https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78176
--- Comment #15 from Doug Gilmore <doug.gilmore at imgtec dot com> --- Created attachment 40631 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40631&action=edit Prototype change to backout r216501. > Bisected the problem to commit r216501: The review discussion of r216501 starts with message: https://gcc.gnu.org/ml/gcc-patches/2014-10/msg00758.html Which contains: The are two implementations of seq_cost. The function bodies are exactly the same. The patch removes one of them and make the other global. This seems the patch was cleanup that shouldn't introduce a functional change. However implementations of seq_cost are different, per final version of the patch: https://gcc.gnu.org/ml/gcc-patches/2014-10/msg00896.html cfgloopanal.c: - cost += set_rtx_cost (set, speed); rtlanal.c: + cost += set_rtx_cost (set, speed); tree-ssa-loop-ivopts.c: - cost += set_src_cost (SET_SRC (set), speed); In general, when computing the cost of a sequence of N INSNs this increases the cost of the sequence by N*4. This really throws off the costing of substituting different IVs on MIPS. The first patch attached (just a prototype) basically reverts this change. The second fixes a problem with r220473, a fix for PR62631 from Eric Botcazou: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62631#c17 This looks a generic problem in get_shiftadd_cost to me, it ought to mimic the algorithms in expmed.c, something like ... This change can lower the cost of a sequence of instruction. However there are situations this (lower) cost is being scaled by an estimated iteration count will cause the adjusted cost to now become zero. For the example attached to the second patch the IV replacement algorithm will determine that the cost using separate IVs for each load will be less than then cost of one IV for all loads. Thus, in the second patch we detect that a non-zero cost being scaled to zero should represented by one instead, which gets us back to IVSOPTS generating just one IV that will be used for all loads.
