WO1987000318A1

WO1987000318A1 - Selective operation of processing elements in a single instruction, multiple data stream (simd) computer system

Info

Publication number: WO1987000318A1
Application number: PCT/US1986/001354
Authority: WO
Inventors: Adam E. Levinthal; Thomas Porter; Thomas D. S. Duff; Loren C. Carpenter
Original assignee: Pixar
Priority date: 1985-06-24
Filing date: 1986-06-23
Publication date: 1987-01-15
Also published as: GB8614907D0; GB2177526A; DE3620982A1; AU6128486A; EP0227811A1; JPS6254359A; FR2583904A1; JPH031699B2; CA1267230A; GB2177526B

Abstract

A plurality of processing elements (11, 13, 15, 17) independently operate in parallel on separate streams of data but in response to common instruction. In order to selectively and individually enable each processing element, a control register stage (37) is provided for each. Each register may be controlled, as between its enabling and disabling states with respect to execution of a common instruction, by the results of a test performed by its associated processor in response to a prior instruction and by the complement of the test results. The system is especially adapted to support flow of control operators, such as IF/THEN constructs, IF/THEN/ELSE constructs and WHILE/DO loop constructs.

Description

SELECTIVE OPERATION OF PROCESSING ELEMENTS IN A SINGLE INSTRUCTION, MULTIPLE DATA STREAM (SIMP) COMPUTER SYSTEM

Background of the Invention

This invention relates generally to parallel data processing techniques and computer systems, and specifically to those of a type where each of a plurality of parallel processors simultaneously executes the same instruction on different data. Such a computer is commonly termed a single instruction, multiple data stream (SIMD) processor.

There are many data processing applications wherein multiple streams of data may be processed in the same manner. An example is in the field of computer graphics where separate video red, green, blue and alpha digital signals may be processed identically. To achieve the highest processing rate, it is thus convenient to process these four data streams simultaneously with the same sequence of instructions. That is, at any given instant, separate red, green, blue and alpha data for a particular color display pixel are being simultaneously processed. Parallel processing is particularly fast if the

^"program being executed on the parallel streams of data is an invariant series of statements. It is more common, however, that the controlling program includes condi¬ tional statements that depend for execution upon the data in each of the parallel processors. Since the data being processed in each stream will be different, provision must be made in this case for those processors whose data does not meet the condition of the program statement to be rendered non-operative during the time that the remaining processors are executing the particular statement. It is known that a WHILE-DO construct is the minimum needed to implement all possible flow control structures.

A common example of such a conditional program instruction is an "IF-THEN" statement: that is, the individual processors are all instructed to perform a certain manipulation of their individual data streams, but only "if" their data meets a certain condition expressed in the program instruction. Those processors whose data at that instant do not meet the condition do not execute that instruction. An "IF-THEN" instruction is often augmented by an "ELSE" modifier; that is, those processors not executing the "IF-THEN" statement are subsequently instructed to execute a different operation on their data at the next instant while those processors who did execute the "IF-THEN" instruction are rendered inoperative.

It is a general object of the present invention to provide improved techniques and circuits for selec¬ tively controlling which of a plurality of parallel processors execute specific conditional instructions.

Summary of the Invention

This and additional objects are accomplished by the present invention, wherein, briefly, each of the parallel processors has a separate control element, such as one bit of a control register, that enables the processor to execute a common, instruction given all processors when the element is in one state and disables the processor from executing that instruction when in its other state. The state of each control element is set to control execution_,of a particular statement dependent upon whether the data for that processor met the test of a previous instruction, such as an "IF-THEN" instruction. In subsequent complementary execution, such as occurs in an "ELSE" instruction, the states of the control elements are reversed so that those processors who did not execute the first statement will execute the subsequent state¬ ment, and vice versa. In addition, in order to provide a capability for nested excecution of such complementary types of instructions, a memory device (a stack memory in a preferred embodiment) is provided to store the states of the individual control elements when the nested condi- tional statement occurs. When execution of the nested instruction is completed, the states of the control elements at the time of the nesting conditional statement are restored so that the processing of them may continue.

Additional objects, features and advantages of the various aspects of the present invention will best be appreciated from a description of its preferred embodi¬ ments, which description should be taken in conjunction wit the accompanying drawings.

Brief Description of the Drawings Figure 1 illustrates in general block diagram form a SIMD processor;

Figure 2 illustrates a first circuit embodiment

"* " ^* "" of the control circuits of the system of. Figure 1;

Figures 3 and 4 are tables which illustrate the operation of the system of Figure 1 when implemented with the control circuit of Figure 2; Figure 5 illustrates a second circuit embodi¬ ment of the control circuits of the system of Figure 1;

Figures 6 and 7 are tables which illustrate the operation of the system of Figure 1 whem implemented with the control circuit of Figure 5; and Figure 8 provides logic details of another portion of the circuit of Figure 1.

