public inbox for gcc-bugs@sourceware.org help / color / mirror / Atom feed
From: "gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/95752] New: Failure to optimize complicated usage of __builtin_ctz with conditionals properly Date: Thu, 18 Jun 2020 20:18:18 +0000 [thread overview] Message-ID: <bug-95752-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95752 Bug ID: 95752 Summary: Failure to optimize complicated usage of __builtin_ctz with conditionals properly Product: gcc Version: 11.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: gabravier at gmail dot com Target Milestone: --- unsigned long f(uint64_t value) { unsigned int result; if ((value & 0xFFFFFFFF) == 0) { result = __builtin_ctz(value >> 32) + 32; } else { if ((unsigned int)value != 0) result = __builtin_ctz((unsigned int)value); } return result; } With -O3 -mbmi, LLVM outputs this : f(unsigned long): mov rax, rdi shr rax, 32 tzcnt ecx, eax or ecx, 32 tzcnt eax, edi cmovb eax, ecx ret GCC outputs this : f(unsigned long): test edi, edi jne .L2 shr rdi, 32 xor eax, eax tzcnt eax, edi add eax, 32 mov eax, eax ret .L2: xor edx, edx mov eax, 0 tzcnt edx, edi test edi, edi cmovne eax, edx mov eax, eax ret This may be related to how GCC handles undefined behaviour in relation to `__builtin_ctz` and uninitialized variables, but this still seems like it could be heavily optimized. At least, it could emit something like this if the `cmovcc` is not the best behaviour here : f(unsigned long): test edi, edi jne .L2 shr rdi, 32 tzcnt eax, edi add eax, 32 ret .L1: tzcnt eax, edi ret Using this code : unsigned long f(uint64_t value) { unsigned int result; if ((value & 0xFFFFFFFF) == 0) { result = __builtin_ctz(value >> 32) + 32; } else { if ((unsigned int)value != 0) result = __builtin_ctz((unsigned int)value); else __builtin_unreachable(); } return result; } (i.e. adding __builtin_unreachable where an undefined value is created) generates better code : f(unsigned long): xor eax, eax tzcnt eax, edi test edi, edi jne .L3 shr rdi, 32 tzcnt edi, edi lea eax, [rdi+32] .L3: mov eax, eax ret This looks like something tree-ssa optimizers could do (inserting __builtin_unreachable when invoking UB through usage of undefined values) since https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94861 indicates that GCC doesn't do this even for the simplest cases (and, looking at tree dumps, tree-ssa doesn't look like it makes any assumptions on the initial value of variables).
next reply other threads:[~2020-06-18 20:18 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-06-18 20:18 gabravier at gmail dot com [this message] 2021-08-20 5:38 ` [Bug tree-optimization/95752] " pinskia at gcc dot gnu.org 2023-11-10 22:47 ` pinskia at gcc dot gnu.org
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=bug-95752-4@http.gcc.gnu.org/bugzilla/ \ --to=gcc-bugzilla@gcc.gnu.org \ --cc=gcc-bugs@gcc.gnu.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).