Dataflow analysis in the compiler to avoid runtime NULL checks #93143

sweeneyde · 2022-05-23T18:32:38Z

LOAD_FAST accounts for 14.6% of all bytecodes executed. Including superinstructions brings this up to 14.6+4.7+4.6+2.4+0.9 = 27.1%

cpython/Python/ceval.c

Lines 1864 to 1872 in 202ed25

    
           TARGET(LOAD_FAST) { 
        
               PyObject *value = GETLOCAL(oparg); 
        
               if (value == NULL) { 
        
                   goto unbound_local_error; 
        
               } 
        
               Py_INCREF(value); 
        
               PUSH(value); 
        
               DISPATCH(); 
        
           }

We can turn this NULL-check into an assertion in many cases, where we can determine at compile time that the local variable is already initialized. Preliminary tests show that almost all LOAD_FAST instructions can be statically analyzed to be loading already-initialized variables.

The one twist is handling del frame.f_locals["x"] or frame.f_lineno = 17, where previously-safe loads could become unsafe. For now, we can just replace all the LOAD_FAST (no null check) with LOAD_FAST_CHECK in that particular code object.

See also faster-cpython/ideas#306

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…iler (GH-93144)

sweeneyde · 2022-06-01T08:18:06Z

TODO: documentation

sweeneyde added type-feature performance interpreter-core labels May 23, 2022

sweeneyde mentioned this issue May 23, 2022

gh-93143: Avoid NULL check in LOAD_FAST based on analysis in the compiler #93144

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sweeneyde added a commit that referenced this issue May 31, 2022

gh-93143: Avoid NULL check in LOAD_FAST based on analysis in the comp…

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…iler (GH-93144)

python / cpython Public

Dataflow analysis in the compiler to avoid runtime NULL checks #93143

Dataflow analysis in the compiler to avoid runtime NULL checks #93143

sweeneyde commented May 23, 2022

sweeneyde commented Jun 1, 2022

python / cpython Public

Dataflow analysis in the compiler to avoid runtime NULL checks #93143

Dataflow analysis in the compiler to avoid runtime NULL checks #93143

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sweeneyde commented May 23, 2022

sweeneyde commented Jun 1, 2022