Technique for speculatively generating an output value in anticipation of its use by downstream processing circuitry

A non-transitory computer-readable medium to store computer-readable code for fabrication of an apparatus comprising:

• a filter circuit comprising storage circuitry to store information indicative of a default number of bits of program counter values associated with selected instructions, the filter circuit configured, in response to a current program counter value, to:
• perform a lookup operation in the storage circuitry based on a chosen number of bits of the current program counter value;
• and
• in response to a hit in the storage circuitry, assert a trigger signal;
• a processing unit including processing circuitry and associated with a given processing stage of a plurality of sequential processing stages of a processing pipeline configured to execute processing instructions, the processing unit configured, in response to the trigger signal, to perform an operation in order to speculatively generate an output value in anticipation of that output value being of use to downstream processing circuitry associated with one or more downstream processing stages later in the plurality of sequential processing stages than the given processing stage;
• and
• resolution circuitry associated with a downstream processing stage of the plurality of sequential processing stages, to determine whether the output value is of use to the downstream processing circuitry, and in that event to assert a false miss indication to the filter circuit in the absence of the processing unit having been triggered to produce the output value,
• wherein the filter circuit is configured to maintain a trigger sensitivity metric that is updated in dependence on the false miss indication, and the chosen number of bits employed when performing the lookup operation is dependent on the trigger sensitivity metric.

A method of operating a data processing apparatus comprising a filter circuit, the filter circuit comprising storage circuitry, the method comprising:

• storing, in the storage circuitry, information indicative of a default number of bits of program counter values associated with selected instructions;
• in response to a current program counter value:
• performing, with the filter circuit, a lookup operation in the storage circuitry based on a chosen number of bits of the current program counter value;
• and
• in response to a hit in the storage circuitry, asserting, with the filter circuit, a trigger signal;
• performing, at a given processing stage of a plurality of processing stages of a processing pipeline configured to execute processing instructions, in response to the trigger signal, an operation in order to speculatively generate an output value in anticipation of that output value being of use to downstream processing circuitry associated with one or more downstream processing stages later in the plurality of sequential processing stages than the given processing stage;
• determining, at a downstream processing stage of the plurality of sequential processing stages, whether the output value is of use to the downstream processing circuitry, and in that event asserting a false miss indication to the filter circuit in the absence of the trigger signal being asserted to produce the output value;
• and
• maintaining, by the filter circuit, a trigger sensitivity metric that is updated in dependence on the false miss indication,
• wherein the chosen number of bits employed when performing the lookup operation is dependent on the trigger sensitivity metric.