On Wed, Sep 3, 2014 at 1:29 AM, Steven Bosscher <stevenb....@gmail.com> wrote: > On Wed, Sep 3, 2014 at 1:35 AM, Carrot Wei wrote: >> 1. It is well known that register renaming is a big help to register >> allocation, but in gcc's backend, the web pass is far before RA, there >> are about 20 passes between them. Does it mean register renaming can >> also heavily benefit other optimizations? > > Yes - sometimes anyway. Most non-SSA data flow analyses can't look > through pseudos that have multiple non-overlapping live ranges. Think > constant/copy propagation, value numbering, etc. After passes that > duplicate basic blocks, and not renaming registers, you get false > dependencies that hide RA and scheduling opportunities. This is why > pass_web is after code-duplication transformations like (RTL) loop > unrolling but before the last RTL CPROP pass. The old RA couldn't do > register renaming, so something before RA had to take care of it. > Enter pass_web. > > But this is less relevant in GCC today, where RTL code transformations > basically should be limited to simple local transformations, with the > more difficult global transformations already done on GIMPLE > (including live range splitting, part of out-of-SSA). On top of that, > IRA knows how to do some forms of live range splitting (but not within > loops, AFAIU, because a loop is a single region in IRA). If someone > would give some TLC to the RTL loop-unroll.c code for > IV-splitting/accumulator-expanding and make them enabled by default, I > doubt pass_web would be doing much at all. > > >> And the passes between them >> usually don't generate more register renaming chances? > > Usually not. Most of them create new pseudos for newly inserted expressions. > > Some passes are actually harmed by pass_web. auto_inc_dec is one of those. > > >> 2. It looks current web pass can't handle AUTOINC expressions, a reg >> operand is used as both use and def reference in an AUTOINC >> expression, so this def side should not be renamed. Pass web doesn't >> explicitly check this case, may rename the reg operand of AUTOINC >> expression. Is this expected because it is before auto_inc_dec pass? > > You already found the DF_REF_READ_WRITE bits. pass_web also handles > match_dup constraints. That should be enough. If it is not, then > please file a bug report (and feel free to assign it to me). > Bug entry filed https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63156. Debugging shows that DF_REF_READ_WRITE is not set for the operand of post_inc, may be a dataflow problem?
thanks Guozhi Wei
