Processor pipeline description - GNU Compiler Collection (GCC) Internals

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14.19.8 Specifying processor pipeline description

To achieve better performance, most modern processors (super-pipelined, superscalar RISC, and VLIW processors) have many functional units on which several instructions can be executed simultaneously. An instruction starts execution if its issue conditions are satisfied. If not, the instruction is stalled until its conditions are satisfied. Such interlock (pipeline) delay causes interruption of the fetching of successor instructions (or demands nop instructions, e.g. for some MIPS processors).

There are two major kinds of interlock delays in modern processors. The first one is a data dependence delay determining instruction latency time. The instruction execution is not started until all source data have been evaluated by prior instructions (there are more complex cases when the instruction execution starts even when the data are not available but will be ready in given time after the instruction execution start). Taking the data dependence delays into account is simple. The data dependence (true, output, and anti-dependence) delay between two instructions is given by a constant. In most cases this approach is adequate. The second kind of interlock delays is a reservation delay. The reservation delay means that two instructions under execution will be in need of shared processors resources, i.e. buses, internal registers, and/or functional units, which are reserved for some time. Taking this kind of delay into account is complex especially for modern RISC processors.

The task of exploiting more processor parallelism is solved by an instruction scheduler. For a better solution to this problem, the instruction scheduler has to have an adequate description of the processor parallelism (or pipeline description). GCC machine descriptions describe processor parallelism and functional unit reservations for groups of instructions with the aid of regular expressions.

The GCC instruction scheduler uses a pipeline hazard recognizer to figure out the possibility of the instruction issue by the processor on a given simulated processor cycle. The pipeline hazard recognizer is automatically generated from the processor pipeline description. The pipeline hazard recognizer generated from the machine description is based on a deterministic finite state automaton (DFA): the instruction issue is possible if there is a transition from one automaton state to another one. This algorithm is very fast, and furthermore, its speed is not dependent on processor complexity¹.

The rest of this section describes the directives that constitute an automaton-based processor pipeline description. The order of these constructions within the machine description file is not important.

The following optional construction describes names of automata generated and used for the pipeline hazards recognition. Sometimes the generated finite state automaton used by the pipeline hazard recognizer is large. If we use more than one automaton and bind functional units to the automata, the total size of the automata is usually less than the size of the single automaton. If there is no one such construction, only one finite state automaton is generated.

     (define_automaton automata-names)

automata-names is a string giving names of the automata. The names are separated by commas. All the automata should have unique names. The automaton name is used in the constructions define_cpu_unit and define_query_cpu_unit.

Each processor functional unit used in the description of instruction reservations should be described by the following construction.

     (define_cpu_unit unit-names [automaton-name])

unit-names is a string giving the names of the functional units separated by commas. Don't use name `nothing', it is reserved for other goals.

automaton-name is a string giving the name of the automaton with which the unit is bound. The automaton should be described in construction define_automaton. You should give automaton-name, if there is a defined automaton.

The assignment of units to automata are constrained by the uses of the units in insn reservations. The most important constraint is: if a unit reservation is present on a particular cycle of an alternative for an insn reservation, then some unit from the same automaton must be present on the same cycle for the other alternatives of the insn reservation. The rest of the constraints are mentioned in the description of the subsequent constructions.

The following construction describes CPU functional units analogously to define_cpu_unit. The reservation of such units can be queried for an automaton state. The instruction scheduler never queries reservation of functional units for given automaton state. So as a rule, you don't need this construction. This construction could be used for future code generation goals (e.g. to generate VLIW insn templates).

     (define_query_cpu_unit unit-names [automaton-name])

unit-names is a string giving names of the functional units separated by commas.

automaton-name is a string giving the name of the automaton with which the unit is bound.

The following construction is the major one to describe pipeline characteristics of an instruction.

