gh-152192: Fix JUMP_BACKWARD passing a truncated oparg to the jit tracer

[deadlovelll]

The _JIT op used oparg as the counter in the while loop, truncating it before reaching _PyJit_TryInitializeTracing.
Fixed it by using a temporary value.

For more details see gh-152192

[deadlovelll]

I want to clarify a little bit about this test.

When the jit counter is enabled we enter into _JIT op with original oparg = 299
Then oparg truncates with buggy loop

while (oparg > 255) {
       oparg >>= 8
       insert_exec_at--;
}

oparg becomes 1, and we pass it to _PyJit_TryInitializeTracing

In _PyJit_TryInitializeTracing we store this broken oparg

tracer->prev_state.instr_oparg = oparg;

And call ENTER_TRACING()

In next calls code uses TRACE_RECORD which in turn calls _PyJit_translate_single_bytecode_to_trace. This is the point where shift exists

    // Rewind EXTENDED_ARG so that we see the whole thing.
    // We must point to the first EXTENDED_ARG when deopting.
    int oparg = tracer->prev_state.instr_oparg;
    int opcode = this_instr->op.code;
    int rewind_oparg = oparg;
    while (rewind_oparg > 255) {
        rewind_oparg >>= 8;
        target--;
    }

We get the broken oparg from tracer->prev_state.instr_oparg and the loop does not executes because 1 < 255. target does not decrements and the instruction is not correct (should be EXTENDED_ARG but actually JUMP_BACKWARD)

P.S: I may not be fully correct in the internals but the test catches the shift on the buggy build

[markshannon]

This looks good. Thanks for fixing this.

Can you link to the issue in the test, then I'll merge.

[markshannon]

Can you add a comment that this is testing for #152192

[deadlovelll]

Hello! Thank you for the review. Just pushed the comment note on test

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