On Fri, Dec 19, 2014 at 6:09 AM, Segher Boessenkool <seg...@kernel.crashing.org> wrote: > On Thu, Dec 18, 2014 at 05:00:01PM +0000, Jiong Wang wrote: >> On 17/12/14 15:54, Richard Biener wrote: >> >ick. I realize we don't have SSA form on RTL but doesn't DF provide >> >at least some help in looking up definition statements for pseudos? >> >In fact we want to restrict the transform to single-use pseudos, thus >> >hopefully it can at least tell us that... (maybe not and this is what >> >LOG_LINKS are for in combine...?) At least loop-invariant alreadly >> >computes df_chain with DF_UD_CHAIN which seems exactly what >> >is needed (apart from maybe getting at single-use info). >> >> thanks very much for these inspiring questions. >> >> yes, we want to restrict the transformation on single-use pseudo only, >> and it's better the transformation could re-use existed info and helper >> function to avoid increase compile time. but I haven't found anything I >> can reuse at the stage the transformation happen. >> >> the info similar as LOG_LINKS is what I want, but maybe simpler. I'd study >> the code about build LOG_LINKS, and try to see if we can do some factor out. > > LOG_LINKs in combine are just historical. combine should be converted > to use DF fully. > > LOG_LINKs have nothing to do with single use; they point from the _first_ > use to its corresponding def. > > You might want to look at what fwprop does instead. Pass rtl fwprop uses df information in single-definition way, it doesn't really take into consideration if register is a single use. This often corrupts other optimizations like post-increment and load/store pair. For example:
add r2, r1, r0 ldr rx, [r2] add r2, r2, #4 is transformed into below form: add r2, r1, r0 ldr rx, [r1, r0] add r2, r2, #4 As a result, post-increment opportunity is corrupted, also definition of r2 can't be deleted because it's not single use. Thanks, bin