     (define_insn_reservation insn-name default_latency
                              condition regexp)

default_latency is a number giving latency time of the instruction. There is an important difference between the old description and the automaton based pipeline description. The latency time is used for all dependencies when we use the old description. In the automaton based pipeline description, the given latency time is only used for true dependencies. The cost of anti-dependencies is always zero and the cost of output dependencies is the difference between latency times of the producing and consuming insns (if the difference is negative, the cost is considered to be zero). You can always change the default costs for any description by using the target hook TARGET_SCHED_ADJUST_COST (see Scheduling).

insn-name is a string giving the internal name of the insn. The internal names are used in constructions define_bypass and in the automaton description file generated for debugging. The internal name has nothing in common with the names in define_insn. It is a good practice to use insn classes described in the processor manual.

condition defines what RTL insns are described by this construction. You should remember that you will be in trouble if condition for two or more different define_insn_reservation constructions is TRUE for an insn. In this case what reservation will be used for the insn is not defined. Such cases are not checked during generation of the pipeline hazards recognizer because in general recognizing that two conditions may have the same value is quite difficult (especially if the conditions contain symbol_ref). It is also not checked during the pipeline hazard recognizer work because it would slow down the recognizer considerably.

regexp is a string describing the reservation of the cpu's functional units by the instruction. The reservations are described by a regular expression according to the following syntax:

            regexp = regexp "," oneof
                   | oneof
     
            oneof = oneof "|" allof
                  | allof
     
            allof = allof "+" repeat
                  | repeat
     
            repeat = element "*" number
                   | element
     
            element = cpu_function_unit_name
                    | reservation_name
                    | result_name
                    | "nothing"
                    | "(" regexp ")"

`,' is used for describing the start of the next cycle in the reservation.
`|' is used for describing a reservation described by the first regular expression or a reservation described by the second regular expression or etc.
`+' is used for describing a reservation described by the first regular expression and a reservation described by the second regular expression and etc.
`*' is used for convenience and simply means a sequence in which the regular expression are repeated number times with cycle advancing (see `,').
`cpu_function_unit_name' denotes reservation of the named functional unit.
`reservation_name' — see description of construction `define_reservation'.
`nothing' denotes no unit reservations.

Sometimes unit reservations for different insns contain common parts. In such case, you can simplify the pipeline description by describing the common part by the following construction

     (define_reservation reservation-name regexp)

reservation-name is a string giving name of regexp. Functional unit names and reservation names are in the same name space. So the reservation names should be different from the functional unit names and can not be the reserved name `nothing'.

The following construction is used to describe exceptions in the latency time for given instruction pair. This is so called bypasses.

     (define_bypass number out_insn_names in_insn_names
                    [guard])

number defines when the result generated by the instructions given in string out_insn_names will be ready for the instructions given in string in_insn_names. The instructions in the string are separated by commas.

guard is an optional string giving the name of a C function which defines an additional guard for the bypass. The function will get the two insns as parameters. If the function returns zero the bypass will be ignored for this case. The additional guard is necessary to recognize complicated bypasses, e.g. when the consumer is only an address of insn `store' (not a stored value).

The following five constructions are usually used to describe VLIW processors, or more precisely, to describe a placement of small instructions into VLIW instruction slots. They can be used for RISC processors, too.

     (exclusion_set unit-names unit-names)
     (presence_set unit-names patterns)
     (final_presence_set unit-names patterns)
     (absence_set unit-names patterns)
     (final_absence_set unit-names patterns)

unit-names is a string giving names of functional units separated by commas.

patterns is a string giving patterns of functional units separated by comma. Currently pattern is one unit or units separated by white-spaces.

The first construction (`exclusion_set') means that each functional unit in the first string can not be reserved simultaneously with a unit whose name is in the second string and vice versa. For example, the construction is useful for describing processors (e.g. some SPARC processors) with a fully pipelined floating point functional unit which can execute simultaneously only single floating point insns or only double floating point insns.

The second construction (`presence_set') means that each functional unit in the first string can not be reserved unless at least one of pattern of units whose names are in the second string is reserved. This is an asymmetric relation. For example, it is useful for description that VLIW `slot1' is reserved after `slot0' reservation. We could describe it by the following construction

     (presence_set "slot1" "slot0")

Or `slot1' is reserved only after `slot0' and unit `b0' reservation. In this case we could write

     (presence_set "slot1" "slot0 b0")

The third construction (`final_presence_set') is analogous to `presence_set'. The difference between them is when checking is done. When an instruction is issued in given automaton state reflecting all current and planned unit reservations, the automaton state is changed. The first state is a source state, the second one is a result state. Checking for `presence_set' is done on the source state reservation, checking for `final_presence_set' is done on the result reservation. This construction is useful to describe a reservation which is actually two subsequent reservations. For example, if we use

     (presence_set "slot1" "slot0")

the following insn will be never issued (because `slot1' requires `slot0' which is absent in the source state).

