From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lindbergh.monkeyblade.net (lindbergh.monkeyblade.net [23.128.96.19]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5FBBB3B7A5 for ; Fri, 10 Nov 2023 21:03:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="SfxLwdGF" Received: from out-176.mta1.migadu.com (out-176.mta1.migadu.com [IPv6:2001:41d0:203:375::b0]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 25829133 for ; Fri, 10 Nov 2023 13:03:09 -0800 (PST) Message-ID: <4e79cf07-cfd0-4662-82cc-cfb0c9f39f4c@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1699650186; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=4YtGBOP2wt4s/AzLZa4K8lpbxPEwW7TsGINgBiHBG20=; b=SfxLwdGF3cCW+P6ykHoGScIisqK5rKK8rzyCPjigCIeIiVVBh6RraboH/hYkxxzToW2tfp hP0aQKKT/BTqKJlKUEUqVRRxLYSYoQhgu0TyRQbvr33di+nPf2RDiK6IQjklGfMkAhRZbc mEP7ZO+iu1Nzm89cUnJWTgGDH3zeYcI= Date: Fri, 10 Nov 2023 13:03:01 -0800 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH bpf-next v2] selftests/bpf: Fix pyperf180 compilation failure with clang18 Content-Language: en-GB To: Andrii Nakryiko Cc: bpf@vger.kernel.org, Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , kernel-team@fb.com, Martin KaFai Lau References: <20231110193644.3130906-1-yonghong.song@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 11/10/23 11:45 AM, Andrii Nakryiko wrote: > On Fri, Nov 10, 2023 at 11:37 AM Yonghong Song wrote: >> With latest clang18 (main branch of llvm-project repo), when building bpf selftests, >> [~/work/bpf-next (master)]$ make -C tools/testing/selftests/bpf LLVM=1 -j >> >> The following compilation error happens: >> fatal error: error in backend: Branch target out of insn range >> ... >> Stack dump: >> 0. Program arguments: clang -g -Wall -Werror -D__TARGET_ARCH_x86 -mlittle-endian >> -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf/tools/include >> -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf -I/home/yhs/work/bpf-next/tools/include/uapi >> -I/home/yhs/work/bpf-next/tools/testing/selftests/usr/include -idirafter >> /home/yhs/work/llvm-project/llvm/build.18/install/lib/clang/18/include -idirafter /usr/local/include >> -idirafter /usr/include -Wno-compare-distinct-pointer-types -DENABLE_ATOMICS_TESTS -O2 --target=bpf >> -c progs/pyperf180.c -mcpu=v3 -o /home/yhs/work/bpf-next/tools/testing/selftests/bpf/pyperf180.bpf.o >> 1. parser at end of file >> 2. Code generation >> ... >> >> The compilation failure only happens to cpu=v2 and cpu=v3. cpu=v4 is okay >> since cpu=v4 supports 32-bit branch target offset. >> >> The above failure is due to upstream llvm patch [1] where some inlining behavior >> are changed in clang18. >> >> To workaround the issue, previously all 180 loop iterations are fully unrolled. >> The bpf macro __BPF_CPU_VERSION__ (implemented in clang18 recently) is used to avoid >> unrolling changes if cpu=v4. If __BPF_CPU_VERSION__ is not available and the >> compiler is clang18, the unrollng amount is unconditionally reduced. >> >> [1] https://github.com/llvm/llvm-project/commit/1a2e77cf9e11dbf56b5720c607313a566eebb16e >> >> Signed-off-by: Yonghong Song >> --- >> tools/testing/selftests/bpf/progs/pyperf180.c | 22 +++++++++++++++++++ >> 1 file changed, 22 insertions(+) >> >> diff --git a/tools/testing/selftests/bpf/progs/pyperf180.c b/tools/testing/selftests/bpf/progs/pyperf180.c >> index c39f559d3100..42c4a8b62e36 100644 >> --- a/tools/testing/selftests/bpf/progs/pyperf180.c >> +++ b/tools/testing/selftests/bpf/progs/pyperf180.c >> @@ -1,4 +1,26 @@ >> // SPDX-License-Identifier: GPL-2.0 >> // Copyright (c) 2019 Facebook >> #define STACK_MAX_LEN 180 >> + >> +/* llvm upstream commit at clang18 >> + * https://github.com/llvm/llvm-project/commit/1a2e77cf9e11dbf56b5720c607313a566eebb16e >> + * changed inlining behavior and caused compilation failure as some branch >> + * target distance exceeded 16bit representation which is the maximum for >> + * cpu v1/v2/v3. Macro __BPF_CPU_VERSION__ is later implemented in clang18 >> + * to specify which cpu version is used for compilation. So a smaller >> + * unroll_count can be set if __BPF_CPU_VERSION__ is less than 4, which >> + * reduced some branch target distances and resolved the compilation failure. >> + * >> + * To capture the case where a developer/ci uses clang18 but the corresponding >> + * repo checkpoint does not have __BPF_CPU_VERSION__, a smaller unroll_count >> + * will be set as well to prevent potential compilation failures. >> + */ >> +#ifdef __BPF_CPU_VERSION__ >> +#if __BPF_CPU_VERSION__ < 4 >> +#define UNROLL_COUNT 90 >> +#endif >> +#elif __clang_major__ == 18 >> +#define UNROLL_COUNT 90 >> +#endif >> + > can it be written as one if? > > #if (defined(__BPF_CPU_VERSION__) && __BPF_CPU_VERSION__ < 4) || > __clang_major >= 18 > > > ? This won't work. For example, using latest upstream clang18, __BPF_CPU_VERSION__ does exist, and user use cpu v4, in this case we do not want to do unrolling but with the above: (defined(__BPF_CPU_VERSION__) && __BPF_CPU_VERSION__ < 4) is false __clang_major >= 18 is true so we do unrolling but we do not need to do since user uses cpu v4. > >> #include "pyperf.h" >> -- >> 2.34.1 >> >>