Re: use of LIKELY() and UNLIKELY() branch predictors

Zefram <zefram@fysh.org> wrote:
:Dave Mitchell wrote:
:>1) Has anyone looked at this recently to know whether gcc does anything
:>useful with it?
:
:Yes, it does.  I'm using this facility in a module that I'm currently
:writing, and I perused some of the generated assembly code to see
:whether it makes a difference.  gcc tends to arrange for the unlikely
:case to be taking a branch, while the likely case is to continue inline.
:This is the same thing that I do when writing assembly code manually.
:(amd64, gcc 4.4.5.)

Do you know how this facility plays out in the presence of hardware branch
prediction? The impression I get is that successful hardware prediction 
of a (cacheline-busting) branch is most likely still slower than successful
hardware prediction of a non-branch, so that software hints are still useful;
but I really can't tell for sure from the online resources I've found.

Hugo

• use of LIKELY() and UNLIKELY() branch predictors by Dave Mitchell
• Re: use of LIKELY() and UNLIKELY() branch predictors by bulk88
• Re: use of LIKELY() and UNLIKELY() branch predictors by Dave Mitchell
• Re: use of LIKELY() and UNLIKELY() branch predictors by Tim Bunce
• Re: use of LIKELY() and UNLIKELY() branch predictors by Dave Mitchell
• Re: use of LIKELY() and UNLIKELY() branch predictors by Steffen Mueller
• Re: use of LIKELY() and UNLIKELY() branch predictors by bulk88
• Re: use of LIKELY() and UNLIKELY() branch predictors by Zefram
• Re: use of LIKELY() and UNLIKELY() branch predictors by Steffen Mueller
• Re: use of LIKELY() and UNLIKELY() branch predictors by Nicholas Clark
• Re: use of LIKELY() and UNLIKELY() branch predictors by Zefram
• Re: use of LIKELY() and UNLIKELY() branch predictors by David Cantrell
• Re: use of LIKELY() and UNLIKELY() branch predictors by Tim Bunce
• Re: use of LIKELY() and UNLIKELY() branch predictors by demerphq
• Re: use of LIKELY() and UNLIKELY() branch predictors by Zefram
• Re: use of LIKELY() and UNLIKELY() branch predictors by hv
• Re: use of LIKELY() and UNLIKELY() branch predictors by Zefram
• Re: use of LIKELY() and UNLIKELY() branch predictors by Tim Bunce
• Re: use of LIKELY() and UNLIKELY() branch predictors by Dave Mitchell
• Re: use of LIKELY() and UNLIKELY() branch predictors by Nicholas Clark