https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71414
Richard Biener <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Target| |x86_64-*-* Status|UNCONFIRMED |NEW Last reconfirmed| |2016-06-07 CC| |jamborm at gcc dot gnu.org, | |mliska at suse dot cz Component|other |tree-optimization Blocks| |53947 Summary|2x slower than clang |2x slower than clang |summing small float array |summing small float array, | |GCC should consider larger | |vectorization factor for | |"unrolling" reductions Ever confirmed|0 |1 --- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> --- An interesting observation is that we clone sum32 for IPA-CP of n == 1024 but for some unknown reason figure Alignment of 'a' as unusable: Lattices: Node: main/35: Node: sum32/34: param [0]: VARIABLE ctxs: VARIABLE Alignment unusable (BOTTOM) AGGS VARIABLE param [1]: VARIABLE 1024 [from: 35(99000)] [loc_time: 65, loc_size: 10, prop_time: 0, prop_size: 0] ctxs: VARIABLE Alignment unusable (BOTTOM) AGGS VARIABLE Evaluating opportunities for sum32/34. - considering value 1024 for param #1 n (caller_count: 1) good_cloning_opportunity_p (time: 65, size: 10, freq_sum: 99000) -> evaluation: 643500, threshold: 500 Creating a specialized node of sum32/34. replacing param #1 n with const 1024 Accounting size:7.00, time:72.78 on predicate:(true) Accounting size:3.00, time:2.00 on new predicate:(not inlined) the new node is sum32.constprop/43. iff LLVM disables IPA CP cloning with 'noinline' the testcase should add 'noclone' as well to be a fair comparison. The vectorizer decides to peel the loop for alignment (as usual...) and thus creates both prologue and epilogue loop. That shouldn't matter in practice but it likely obfuscates code enough to make the Job for IVOPTs harder. If the desire was to have nothing known about alignment and 'n' in sum32 the above cannot be avoided anyway. We also peel both prologue and epilogue loop. clang 3.6 (the one I have locally) doesn't peel for alignment and thus uses unaligned loads and unrolls the loop by 2 only. It doesn't do any IPA CP with -Ofast. Note that the difference WRT clangs unrolling and GCCs unrolling is that clang uses two accumulators while GCC just processes multiple loads on the same accumulator with its unrolling. Thus clang can exploit parallelism in the pipeline of the CPU while GCC restricts the CPU due to the dependences. This means that it is the vectorizer that needs to consider using a larger vectorization factor rather than post-vectorization unrolling (that's likely to pay off only for reductions). I wonder what LLVMs heuristic is here. The IPA-CP alignment question remains though. Martins? Referenced Bugs: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947 [Bug 53947] [meta-bug] vectorizer missed-optimizations