95d94b52ea
The following puts a limit on the number of alias tests we do in terminate_all_aliasing_chains which is quadratic in the number of overall stores currentrly tracked. There is already a limit in place on the maximum number of stores in a single chain so the following adds a limit on the number of chains tracked. The worst number of overall stores tracked from the defaults (64 and 64) is then 4096 which when imposed as the sole limit for the testcase still causes store merging : 71.65 ( 56%) because the testcase is somewhat degenerate with most chains consisting only of a single store (and 25% of exactly three stores). The single stores are all CLOBBERs at the point variables go out of scope. Note unpatched we have store merging : 308.60 ( 84%) Limiting the number of chains to 64 brings this down to store merging : 1.52 ( 3%) which is more reasonable. There are ideas on how to make terminate_all_aliasing_chains cheaper but for this degenerate case they would not have any effect so I'll defer for GCC 12 for those. I'm not sure we want to have both --params, just keeping the more to-the-point max-stores-to-track works but makes the degenerate case above slower. I made the current default 1024 which for the testcasse (without limiting chains) results in 25% compile time and 20s putting it in the same ballpart as the next offender (which is PTA). This is a regression on trunk and the GCC 10 branch btw. 2021-02-11 Richard Biener <rguenther@suse.de> PR tree-optimization/38474 * params.opt (-param=max-store-chains-to-track=): New param. (-param=max-stores-to-track=): Likewise. * doc/invoke.texi (max-store-chains-to-track): Document. (max-stores-to-track): Likewise. * gimple-ssa-store-merging.c (pass_store_merging::m_n_chains): New. (pass_store_merging::m_n_stores): Likewise. (pass_store_merging::terminate_and_process_chain): Update m_n_stores and m_n_chains. (pass_store_merging::process_store): Likewise. Terminate oldest chains if the number of stores or chains get too large. (imm_store_chain_info::terminate_and_process_chain): Dump chain length.
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