     (define_reservation "insn_and_nop" "slot0 + slot1")

but it can be issued if we use analogous `final_presence_set'.

The forth construction (`absence_set') means that each functional unit in the first string can be reserved only if each pattern of units whose names are in the second string is not reserved. This is an asymmetric relation (actually `exclusion_set' is analogous to this one but it is symmetric). For example it might be useful in a VLIW description to say that `slot0' cannot be reserved after either `slot1' or `slot2' have been reserved. This can be described as:

     (absence_set "slot0" "slot1, slot2")

Or `slot2' can not be reserved if `slot0' and unit `b0' are reserved or `slot1' and unit `b1' are reserved. In this case we could write

     (absence_set "slot2" "slot0 b0, slot1 b1")

All functional units mentioned in a set should belong to the same automaton.

The last construction (`final_absence_set') is analogous to `absence_set' but checking is done on the result (state) reservation. See comments for `final_presence_set'.

You can control the generator of the pipeline hazard recognizer with the following construction.

     (automata_option options)

options is a string giving options which affect the generated code. Currently there are the following options:

no-minimization makes no minimization of the automaton. This is only worth to do when we are debugging the description and need to look more accurately at reservations of states.
time means printing additional time statistics about generation of automata.
v means a generation of the file describing the result automata. The file has suffix `.dfa' and can be used for the description verification and debugging.
w means a generation of warning instead of error for non-critical errors.
ndfa makes nondeterministic finite state automata. This affects the treatment of operator `|' in the regular expressions. The usual treatment of the operator is to try the first alternative and, if the reservation is not possible, the second alternative. The nondeterministic treatment means trying all alternatives, some of them may be rejected by reservations in the subsequent insns.
progress means output of a progress bar showing how many states were generated so far for automaton being processed. This is useful during debugging a DFA description. If you see too many generated states, you could interrupt the generator of the pipeline hazard recognizer and try to figure out a reason for generation of the huge automaton.

As an example, consider a superscalar RISC machine which can issue three insns (two integer insns and one floating point insn) on the cycle but can finish only two insns. To describe this, we define the following functional units.

     (define_cpu_unit "i0_pipeline, i1_pipeline, f_pipeline")
     (define_cpu_unit "port0, port1")

All simple integer insns can be executed in any integer pipeline and their result is ready in two cycles. The simple integer insns are issued into the first pipeline unless it is reserved, otherwise they are issued into the second pipeline. Integer division and multiplication insns can be executed only in the second integer pipeline and their results are ready correspondingly in 8 and 4 cycles. The integer division is not pipelined, i.e. the subsequent integer division insn can not be issued until the current division insn finished. Floating point insns are fully pipelined and their results are ready in 3 cycles. Where the result of a floating point insn is used by an integer insn, an additional delay of one cycle is incurred. To describe all of this we could specify

     (define_cpu_unit "div")
     
     (define_insn_reservation "simple" 2 (eq_attr "type" "int")
                              "(i0_pipeline | i1_pipeline), (port0 | port1)")
     
     (define_insn_reservation "mult" 4 (eq_attr "type" "mult")
                              "i1_pipeline, nothing*2, (port0 | port1)")
     
     (define_insn_reservation "div" 8 (eq_attr "type" "div")
                              "i1_pipeline, div*7, div + (port0 | port1)")
     
     (define_insn_reservation "float" 3 (eq_attr "type" "float")
                              "f_pipeline, nothing, (port0 | port1))
     
     (define_bypass 4 "float" "simple,mult,div")

To simplify the description we could describe the following reservation

     (define_reservation "finish" "port0|port1")

and use it in all define_insn_reservation as in the following construction

     (define_insn_reservation "simple" 2 (eq_attr "type" "int")
                              "(i0_pipeline | i1_pipeline), finish")

Footnotes

[1] However, the size of the automaton depends on processor complexity. To limit this effect, machine descriptions can split orthogonal parts of the machine description among several automata: but then, since each of these must be stepped independently, this does cause a small decrease in the algorithm's performance.