Description of the Preferred Embodiments

Referring to Figure 1, the overall architecture of a computer system utilizing the various aspects of the present invention will be described. Separate pro¬ cessors 11, 13, 15 and 17 receive, respectively, inde¬ pendent data streams in input lines 19, 21, 23, and 25. Similarly, independent lines 27, 29, 31, and 33 carry, respectively, the outputs of the processing elements. Four parallel data processors are illustrated in this example, but it will be understood that the principles of the present invention apply to a parallel system contain¬ ing arbitrarily many parallel processing elements. Four processors are conveniently used in a graphics computer system, one channel used to process data of the red component of a video signal, another for the green component, a third for the blue, and a fourth for an alpha component that provides other information of the image. Parallel processing is particularly adapted for a gra- phics application since high speed processing is a requirement and the same sequence of program instructions is executed simultaneously on all four data paths. There are certain program instructions, how¬ ever, that require one or more processing elements to not participate in executing a particular program instruction that is applied simultaneously through an instruction bus 35 to all four of the processing elements 11, 13, 15 and 17. In order to control which of the four processing elements are active to execute a particular instruction, a control circuit is provided in association with each of them, such as a circuit 37 which controls operation of the processing element 11. A line 39 carries a signal to the processing element 11 which controls whether it is enabled to execute an instruction on the bus 35. For example, a voltage in line 39 representative of a logical "1" will cause the processing element to execute the instruction, while a voltage representative of a logical "0" will disable the processing element during execution of that particular instruction by other of the processing elements.

Each of the four control circuits of the system of Figure 1, such as the circuit 37, determines whether to enable its associated processing element, such as pro¬ cessor 11, on the basis of several pieces of information. One is an initial condition which is presented external of the circuits of Figure 1 in a set line 41. Another piece of information is a status instruction in a bus 43 which specifies, for those processor instructions on bus 35 that may require less than all of the processing elements to execute the instruction, additional instructions for determining the state of the enable signal in the line 39. A final piece of information is a true "1" or false "0" signal in a line 45 which gives the result of a test performed by the processing element 11 on its data in response to a current or immediately preceding instruc-

SUBSTi t « tion on the bus 35. Each of the four control circuits shown in Figure 1 operates similarly, except that the test result input received from its associated processing element can be different and thus result in some processors being enabled and others being disabled at a given instant in time.

The function of the control circuits in the system of Figure 1 is explained more fully with respect to its two preferred embodiments, one embodiment illustrated in Figures 2-4 and another in Figures 5-7. But before proceeding to those embodiments, some general items of the system of Figure 1 are first explained. The pro¬ cessor instructions in the bus 35 and the status instructions 43 originate from a micro-programmed control unit such as micro-sequencer 47. A micro-programmed control unit consists of the micro-program memory and the structure required to determine the address of the next microinstruction, specific implementations being well known. A logic circuit 49 has as inputs the invidiual test result lines of each of the processing elements. The logic circuit 49 generates a condition code in an output line 51 when the signals in the input test result lines are a particular one or more combinations. The signal in the line 51 is connected to the condition code input of the micro-sequencer 47, thus enabling a change in the sequence of instructions in response to a particular combination of test result outputs. Another input to the logic circuits 49 is by way of a line 53, an instruction field of the micro-sequencer 47.

In a particular implementation of the system of Figure 1 for color computer graphics processing, each of the processing elements contains as primary components a 16-bit multiplier and a 16-bit arithmetic and logic unit (ALU). Extremely fast processing is desired in computer graphics applications because of the large number of pixels in each frame of a picture, each pixel being defined by four 16-bit words.

Referring to Figure 2, a circuit is shown that- is suitable for use, according to one embodiment, as each of the control circuits shown in Figure 1, such as the circuit 37. A flip-flop circuit 61 has its output connected to the enable line 39. An input line 63 is connected to an output of a four-position multiplexer 65. The multiplexer 65 has four separate inputs 0-3. The status instruction in the bus 43 selects which of the inputs 0-3 is connected to the output 63. The 0 input of the multiplexer is connected directly to the output of the flip-flop 61, thereby allowing the current state of the flip-flop 61 to be held when the multiplexer 65 is switched to its 0 input. Conversely, when switched to its number 3 input, the state of the flip-flop 61 is changed since its output is connected through an inverter 67 back to its input. The number 1 and number 2 position input positions of the multiplexer 65 are the test result line 45 and the set line 41, respectively, previously discussed with respect to Figure 1. The specific circuit examples being described are particularly adapted for executing IF-THEN-ELSE program instructions. The table of Figure 3 summarizes the four possible states of the control circuit of Figure 2, depending upon the status instruction on the bus 43. When the multiplexer 65 is switched to its 0 input, the output in the line 39 is held, the condition desired when the logical operation commanded by the instruction on the bus 35 of Figure 1 is to execute a statement. The next status instruction, selecting the 1 input of the multi¬ plexer 65, causes the test result of its associated processing element to be stored, as previously described, an operation that accompanies an IF instruction in the bus 35. The status instruction 2 causes the flip-flop 61 to be set, a status instruction on bus 43 that accompanies an END IF instruction in the processing element instruction bus 35. Lastly, a status instruction 3 causes the flip- flop element 61 to change state in order to enable those processors previously disabled, and conversely to disable those processors previously enabled. The status in¬ struction 3 is presented in the bus 43 simultaneously with the ELSE instruction in the bus 35. Micro-code in the micro-sequencer 47 assures that the instructions in the buses 35 and 43 correspond according to the table of Figure 3 in accordance with other particular requirements of any application.

