From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from nikam.ms.mff.cuni.cz (nikam.ms.mff.cuni.cz [195.113.20.16]) by sourceware.org (Postfix) with ESMTPS id F09C63858D32 for ; Sun, 18 Jun 2023 15:54:42 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org F09C63858D32 Authentication-Results: sourceware.org; dmarc=fail (p=none dis=none) header.from=ucw.cz Authentication-Results: sourceware.org; spf=none smtp.mailfrom=kam.mff.cuni.cz Received: by nikam.ms.mff.cuni.cz (Postfix, from userid 16202) id 5E3C628AEBE; Sun, 18 Jun 2023 17:54:41 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ucw.cz; s=gen1; t=1687103681; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type; bh=vZJIifOtm4qcAFX3vCs9YX7UundJ0fouLad5qbCtkfY=; b=b9KTkldbYd5aRESe3xHG1KYdTio9Slx+Gss7hIPLop3tIWl8hslS+hbmAFgZOflkWybx08 JBscwu7Ke9ESA3a5+R2sdLHNVlHVcA8vu47zoyPCDj/CuqxUuBgErXMuTQmmRfN4GbExKO kAwHRIwduuXnChd0Idpp/VHkpn4QLeM= Date: Sun, 18 Jun 2023 17:54:41 +0200 From: Jan Hubicka To: gcc-patches@gcc.gnu.org, rguenther@suse.cz Subject: Optimize std::max early Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Spam-Status: No, score=-11.1 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,GIT_PATCH_0,HEADER_FROM_DIFFERENT_DOMAINS,RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_NONE,TXREP,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: Hi, we currently produce very bad code on loops using std::vector as a stack, since we fail to inline push_back which in turn prevents SRA and we fail to optimize out some store-to-load pairs (PR109849). I looked into why this function is not inlined and it is inlined by clang. We currently estimate it to 66 instructions and inline limits are 15 at -O2 and 30 at -O3. Clang has similar estimate, but still decides to inline at -O2. I looked into reason why the body is so large and one problem I spotted is the way std::max is implemented by taking and returning reference to the values. const T& max( const T& a, const T& b ); This makes it necessary to store the values to memory and load them later (Max is used by code computing new size of vector on resize.) Two stores, conditional and load accounts as 8 instructions, while MAX_EXPR as 1 and has a lot better chance to fold with the surrounding code. We optimize this to MAX_EXPR, but only during late optimizations. I think this is a common enough coding pattern and we ought to make this transparent to early opts and IPA. The following is easist fix that simply adds phiprop pass that turns the PHI of address values into PHI of values so later FRE can propagate values across memory, phiopt discover the MAX_EXPR pattern and DSE remove the memory stores. Bootstrapped/regtested x86_64-linux, does this look resonable thing to do? Looking into how expensive the pass is, I think it is very cheap, except that it computes postdominator and updates ssa even if no patterns are matched. I will send patch to avoid that. gcc/ChangeLog: PR tree-optimization/109811 PR tree-optimization/109849 * passes.def: Add phiprop to early optimization passes. * tree-ssa-phiprop.cc: Allow clonning. gcc/testsuite/ChangeLog: PR tree-optimization/109811 PR tree-optimization/109849 * gcc.dg/tree-ssa/phiprop-1.c: New test. diff --git a/gcc/passes.def b/gcc/passes.def index c9a8f19747b..faa5208b26b 100644 --- a/gcc/passes.def +++ b/gcc/passes.def @@ -88,6 +88,8 @@ along with GCC; see the file COPYING3. If not see /* pass_build_ealias is a dummy pass that ensures that we execute TODO_rebuild_alias at this point. */ NEXT_PASS (pass_build_ealias); + /* Do phiprop before FRE so we optimize std::min and std::max well. */ + NEXT_PASS (pass_phiprop); NEXT_PASS (pass_fre, true /* may_iterate */); NEXT_PASS (pass_early_vrp); NEXT_PASS (pass_merge_phi); diff --git a/gcc/testsuite/gcc.dg/tree-ssa/phiprop-1.c b/gcc/testsuite/gcc.dg/tree-ssa/phiprop-1.c new file mode 100644 index 00000000000..9f52c2a7298 --- /dev/null +++ b/gcc/testsuite/gcc.dg/tree-ssa/phiprop-1.c @@ -0,0 +1,14 @@ +/* { dg-do compile } */ +/* { dg-options "-O1 -fdump-tree-phiprop1-details -fdump-tree-release_ssa" } */ +int max(int a, int b) +{ + int *ptr; + if (a > b) + ptr = &a; + else + ptr = &b; + return *ptr; +} + +/* { dg-final { scan-tree-dump-times "Inserting PHI for result of load" 1 "phiprop1"} } */ +/* { dg-final { scan-tree-dump-times "MAX_EXPR" 1 "release_ssa"} } */ diff --git a/gcc/tree-ssa-phiprop.cc b/gcc/tree-ssa-phiprop.cc index 3cb4900b6be..5dc505df420 100644 --- a/gcc/tree-ssa-phiprop.cc +++ b/gcc/tree-ssa-phiprop.cc @@ -476,6 +476,7 @@ public: {} /* opt_pass methods: */ + opt_pass * clone () final override { return new pass_phiprop (m_ctxt); } bool gate (function *) final override { return flag_tree_phiprop; } unsigned int execute (function *) final override;