Conditional moves vs. branching in unrolled loops

[Paul Sandoz]

> On 6 Jan 2016, at 13:38, Vitaly Davidovich <vitalyd at gmail.com> wrote:
> 
> Perhaps for conditional moves data dependency chains are more costly?
> 
> cmov carries a dependency on both inputs, making it more likely to stall when at least one isn't available whereas the branch still allows cpu to continue with speculative execution.  In a tight loop with a memory access as one input to cmov, the memory op has to retire before cmov can proceed; using cmov when both inputs are already ready (e.g. values in registers) is pretty harmless though and avoids a branch entirely.  cmov also has larger encoding than a branch.
> 

Ok. The generated code for an unrolled loop firsts load array elements into registers before performing the cmovs.


> As the original jira on this issue states, cmov should only be used when the branch is profiled to be unpredictable.  I'm not sure why loops with a max/min accumulator need to be called out separately in this regard - wouldn't the branch profile dictate this anyway?

Yes, that was me not understanding the underlying branch profiling mechanisms.

Paul.
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