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* [Bug c/15823] New: optimise >>> style calculation
@ 2004-06-04 16:41 ajrobb at bigfoot dot com
2004-06-04 16:48 ` [Bug c/15823] " pinskia at gcc dot gnu dot org
2004-06-24 8:48 ` ajrobb at bigfoot dot com
0 siblings, 2 replies; 3+ messages in thread
From: ajrobb at bigfoot dot com @ 2004-06-04 16:41 UTC (permalink / raw)
To: gcc-bugs
The C code fragment (i & (~0<<n))/(1<<n) is equivalent to Java's i>>>n
arithmetic shift (at least for n < sizeof(i)*CHAR_BIT).
There are two problems:
1) if n is not a manifest constant, i/(1<<n) uses divide.
2) Even when n is a constant, say 2:
(i & ~3)/4
still does not compile to a single SAR (x86) it still checks the sign and adds
3 if negative.
--
Summary: optimise >>> style calculation
Product: gcc
Version: 3.3.1
Status: UNCONFIRMED
Severity: enhancement
Priority: P2
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: ajrobb at bigfoot dot com
CC: gcc-bugs at gcc dot gnu dot org
GCC build triplet: i586-suse-linux
GCC host triplet: i586-suse-linux
GCC target triplet: i586-suse-linux
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15823
^ permalink raw reply [flat|nested] 3+ messages in thread* [Bug c/15823] optimise >>> style calculation 2004-06-04 16:41 [Bug c/15823] New: optimise >>> style calculation ajrobb at bigfoot dot com @ 2004-06-04 16:48 ` pinskia at gcc dot gnu dot org 2004-06-24 8:48 ` ajrobb at bigfoot dot com 1 sibling, 0 replies; 3+ messages in thread From: pinskia at gcc dot gnu dot org @ 2004-06-04 16:48 UTC (permalink / raw) To: gcc-bugs ------- Additional Comments From pinskia at gcc dot gnu dot org 2004-06-04 16:48 ------- i & ~3 still can be postive. So you still have to do the sign check, if you want te equivalent of Java's i >> n the do ((unsigned int)i) >> n. -- What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |RESOLVED Resolution| |INVALID http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15823 ^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug c/15823] optimise >>> style calculation 2004-06-04 16:41 [Bug c/15823] New: optimise >>> style calculation ajrobb at bigfoot dot com 2004-06-04 16:48 ` [Bug c/15823] " pinskia at gcc dot gnu dot org @ 2004-06-24 8:48 ` ajrobb at bigfoot dot com 1 sibling, 0 replies; 3+ messages in thread From: ajrobb at bigfoot dot com @ 2004-06-24 8:48 UTC (permalink / raw) To: gcc-bugs ------- Additional Comments From ajrobb at bigfoot dot com 2004-06-24 08:39 ------- Subject: Re: optimise >>> style calculation Thanks for getting back to me, I am not looking for Java's i>>n rather i>>>n (e.g. x86 SAR assembler) I want to duplicate the most significant bit into the next n most significant bits. However, I want to avoid assembler and use a C code fragment that compiles to a single SAR instruction on an x86. Regards, Andy Robb. On Friday 04 June 2004 17:48, pinskia at gcc dot gnu dot org wrote: > ------- Additional Comments From pinskia at gcc dot gnu dot org 2004-06-04 > 16:48 ------- i & ~3 still can be postive. So you still have to do the > sign check, if you want te equivalent of Java's i >> n the do ((unsigned > int)i) >> n. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15823 ^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2004-06-24 8:39 UTC | newest] Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2004-06-04 16:41 [Bug c/15823] New: optimise >>> style calculation ajrobb at bigfoot dot com 2004-06-04 16:48 ` [Bug c/15823] " pinskia at gcc dot gnu dot org 2004-06-24 8:48 ` ajrobb at bigfoot dot com
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