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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug middle-end/78103] Failure to optimize with __builtin_clzl Date: Sat, 24 Jul 2021 12:28:42 +0000 [thread overview] Message-ID: <bug-78103-4-snH3qRm372@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-78103-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78103 --- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> --- I actually think this should be doable with combiner splitters. For the unsigned int findLastSet3(unsigned long x) { return x ? 8 * sizeof(unsigned long) - __builtin_clzl(x) : 0; } or unsigned int findLastSet4(unsigned int x) { return x ? 8 * sizeof(unsigned int) - __builtin_clz(x) : 0; } the combiner sees everything together, in the first case (parallel [ (set (reg:SI 84 [ <retval> ]) (minus:SI (const_int 64 [0x40]) (xor:SI (minus:SI (const_int 63 [0x3f]) (subreg:SI (clz:DI (reg/v:DI 85 [ x ])) 0)) (const_int 63 [0x3f])))) (clobber (reg:CC 17 flags)) ]) and in the latter case (parallel [ (set (reg/v:SI 85 [ x ]) (minus:SI (const_int 32 [0x20]) (xor:SI (minus:SI (const_int 31 [0x1f]) (clz:SI (reg/v:SI 85 [ x ]))) (const_int 31 [0x1f])))) (clobber (reg:CC 17 flags)) ]) so we just need to split it - into bsr and add 1 (of course only if !TARGET_LZCNT, otherwise it is defined even for 0 and we don't know that it will be UB on zero). For the #c0 findLastSet we don't know that, but again we should know that it is UB at zero when !TARGET_LZCNT and so we could first split (set (reg:DI 87 [ _1 ]) (xor:DI (sign_extend:DI (minus:SI (const_int 63 [0x3f]) (subreg:SI (clz:DI (reg/v:DI 85 [ x ])) 0))) (const_int 63 [0x3f]))) into bsr and xor (i.e. get rid of the sign extension, if it is UB at zero, then we know clz will be in range 0 to 63 and sign extension or zero extension are the same and performed already when using the bs{q,l} instruction.
next prev parent reply other threads:[~2021-07-24 12:28 UTC|newest] Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top [not found] <bug-78103-4@http.gcc.gnu.org/bugzilla/> 2021-07-24 6:20 ` pinskia at gcc dot gnu.org 2021-07-24 12:28 ` jakub at gcc dot gnu.org [this message] 2021-07-26 10:57 ` jakub at gcc dot gnu.org 2021-07-26 11:14 ` jakub at gcc dot gnu.org 2021-07-26 11:22 ` jakub at gcc dot gnu.org 2021-07-26 12:11 ` jakub at gcc dot gnu.org 2021-07-26 21:01 ` segher at gcc dot gnu.org 2021-07-26 21:08 ` jakub at gcc dot gnu.org 2021-07-26 21:26 ` segher at gcc dot gnu.org 2021-07-27 8:52 ` jakub at gcc dot gnu.org 2021-07-27 12:35 ` segher at gcc dot gnu.org 2021-07-27 12:41 ` jakub at gcc dot gnu.org 2021-07-27 13:53 ` jakub at gcc dot gnu.org 2021-07-27 15:25 ` segher at gcc dot gnu.org 2021-07-27 15:37 ` segher at gcc dot gnu.org 2021-07-31 7:21 ` cvs-commit at gcc dot gnu.org 2021-07-31 8:24 ` jakub at gcc dot gnu.org 2021-08-01 20:33 ` cvs-commit at gcc dot gnu.org
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