The table of Figure 4 better explains the operation of the circuit of Figure 1, when using a control circuit of Figure 2, by a specific example. Consider the example of an IF statement asking whether the data input to each processing element .(D ) is greater than 1. As shown in line 2 of the table of Figure 4, it is assumed in the "test result" column that the first and third processing elements have passed the test, thus showing the logical "1" in their test result output lines 45, while the second and fourth processors have failed the test, and thus show a test result logical signal of "0". Even though each processor is executing the same _. IF instruction, the results of the test performed by each can be different because the data being processed by each is generally different.

At the same time the IF instruction is being executed, the status instruction on the bus 43 causes the multiplexer 65 of each of the control circuits of the system of Figure 1 to switch to its position 1 to receive the test results from their corresponding processors. These test results, whether a test pass "1" or fail "0", are then stored in the individual flip-flop elements. The enable signal outputs of the four flip-flops are given as the enable signals in the table of Figure 4, referred to interchangably in this example as "run flags". At line 2 of the table of Figure 4, the runflags are causing those processing elements who pass the test to be enabled and those who did not to be disabled. Those which are enabled are then caused, as shown in the line 3 of the table of Figure 4, to execute a statement, in this example chosen to be to set the data output (D ) equal to 1 of the enabled processing elements. The disabled processing elements do nothing at this time.

An ELSE instruction is next presented to all the processing elements for execution, which is to say that those processors who failed the IF test are now going to be called upon to do something different, as illus¬ trated in lines 4 and 5 of the. table of Figure 4. The ELSE processor instruction is accompanied by the status instruction 3 which causes the control circuits, illus- trated in Figure 2, to all invert the states of their flip-flops. That can be seen by comparing the run flags of lines 3 and 4 of Figure 4, one being the complement of the other. Once the processors previously disabled are enabled, a statement is executed, as shown in line 5 of Figure 4, wherein in this example the output data value is set equal to the input data value. The result of the routine illustrated in Figure 4 is thus to set the value of the data output lines 27 and 31 equal to 1, and output lines 29 and 33 equal to the value of the corresponding data input. Complementary operation of the processors to execute the IF and ELSE instructions is made possible by a simple provision in each of the control circuits for inverting all of their states in response to a single status instruction.

The logic circuits 49 of Figure 1 are useful for detecting conditions where, because of a particular combination of input data, certain instructions need not be executed. In such a case, the micro-sequencer 47 is then caused to skip the unexecutable instructions. Logic circuits 49 may be omitted in implementations where unexecuted instruction sequences may be allowed to occur. In the example of Figure 4, if the test results shown in line 2 had all been 0, then there is no need to execute the statement of line 3 since all processors would be disabled. For this particular example, therefore, the logic circuits 49 are designed to detect when all processor test results are false (0) and causes the condition code in the line 51 to change, with the resultant change of the instruction sequence issued by the micro-sequencer 47. Additionally, if the test re¬ sults are all true (1), then the instructions at lines 4 and 5 of Figure 4 do not need to be executed, so the condition code in the line 51 can cause that instruction sequence to be bypassed, as well. A signal in line 53 functions to allow testing for any false (0) condition or any true (1) condition. Thus, the ability is provided (in conjunction with the status instruction on the bus 43) for testing for any or all conditions true or false.

An example of specific logic for carrying out these functions is given in Figure 8. An OR gate 52 has as its inputs the test result lines from all of the processing elements. The gate's output is one input of an exclusive OR gate 54, the select line 53 being the second input. The output of the gate 54 is the condition code line 51. The gate 54 operates to pass through the output of the gate 52 when the select line 53 is false (0), and to pass a complement of that output when the line 53 is true (1).

Certain applications will require the ability of the individual processing element control circuits to handle a set of instructions that is nested within an IF- THEN-ELSE series of instructions. When this is required, the run flags determined as the result of executing the IF instruction are stored while the nested set of instruc¬ tions is being executed. Once the nested instructions have been executed, the stored run flags are called out of memory so that the remainder of the IF-THEN-ELSE set of instructions can be executed.

