ReductionOrInfo Struct Reference

#include <reduction_or.h>

Public Types
using	SaluReductionOrGroup = std::map<cstring, ordered_set<const IR::MAU::StatefulAlu *>>
using	TblReductionOrGroup = std::map<cstring, ordered_set<const IR::MAU::Table *>>

Public Member Functions
void	clear ()
bool	is_reduction_or (const IR::MAU::Instruction *, const IR::MAU::Table *, cstring &red_or_key) const

Public Attributes
SaluReductionOrGroup	salu_reduction_or_group
TblReductionOrGroup	tbl_reduction_or_group

Detailed Description

A reduction or is an optimization where an binary OR operation is implemented by ORing action data on the action data bus, and using the results of this in an ALU operation. The action data bus is a large OXBar, so the sending two action data sources to the same bytes on the action bus results in the ORing of these two sources. This can ORed by up to as many sources are possible per stage.

The standard use case for this is a bloom filter. In order to reduce the dependency chain, multiple stateful ALUs are ORed to the same place in the action data bus. The current way this is delineated is through an extra property on a stateful ALU, "reduction_or_group". Stateful ALUs that are in the same reduction_or_group use this optimization, and thus have no penalty.

TODO: In order to fully gain the power of this optimization, instead of looking for a "reduction_or_group", the compiler could do this lookup automatically based on the P4 code, and the use of stateful ALU outputs in OR based AssignmentStatements. This is a holdover until we have the time to implement this.

Member Function Documentation

is_reduction_or()

bool ReductionOrInfo::is_reduction_or	(	const IR::MAU::Instruction *	instr,
		const IR::MAU::Table *	tbl,
		cstring &	reduction_or_key ) const

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SPDX-License-Identifier: Apache-2.0 A check to see if an instruction is part of a reduction or. The operation is part of a binary OR, i.e.:

f = f | salu.execute();

where the stateful ALU is part of a reduction or group found in the map. This will return the name of the key used by the stateful ALU.