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instructions. Conditional move instructions write the contents of one register over another only if the predicate's value is true, whereas conditional select instructions choose which of two registers has its contents written to a third based on the predicate's value. A more generalized and capable
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used by the instruction to control whether the instruction is allowed to modify the architectural state or not. If the predicate specified in the instruction is true, the instruction modifies the architectural state; otherwise, the architectural state is unchanged. For example, a predicated move
409:. Full predication has a set of predicate registers for storing predicates (which allows multiple nested or sequential branches to be simultaneously eliminated) and most instructions in the architecture have a register specifier field to specify which predicate register supplies the predicate.
646:(2003) overcame this problem by using a special instruction which has no effect other than to supply predicates for the following four instructions. The 64-bit instruction set introduced in ARMv8-A (2011) replaced conditional execution with conditional selection instructions.
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Predication's primary drawback is in increased encoding space. In typical implementations, every instruction reserves a bitfield for the predicate specifying under what conditions that instruction should have an effect. When available memory is limited, as on
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With predication, all possible branch paths are coded inline, but some instructions execute while others do not. The basic idea is that each instruction is associated with a predicate (the word here used similarly to its usage in
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based on the predicate that controls whether the branch occurs, the instructions to be executed are associated with that predicate, so that they will be executed, or not executed, based on whether that predicate is true or false.
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Unlike with control dependencies (branches), they don't predict or speculate what the flags will be, so a cmovcc instead of a jcc can create a loop-carried dependency chain and end up being worse than a predictable branch.
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instructions that allow a program to conditionally branch to a different section of code, thus changing the next step in the sequence. This was sufficient until designers began improving performance by implementing
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to the corresponding elements in the vector registers being processed, whereas scalar predication in scalar instruction sets only need the one predicate bit. Where predicate masks become particularly powerful in
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to conditionally load/store values to memory, a parallel form of the conditional move, and may also apply individual mask bits to individual arithmetic units executing a parallel operation. The technique
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Predication is most effective when paths are balanced or when the longest path is the most frequently executed, but determining such a path is very difficult at compile time, even in the presence of
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instruction (a conditional move) will only modify the destination if the predicate is true. Thus, instead of using a conditional branch to select an instruction or a sequence of instructions to
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Besides eliminating branches, less code is needed in total, provided the architecture provides predicated instructions. While this does not guarantee faster execution in general, it will if the
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design of 1967 allocated a "skip" bit in its instruction formats, and the CDC Flexible
Processor in 1976 allocated three conditional execution bits in its microinstruction formats.
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code, which will be executed only under specific conditions depending on factors that cannot be determined beforehand, for example depending on user input. As the majority of
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that allows most instructions to be predicated by one of 13 predicates that are based on some combination of the four condition codes set by the previous instruction. ARM's
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Predication is not usually speculated and causes a longer dependency chain. For ordered data this translates to a performance loss compared to a predictable branch.
347:) and that the instruction will only be executed if the predicate is true. The machine code for the above example using predication might look something like this:
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Elimination of unnecessary branch instructions can make the execution of necessary branches, such as those that make up loops, faster by lessening the load on
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instructions copied the contents of the source register to the destination register depending on a predicate supplied by the value of the flag register.
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GPU computing. All the techniques, advantages and disadvantages of single scalar predication apply just as well to the parallel processing case.
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instruction set (1994) dropped conditional execution to reduce the size of instructions so they could fit in 16 bits, but its successor,
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Luckily, one of the more common patterns of code that normally relies on branching has a more elegant solution. Consider the following
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instructions are simply instructions predicated with the value true. The use of predication is essential in IA-64's implementation of
253:, a method which is slowed down by branches. For a more thorough description of the problems which arose, and a popular solution, see
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Predicated instructions with different predicates can be mixed with each other and with unconditional code, allowing better
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The main purpose of predication is to avoid jumps over very small sections of program code, increasing the effectiveness of
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generated by instructions may reduce code size further by directly using the
Condition Registers in or as predication.
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was extended in
Version 9 (1994) with conditional move instructions for both integer and floating-point registers.
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architecture, most instructions are predicated. The predicates are stored in 64 special-purpose predicate
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Elimination of the cost of a branch misprediction which can be high on deeply pipelined architectures.
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Mahlke, Scott A.; Hank, Richard E.; McCormick, James E.; August, David I.; Hwn, Wen-mei W. (1995).
