On Thu, Mar 2, 2017 at 7:58 PM, Dibyendu Majumdar <mobile@xxxxxxxxxxxxxxx> wrote: > On 2 March 2017 at 17:43, Luc Van Oostenryck > <luc.vanoostenryck@xxxxxxxxx> wrote: >> On Thu, Mar 02, 2017 at 05:18:16PM +0000, Dibyendu Majumdar wrote: >>> Hi Luc, >>> >>> I was looking at whether I can use the standard LLVM GEP api to access >>> struct and array members in sparse-llvm. I noticed that the output >>> from sparse has byte offsets of each member - but not the member's >>> ordinal position. >> >> Indeed, when the offset is a compile constant otherwise the offset >> is set to zero and there is a pair OP_MUL/OP_ADD before that does >> the needed pointer arithmetic. >> >>> To use the native LLVM methods we would have to >>> allow LLVM to work out byte offsets, and just use the struct or array >>> definition. This would I think make the code more robust and also lead >>> to better optimisation. >> >> I'm not sure to understand here: which byte offsets need to be worked out? > > You use the GEP API which allows you say that you want to access field > 5 from a struct, and so on. Here 5 is the fifth field. LLVM works out > the rest. This is better than casting the struct to char* and then > doing pointer arithmetic. OK. Yes. >> Also, for the optimization, I wouldn't worry for now because: >> - the linearized code is already optimized (nothing sophisticated but the >> essential is already there) >> - you can always later call LLVM's optimization passes >> - the code selector will also do some optimization. >> > > Sure but my experience with LLVM is that it needs to have type > metadata for each store and load instruction to properly do type based > alias analysis. Otherwise it will miss optimisation opportunities. > Maybe we can still provide type metadata as we do have the symbol > definition etc. There is certainly no reasons to not give easily available information. > But I agree that if sparse-llvm works correctly for all inputs then > that is a big step forward (for me) ! Indeed :) Luc -- To unsubscribe from this list: send the line "unsubscribe linux-sparse" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
