MPSGraph 分配变量操作不更新值

Question

我试图了解MPSGraph api。为什么以下代码在第二次运行调用后无法更改var的值？

#import <Foundation/Foundation.h>
#import <MetalPerformanceShadersGraph/MetalPerformanceShadersGraph.h>

void run(void);

void run(void) {
    MPSGraph *graph = [MPSGraph new];

    float test1 = 2.0;
    float test2 = 3.0;
    MPSGraphTensor *constant = [graph constantWithScalar:test1
                                                dataType:MPSDataTypeFloat32];
    MPSGraphTensor *var = [graph variableWithData:[NSData dataWithBytes:&test2 length:sizeof(test2)]
                                            shape:@[@1]
                                         dataType:MPSDataTypeFloat32
                                             name:@"var"];

    MPSGraphTensorDataDictionary *result = [graph runWithFeeds:@{}
                                                 targetTensors:@[constant, var]
                                              targetOperations:NULL];

    float test3;
    NSInteger temp = sizeof(test3);
    [result[var].mpsndarray readBytes:&test3
                          strideBytes:&temp];
    NSLog(@"%f", test3);

    [result[constant].mpsndarray readBytes:&test3
                               strideBytes:&temp];
    NSLog(@"%f", test3);
    
    MPSGraphOperation *op = [graph assignVariable:var
                                withValueOfTensor:constant
                                             name:NULL];
    
    result = [graph runWithFeeds:@{}
                   targetTensors:@[var]
                targetOperations:@[op]];

    [result[var].mpsndarray readBytes:&test3
                          strideBytes:&temp];
    
    NSLog(@"%f", test3);
}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        run();
    }
    return 0;
}

Output：

2022-09-27 13:26:35.708641-0400 MPSGraphExample[9643:2669233] Metal API Validation Enabled
2022-09-27 13:26:35.732058-0400 MPSGraphExample[9643:2669233] 3.000000
2022-09-27 13:26:35.732097-0400 MPSGraphExample[9643:2669233] 2.000000
2022-09-27 13:26:35.733821-0400 MPSGraphExample[9643:2669233] 3.000000
Program ended with exit code: 0

Answer 1

事实证明，该操作是在评估张量之后应用的。 将其更改为

[graph runWithFeeds:@{}
      targetTensors:@[]
   targetOperations:@[op]];

result = [graph runWithFeeds:@{}
               targetTensors:@[var]
            targetOperations:@[]];

现在显示更新的值。