On Fri, May 20, 2016 at 05:11:59PM +0200, Marc Glisse wrote: > On Fri, 20 May 2016, Andi Kleen wrote: > > >Richard Biener <rguent...@suse.de> writes: > > > >>The following patch adds BIT_FIELD_INSERT, an operation to > >>facilitate doing bitfield inserts on registers (as opposed > >>to currently where we'd have a BIT_FIELD_REF store). > > > >I wonder if these patches would make it easier to use the Haswell > >bit manipulations instructions on x86 (which act on registers). > > > >I found that gcc makes significantly less use of them than LLVM, > >sometimes leading to much bigger code. > > Could you point at some bugzilla entries? I don't really see which > BMI* instruction could be helped by BIT_FIELD_INSERT (PDEP seems too > hard). There is one BMI1 instruction we don't use much, bextr (only > defined with an UNSPEC in i386.md, unlike the TBM version), but it > is about extracting.
Ok. Yes I was thinking of BEXTR. I thought I had filed a bugzilla at some point, but can't find it right now. If you compare bitfield code compiled for Haswell on LLVM and GCC it is very visible how much worse gcc is. So perhaps it only needs changes in the backend. -Andi
