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Guy Harris (talk | contribs) Fix capitalization. |
Guy Harris (talk | contribs) →Disadvantages: At least make the external link in the quote look and work the same way it does in the Stack Overflow post whence it came, even if that doesn't prevent the Mediawiki engine from whining about it. |
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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.
*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>
Predication is most effective when paths are balanced or when the longest path is the most frequently executed,<ref name="Fisher04"/> but determining such a path is very difficult at compile time, even in the presence of [[profiling (computer programming)|profiling information]].
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