On Fri, Dec 18, 2015 at 05:09:14PM +0300, Yuri Rumyantsev wrote: > James, > > We implemented slightly different patch - we restrict number of SET > instructions for if-conversion through new parameter and add check in > bb_ok_for_noce_convert_multiple_sets: > > + unsigned limit = MIN (ii->branch_cost, > + (unsigned) PARAM_VALUE (PARAM_MAX_IF_CONV_SET_INSNS)); > .. > - if (count > ii->branch_cost) > - return FALSE; > + if (count > limit) > + return false; > > Now we are testing it for different suites/chips but I saw that real > benchmark for which we saw huge performance degradation gets speed-ip > on 65% for -march=slm -m32.
Yes, that will work too. I put it where I did to give back-ends the choice to turn off all if-conversion by setting the param to zero. Your fix is more targeted to fixing just the one regression. I don't have a preference as to which direction we take this. I saw a similar performance boost for your testcase on my development box with my patch - so at least we now have two candidate patches to fix the performance regression. Thanks, James > > 2015-12-18 16:52 GMT+03:00 James Greenhalgh <james.greenha...@arm.com>: > > > > Hi, > > > > PR68920 talks about undesirable if-conversion in the x86 back-end. > > The if-conversion cost model simply uses BRANCH_COST (I want to revisit > > this for GCC 7), but BRANCH_COST is overloaded in the compiler and reducing > > it causes other optimisations to be disabled. > > > > Consequently, to fix the PR we want something new that the target can set > > to override BRANCH_COST and reduce the number of instructions that get > > if-converted. This patch adds this mechanism through a param. > > > > Bootstrapped on x86_64-none-linux-gnu and aarch64-none-linux-gnu. > > > > OK for trunk? > > > > Thanks, > > James > > > > --- > > gcc/ > > > > 2015-12-17 James Greenhalgh <james.greenha...@arm.com> > > > > PR rtl-optimization/68920 > > * params.def (PARAM_MAX_RTL_IF_CONVERSION_INSNS): New. > > * ifcvt.c (noce_find_if_block): Limit by new param. > > * doc/invoke.texi (max-rtl-if-conversion-insns): Document it. > > > > gcc/testsuite/ > > > > 2015-12-17 James Greenhalgh <james.greenha...@arm.com> > > > > PR rtl-optimization/68920 > > * gcc.deg/ifcvt-5.c: New. > > >