The control circuit of Figure 5 allows such ^•nested program instruction operation. Added to the system circuit of Figure 1 is a stacked memory 81, and associated controlling decoder circuits 83. The cir¬ cuits within the dotted outline of Figure 5 are not repeated within each of the four control circuits of Figure 1, but rather are shared by them. The decoding circuits 83 respond to status instructions in the bus 43 to cause the current enable signals (run flags) of each of the control circuits to be stored in the stack memory 81 (a "push") through lines 85 or to be read from memory (a "pop") through lines 87. As is well known, stack memories read ("pop") the last written ("pushed") data. And each time data is written when there already is data in the stack memory, the existing data is pushed to a lower level in a manner that it can be read out of the memory only after the most recently written data is read out. In other words, data is read out in a first-in, last-out sequence.

Returning to Figure 5, the circuitry of each of the four control circuits of Figure 1 is described for this embodiment. A flip-flop 91 of the same type used in the embodiment of Figure 2 is employed, with this output being the enable signal, one bit of the four-bit run flag. Its input in a line 93 is also connected to an output of a multiplexer 95. The multiplexer, however, has five positions 0-4, one more than used in the embodiment of Figure 2. One of these inputs is selected at a time for connection to the input of the flip-flop 91 by the status instruction in the bus 43. The 0 input is connected directly to the flip-flop output, thus serving to hold the flip-flop in whatever state it is found when switched to that position. Input 1 of the multiplexer receives the output of AND gate 97, having as one input the output of the flip-flop 91 and as the other input test result line 45 of its associated processor. As indicated in the table of Figure 6, the status instruction 1 is also decoded by circuits 83 to store ("push") at the top of the stack memory 81 the output (run flags) of the flip-flops within the control circuits of Figure 1. Multiplexer input 2 is connected to the set line 41. Input number 3 is connected to the stack memory 81 for setting the flip-flops in accordance with what has previously been recorded at the top of the stack. The decoding circuits 83 cause the top stack data of the memory 81 to pop when the status instruction 3 is received.

The last input of the multiplexer 95, switched in response to a status instruction number 4, receives the output of another AND gate 99 whose two inputs are connected to the stack memory output and the output of the flip-flop 91 through an inverter 101. The result is to AND together the data stored at the top of the stack and a complement of the current run flags.

The control circuit of Figure 5, whose logical operation is shown in the table of Figure 6, is especially adapted for carrying out the sequence of operations given in Figure 7. In that sequence, an IF-THEN-ELSE sequence of program instructions is executed at lines 1, 2, 3, 9, 10, 11, 17, 18, and 19. Nested inside the IF or ELSE portions of that set of instructions is yet another IF- THEN-ELSE series of instructions, at lines 4-8. Similarly, a second set of such statements is nested at lines 12-16 within the basic sequence of instructions. In each of the three IF-THEN-ELSE series of instructions, a different test result is assumed, as shown in the "test result" column of Figure 7. These different test results cause different run flags for each of the three IF-THEN- ELSE series of instructions. The dotted arrows show the flow of run flag bits in the course of the operation of the stack memory 81, those arrows pointing generally to the right being the result of a push operation and those generally to the left the result of a pop operation. Although the various aspects of the present invention have been described with respect to its preferred embodiments, it will be understood that this invention is entitled to protection within the full scope of the appended claims.

Claims

IT IS CLAIMED;

1. In a computer system having a plurality of processors that each operate in a different data path, all of said processors connected to receive a common instruc- tion at. any instant of time, a system for executing at least first and second instructions in time sequence wherein the first instruction causes the data in said paths to be tested against a common condition and the second instruction is executed only in those paths where the test of the first instruction has provided a certain pre-defined result, comprising: a register associated with each of said pro¬ cessors in a manner that the processor is enabled to execute an instruction when the register, is in a first state and disabled when in a second state, means connecting each processor with its asso¬ ciated register for forcing said register into its said second state when the test of the first instruction gives said pre-defined result and into said first state when said test does not give said pre-defined result, and means responsive to said second instruction for changing the state of said registers prior to any of said processors executing said second instruction, whereby the processors executing the second instruction are those where the condition test of the first instruction was said certain pre-defined result and the other processors are disabled during the second instruction.

2. The computer system according to claim 1 wherein said first instruction includes an IF instruc- tion, and said second instruction includes an ELSE -instruction.

3. The computer system according to claim 1 that additionally comprises a memory means connected to said registers for temporarily storing and replacing the contents of said registers, whereby other instructions may be nested between execution of said first and second instructions without losing the results of the test of the first instruction for use when execution of the second instruction resumes.