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Sun, 28 Apr 2024 00:24:06 +0000 (GMT) Message-ID: <71a749ba-d843-424a-9a41-1d20f6be685c@linux.ibm.com> Date: Sat, 27 Apr 2024 19:24:05 -0500 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Maybe we should get rid of ifuncs To: Zack Weinberg , Richard Henderson , libc-alpha@sourceware.org References: Content-Language: en-US Cc: Michael Meissner From: Peter Bergner In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: N2GoDo_S20QzeLyomxOU-uynQrwqi5p0 X-Proofpoint-GUID: N2GoDo_S20QzeLyomxOU-uynQrwqi5p0 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1011,Hydra:6.0.650,FMLib:17.11.176.26 definitions=2024-04-27_22,2024-04-26_02,2023-05-22_02 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1011 mlxscore=0 priorityscore=1501 malwarescore=0 phishscore=0 impostorscore=0 mlxlogscore=872 adultscore=0 spamscore=0 bulkscore=0 suspectscore=0 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2404010000 definitions=main-2404280000 X-Spam-Status: No, score=-3.1 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_EF,KAM_SHORT,RCVD_IN_MSPIKE_H4,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_PASS,TXREP 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: On 4/24/24 9:43 AM, Zack Weinberg wrote: > I'm very curious what the plan for function multiversioning in GCC > and LLVM is, and how close to declarative it gets. GCC (at least on powerpc) already supports it via the target_clones attribute. See gcc/testsuite/gcc.target/powerpc/clone*.c for examples. Basically, it looks like (from clone3.c): __attribute__((target_clones("cpu=power10,cpu=power9,default"))) long mod_func (long a, long b) { return (a % b) + s; } long mod_func_or (long a, long b, long c) { return mod_func (a, b) | c; } Mike knows how this works better than I, but GCC automatically emits an ifunc resolver for the different clones and looks to use the HWCAP* architecture mask associated with the cpu we're compiling for. The "default" function being called in the case our ifunc resolver doesn't match any of the HWCAP* masks from the cpus we're compiling for. Mike, it seems like this is more of a "cpu" clone and not a true HWCAP test, so this specific thing doesn't (at least currently) work for something like __attribute__((target_clones("vsx,mma,default"))) ? Or did I misread the code? I'll note I'm pretty sure we (IBM/powerpc) have added ifunc usage to OpenBLAS and some other libraries outside of glibc. Peter