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→Overview: optional predication! Tags: Mobile edit Mobile web edit Advanced mobile edit |
→SIMD, SIMT and vector predication: ILLIAC IV had masked Predicated SWAR! only 2 bits (2x32 or 1x64) but still! Tags: Mobile edit Mobile web edit Advanced mobile edit |
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*Predication complicates the hardware by adding levels of [[control unit|logic]] to critical [[datapath|paths]] and potentially degrades clock speed.
*A predicated block includes cycles for all operations, so shorter [[control-flow graph|paths]] may take longer and be penalized.
* An extra register read is required. A non-predicated ADD would read two registers from a register file, where a Predicated ADD would need to also read the predicate register file. This increases Hazards in [[Out-of-order execution]].
*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.<ref>{{cite web |last1=Cordes |first1=Peter |title=assembly - How does Out of Order execution work with conditional instructions, Ex: CMOVcc in Intel or ADDNE (Add not equal) in ARM |url=https://stackoverflow.com/a/50960323 |website=Stack Overflow |quote=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. [https://stackoverflow.com/questions/50959808 gcc optimization flag -O3 makes code slower than -O2] is an example of that.}}</ref>
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</syntaxhighlight>
Masking is an integral part of [[Flynn's taxonomy|Array Processors]] such as the [[ILLIAC IV]]. Array Processors are known today as [[single instruction, multiple threads]] (SIMT), and a predicate bit ''per PE'' used to activate or de-activate each Processing Element. When the PE has no [[SIMD within a register]] instructions, each PE may be individually Predicated:
<syntaxhighlight lang="c">
for each (PE j) // of
(active-maskbit j) broadcast_scalar_instruction_to(PE j)
</syntaxhighlight>
Modern SIMT [[GPUs]] use (or used, but ILLIAC IV documentation termed it [[ILLIAC IV#Branches|"branching"]]) predication to enable/disable individual Processing Elements ''and'', separately and furthermore, to ''also'' mask-out sub-words within any given PE's SWAR ALU.
<syntaxhighlight lang="c">
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