| Is -O required for optimization across module bounds? Yes, without -O GHC doesn’t put inlinings into interface files, so an importing module can't see them. Use -fno-omit-interface-pragmas to tell GHC to put the stuff in the interface file regardless. | Also, since I really want a certain level of inlining for a plugin I'm | working on; is there a way to force (from the plugin, i.e. using the | API) to force inlining of a term at its call-site? Yes, use the magic function 'inline'. http://www.haskell.org/ghc/docs/7.8.2/html/libraries/base-4.7.0.0/GHC-Exts.h... (But I don’t know how much people have exercised this recently, and I think it's a bit fragile because it vanishes after a single inlining.) | Alternatively | (weaker), can I force - from a compiler plugin - the inliner to behave | as if -O was set even if it wasn't? Well, you could invoke the simplifier from the plugin, setting the DynFlags to -O first. But I don’t think the plugin can side-effect the DynFlags for downstream passes. Simon