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Predicated instructions were popular in
European computer designs of the 1950s, including the
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A Comparison of Full and
Partial Predicated Execution Support for ILP Processors
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because it avoids the need for writing separated code for prologs and epilogs.
795:. The 22nd International Symposium on Computer Architecture, 22–24 June 1995.
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are able to avoid this issue (see below). Other detriments are the following:
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in general make heavy use of predication, applying one bit of a conditional
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gained conditional move instructions in 1994 with the MIPS IV version; and
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Embedded
Computing — A VLIW Approach to Architecture, Compilers, and Tools
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471:, this space cost can be prohibitive. However, some architectures such as
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Functions that are traditionally computed using simple arithmetic and
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On a system that uses conditional branching, this might translate to
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555:(1990) featured conditional move instructions. POWER's successor,
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A predicated block includes cycles for all operations, so shorter
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architecture (1992) also featured conditional move instructions.
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instruction sets, like AVX2, have the ability to use a logical
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Fisher, Joseph A.; Faraboschi, Paolo; Young, Cliff (2004).
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architecture, a family of conditional move instructions (
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Predication complicates the hardware by adding levels of
839:"4.5.2 Predication § Predication in the Embedded Domain"
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is known in Flynn's taxonomy as "associative processing"
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may be quicker to compute using predicated instructions.
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in a sequence, the traditional solution is to insert
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gcc optimization flag -O3 makes code slower than -O2
60:. Unsourced material may be challenged and removed.
425:. It also has a number of more subtle benefits:
147:is a feature that provides an alternative to
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167:) non-branch instructions associated with a
27:Form of conditionals in computer programming
163:. Predication works by having conditional (
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670:This form of predication is also used in
543:architecture (1986) had a feature called
453:Instruction sets that have comprehensive
421:execution and avoiding problems with the
120:Learn how and when to remove this message
616:) were added to the architecture by the
69:"Predication" computer architecture
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897:
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487:and potentially degrades clock speed.
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928:. Cengage Learning. pp. 532–9.
676:single instruction, multiple threads
559:(1993), dropped these instructions.
58:adding citations to reliable sources
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650:SIMD, SIMT and vector predication
494:may take longer and be penalized.
389:blocks of code are short enough.
155:, as implemented by conditional
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440:and so even better performance.
392:Predication's simplest form is
45:needs additional citations for
1:
704:Instruction-level parallelism
561:Digital Equipment Corporation
396:, where the architecture has
761:Rick Vinyard (2000-04-26).
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734:Very long instruction word
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845:. Elsevier. p. 172.
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240:simply execute the next
132:Not to be confused with
920:Clements, Alan (2013).
958:Instruction processing
953:Conditional constructs
881:is an example of that.
623:(1995) processor. The
438:instruction scheduling
307:branch_if_condition_to
251:instruction pipelining
811:10.1145/223982.225965
724:Speculative execution
636:conditional execution
503:profiling information
141:computer architecture
302:looking similar to:
300:machine instructions
54:improve this article
922:"8.3.7 Predication"
719:Software pipelining
709:Optimizing compiler
599:software pipelining
394:partial predication
553:POWER architecture
431:bitwise operations
402:conditional select
935:978-1-285-41542-0
895:External link in
672:vector processors
445:branch prediction
387:do_something_else
376:do_something_else
313:do_something_else
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230:computer programs
210:vector processing
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525:(1958), and the
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52:Please help
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767:cs.nmsu.edu
621:Pentium Pro
447:mechanisms.
242:instruction
234:conditional
205:mask vector
149:conditional
145:predication
947:Categories
777:2014-04-22
741:References
699:Delay slot
515:Mailüfterl
413:Advantages
262:pseudocode
238:processors
165:predicated
110:March 2014
80:newspapers
797:CiteSeerX
591:registers
531:IBM ACS-1
419:pipelined
370:condition
355:condition
272:condition
212:is if an
169:predicate
888:cite web
682:See also
519:Zuse Z22
405:form is
232:contain
224:Overview
159:machine
644:Thumb-2
630:In the
604:In the
578:In the
557:PowerPC
541:PA-RISC
509:History
473:Thumb-2
197:AVX-512
187:, some
178:execute
153:control
94:scholar
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618:Intel
614:FCMOV
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584:Intel
573:SPARC
565:Alpha
523:ZEBRA
492:paths
485:paths
481:logic
423:cache
228:Most
214:array
101:JSTOR
87:books
930:ISBN
903:help
847:ISBN
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674:and
660:mask
656:SIMD
625:CMOV
612:and
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569:MIPS
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284:else
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