) error

//create from the URL
- (id<MTLDynamicLibrary>) newDynamicLibraryWithURL:(NSURL *) url
                                             error:(NSError **) error

12:18 - Dynamically link shaders

//Pipeline state
MTLRenderPipelineDescriptor* dsc = [MTLRenderPipelineDescriptor new];
dsc.vertexPreloadedLibraries   = @[dylib_Math, dylib_Shadows];
dsc.fragmentPreloadedLibraries = @[dylib_Math, dylib_Shadows];

//Compile options
MTLCompileOptions* options = [MTLCompileOptions new];
options.libraries = @[dylib_Math, dylib_Shadows];
[device newLibraryWithSource:programString
                     options:options
                       error:&error];

13:45 - Specify desired max total threads per threadgroup

@interface MTLComputePipelineDescriptor : NSObject
@property (readwrite, nonatomic) NSUInteger maxTotalThreadsPerThreadgroup;

14:12 - Match desired max total threads per threadgroup

@interface MTLCompileOptions : NSObject
@property (readwrite, nonatomic) NSUInteger maxTotalThreadsPerThreadgroup;

14:25 - Tune Metal dynamic libraries

MTLCompileOptions* options = [MTLCompileOptions new];
options.libraryType = MTLLibraryTypeDynamic;
options.installName = @"executable_path/dylib_Math.metallib";
if(@available(macOS 13.3, *)) {
    options.maxTotalThreadsPerThreadgroup = 768;
}

id<MTLLibrary> lib = [device newLibraryWithSource:programString
                                          options:options
                                            error:&error];
       
id<MTLDynamicLibrary> dynamicLib = [device newDynamicLibrary:lib
                                                       error:&error];