Date: Sat, 17 Mar 2007 20:54:18 -0600 (MDT) From: "M. Warner Losh" <imp@bsdimp.com> To: jroberson@chesapeake.net Cc: max@love2party.net, jeff@freebsd.org, src-committers@freebsd.org, cvs-all@freebsd.org, cvs-src@freebsd.org Subject: Re: cvs commit: src/sys/kern sched_ule.c Message-ID: <20070317.205418.179961042.imp@bsdimp.com> In-Reply-To: <20070317121427.H560@10.0.0.1> References: <20070317110821.I560@10.0.0.1> <200703172100.13218.max@love2party.net> <20070317121427.H560@10.0.0.1>
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In message: <20070317121427.H560@10.0.0.1>
Jeff Roberson <jroberson@chesapeake.net> writes:
:
: On Sat, 17 Mar 2007, Max Laier wrote:
:
: > On Saturday 17 March 2007 20:09, Jeff Roberson wrote:
: >> Any language lawyers care to comment on this?
: >
: > I find this strange. According to the spec "(Decrementing is equivalent
: > to subtracting 1.)", but "pri = --pri % RQ_NQS;" will behave like you
: > expect, while "pri = (pri - 1) % RQ_NQS;" clearly didn't.
:
: I noticed this as well.
:
: When you do --pri, pri is promoted to int for the math and then demoted
: back to char wich truncates the value. Subsequently this value is used
: in the % operation, which gives the expected results.
:
: When you do pri - 1 the intermediate result is promoted to a signed int
: which doesn't yield the result you'd like when you mod with 64.
(pri - 1u) % 64
is likely what you want. That way the (pri - 1u) turns out to be
unsigned because it is (unsigned - unsigned) % signed. which winds up
being unsigned % signed. The - 1 version is (unsigned - signed) %
signed, which breaks down to signed % signed, which yields the strange
result that you saw.
(unsigned char)(pri - 1) % 64 is another way to fry this fish.
Warner
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