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From: "gjl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug middle-end/109907] Missed optimization for bit extraction (uses shift instead of single bit-test) Date: Fri, 26 May 2023 08:49:38 +0000 [thread overview] Message-ID: <bug-109907-4-xuj3caCLZf@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-109907-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109907 --- Comment #20 from Georg-Johann Lay <gjl at gcc dot gnu.org> --- Here is a testcase similar to the one from PR55181, where the first test is for the sign bit: unsigned char lfsr32_mpp_sign (unsigned long number) { unsigned char b = 0; if (number & (1UL << 31)) b--; if (number & (1UL << 29)) b++; if (number & (1UL << 13)) b++; return b; } unsigned char lfsr32_ppp_sign (unsigned long number) { unsigned char b = 0; if (number & (1UL << 31)) b++; if (number & (1UL << 29)) b++; if (number & (1UL << 13)) b++; return b; } What then happens is: expr.cc::do_store_flag() expmed.cc::emit_store_flag_force() expmed.cc::emit_store_flag() expmed.cc::emit_store_flag_1() the latter then does: if (STORE_FLAG_VALUE == 1 || normalizep) /* If we are supposed to produce a 0/1 value, we want to do a logical shift from the sign bit to the low-order bit; for a -1/0 value, we do an arithmetic shift. */ op0 = expand_shift (RSHIFT_EXPR, int_mode, op0, GET_MODE_BITSIZE (int_mode) - 1, subtarget, normalizep != -1); "normalizep" is true because ops->type has a precision of 1, and STORE_FLAG_VALUE is the default of 1. Nowhere is there any cost computation or consideration whether extzv could do the trick.
next prev parent reply other threads:[~2023-05-26 8:49 UTC|newest] Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-05-19 10:10 [Bug middle-end/109907] New: [avr] " gjl at gcc dot gnu.org 2023-05-19 14:14 ` [Bug middle-end/109907] " pinskia at gcc dot gnu.org 2023-05-19 15:42 ` [Bug middle-end/109907] " pinskia at gcc dot gnu.org 2023-05-19 15:45 ` pinskia at gcc dot gnu.org 2023-05-19 20:03 ` pinskia at gcc dot gnu.org 2023-05-19 20:12 ` pinskia at gcc dot gnu.org 2023-05-19 20:41 ` gjl at gcc dot gnu.org 2023-05-19 20:55 ` pinskia at gcc dot gnu.org 2023-05-19 20:59 ` gjl at gcc dot gnu.org 2023-05-20 0:46 ` pinskia at gcc dot gnu.org 2023-05-20 5:09 ` pinskia at gcc dot gnu.org 2023-05-20 7:36 ` gjl at gcc dot gnu.org 2023-05-20 7:50 ` gjl at gcc dot gnu.org 2023-05-20 22:23 ` pinskia at gcc dot gnu.org 2023-05-20 22:40 ` pinskia at gcc dot gnu.org 2023-05-21 5:15 ` pinskia at gcc dot gnu.org 2023-05-21 5:36 ` pinskia at gcc dot gnu.org 2023-05-21 9:09 ` gjl at gcc dot gnu.org 2023-05-21 9:55 ` gjl at gcc dot gnu.org 2023-05-23 9:58 ` gjl at gcc dot gnu.org 2023-05-26 8:49 ` gjl at gcc dot gnu.org [this message] 2023-05-26 11:14 ` gjl at gcc dot gnu.org 2023-05-26 15:21 ` pinskia at gcc dot gnu.org 2023-05-26 16:08 ` gjl at gcc dot gnu.org 2023-05-26 23:11 ` pinskia at gcc dot gnu.org 2023-05-26 23:34 ` pinskia at gcc dot gnu.org 2023-05-26 23:49 ` pinskia at gcc dot gnu.org 2023-05-27 0:36 ` pinskia at gcc dot gnu.org 2023-05-27 4:02 ` pinskia at gcc dot gnu.org 2023-05-27 20:09 ` pinskia at gcc dot gnu.org 2023-06-11 9:22 ` cvs-commit at gcc dot gnu.org 2023-06-11 9:26 ` gjl at gcc dot gnu.org
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