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[Bug target/20781] 64 bit shift by non-constant implemented as libcall on PPC32
- From: "segher at kernel dot crashing dot org" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 7 Apr 2005 23:34:32 -0000
- Subject: [Bug target/20781] 64 bit shift by non-constant implemented as libcall on PPC32
- References: <20050406023153.20781.dberlin@gcc.gnu.org>
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
------- Additional Comments From segher at kernel dot crashing dot org 2005-04-07 23:34 -------
Subject: Re: New: 64 bit shift by non-constant implemented as libcall on PPC32
> (O5 will give slightly better code for both, but let's start somewhere
> :P)
Well the code for the left shift can be replaced with
subfic r6,r5,32
addi r7,r5,-32
slw r3,r3,r5
srw r6,r4,r6
slw r7,r4,r7
slw r4,r4,r5
or r5,r6,r7
or r3,r3,r5
and similar for the right shift (and also for signed shift counts),
as shift counts < 0 and >= 64 are undefined here in C. If you want
to verify this, do take care with count == 32, as it's "special";
but you can also just look in Book I appendix C.1 and trust that.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20781