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From: "segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/102239] powerpc suboptimal boolean test of contiguous bits Date: Mon, 29 Nov 2021 10:05:53 +0000 [thread overview] Message-ID: <bug-102239-4-s2HRs0KABT@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-102239-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102239 --- Comment #8 from Segher Boessenkool <segher at gcc dot gnu.org> --- (In reply to luoxhu from comment #6) > > > foo: > > > .LFB0: > > > .cfi_startproc > > > rldicr. 3,3,29,1 > > > beq 0,.L2 > > > > This is fine, but only because it tests the EQ bit (not the LT or GT bits). > > So the generated RTL for this insn (the 2insn one) is not correct. > > The generated RTL in pr102239.c.300r.split2 is: > > (insn 32 8 33 2 (parallel [ > (set (reg:CC 100 0 [123]) > (compare:CC (and:DI (ashift:DI (reg:DI 3 3 [124]) > (const_int 29 [0x1d])) > (const_int -4611686018427387904 > [0xc000000000000000])) > (const_int 0 [0]))) > (clobber (reg:DI 3 3 [125])) > ]) "pr102239.c":4:6 238 {*rotldi3_mask_dot} > (nil)) > (insn 33 32 10 2 (set (reg:DI 3 3 [125]) > (lshiftrt:DI (reg:DI 3 3 [125]) > (const_int 29 [0x1d]))) "pr102239.c":4:6 278 {lshrdi3} > (nil)) > (jump_insn 10 33 11 2 (set (pc) > (if_then_else (eq (reg:CC 100 0 [123]) > (const_int 0 [0])) > (label_ref 15) > (pc))) "pr102239.c":4:6 868 {*cbranch} > (int_list:REG_BR_PROB 536870916 (nil)) > -> 15) So combine will have to look at insn 10 as well when it does the combination (it often already does, via "other_insn") -- but also it does have to know an "eq" is okay here, and that requires a new pattern. > rotldi3_mask_dot is what you mentioned in c#1, it is a shifted result and > not matter for comparing to 0: It does matter, if what you are want to see is if it is smaller than zero or greater than zero. CCmode includes those things. There is a CCEQmode for if only the EQ bit is set correctly. > > *rotl<mode>3_mask_dot cannot do this either; the base and the dot2 of that > > cannot be done, they return a shifted result, but that doesn't matter for > > comparing it to 0. So we should add a specialised version. > > What specialized version to add? Some pattern that just does this as an rldicr, as a single insn. It will have to be excluded by the 2insn thing (it is only a single insn itself!), and it will have to have comparison mode CCEQ only.
next prev parent reply other threads:[~2021-11-29 10:05 UTC|newest] Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-09-08 11:55 [Bug c/102239] New: " npiggin at gmail dot com 2021-09-09 19:40 ` [Bug target/102239] " segher at gcc dot gnu.org 2021-11-24 7:17 ` luoxhu at gcc dot gnu.org 2021-11-24 23:22 ` segher at gcc dot gnu.org 2021-11-26 8:50 ` luoxhu at gcc dot gnu.org 2021-11-26 15:53 ` segher at gcc dot gnu.org 2021-11-29 3:31 ` luoxhu at gcc dot gnu.org 2021-11-29 6:36 ` luoxhu at gcc dot gnu.org 2021-11-29 10:05 ` segher at gcc dot gnu.org [this message] 2021-11-30 5:28 ` luoxhu at gcc dot gnu.org 2021-11-30 22:59 ` segher at gcc dot gnu.org 2021-12-01 7:46 ` luoxhu at gcc dot gnu.org 2022-01-11 9:23 ` cvs-commit at gcc dot gnu.org 2022-01-12 0:27 ` luoxhu at gcc dot gnu.org
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