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From: "roger at nextmovesoftware dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/113764] [X86] Generates lzcnt when bsr is sufficient Date: Thu, 08 Feb 2024 00:35:12 +0000 [thread overview] Message-ID: <bug-113764-4-K4NeitXwgQ@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-113764-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113764 Roger Sayle <roger at nextmovesoftware dot com> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |roger at nextmovesoftware dot com Status|UNCONFIRMED |NEW Ever confirmed|0 |1 Last reconfirmed| |2024-02-08 --- Comment #1 from Roger Sayle <roger at nextmovesoftware dot com> --- Confirmed. This issue has two parts. The first is that the bsr_1 pattern (and variants) is (are) conditional on !TARGET_LZCNT, so the bsrl instruction isn't currently available with -mlzcnt. The second is that the middle-end doesn't have a preferred canonical RTL representation for this idiom, but all three of the following equivalent functions should generate identical code: unsigned bsr1(unsigned x) { return __builtin_clz(x) ^ 31; } unsigned bsr2(unsigned x) { return 31 - __builtin_clz(x); } unsigned bsr3(unsigned x) { return ~__builtin_clz(x) & 31; } [Note that the tree-ssa optimizers do transform bsr3 into bsr1]. A suitable fix would be to add the equivalent clz(x)^31 variant pattern to i386.md as a "synonymous" define_insn pattern.
next prev parent reply other threads:[~2024-02-08 0:35 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2024-02-05 11:38 [Bug target/113764] New: " chfast at gmail dot com 2024-02-08 0:35 ` roger at nextmovesoftware dot com [this message] 2024-02-09 18:35 ` [Bug target/113764] " roger at nextmovesoftware dot com 2024-02-09 21:58 ` jakub at gcc dot gnu.org 2024-02-11 11:34 ` [Bug target/113764] [X86] __builtin_clz generates " roger at nextmovesoftware dot com
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