#9661: Branchless ==# is compiled to branchy code -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: bgamari Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.9 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: D854 -------------------------------------+------------------------------------- Comment (by jstolarek): Replying to [comment:25 rwbarton]:
Again there is a distinction between the compiler being able to generate branchless code and the compiler giving the user control over whether to generate branchless code. For example in that benchmark, it would be even better if the user could simply write
{{{ let inc = if v >= 0 then 1 else 0 }}}
rather than
{{{ let !(I# v) = x inc = I# (v >=$# 0#) }}} I wholeheartedly agree - it would be better if the compiler could generate optimized branchless code from higher level abstract code. That said, we expose low-level primops and it would be a real shame if the users couldn't get branchless behaviour from them. Besides, if we want GHC to optimize high-level code to branchless code then I suspect it will be much easier to implement if primops compiled to branchless code.
So, this isn't an example of the kind that I am talking about.
I don't follow. Why not? Surely, this is an ugly piece of code written by hand but at the moment GHC can't produce such code from high-level code - it had to be written by hand. So, I don't claim that compiling `==#` to branchless code is the final word on this topic. It's just fixing a bug that I didn't notice when originally implementing #6135 - `>=#`, `>#`, `<=#` and `<#` already compile to branchless code as they were intended to. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9661#comment:26 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler