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Re: [PATCH v2][BZ #12515] Improve precision of clock function
- From: Paul Eggert <eggert at cs dot ucla dot edu>
- To: munroesj at us dot ibm dot com
- Cc: Steven Munroe <munroesj at linux dot vnet dot ibm dot com>, Siddhesh Poyarekar <siddhesh at redhat dot com>, libc-alpha at sourceware dot org
- Date: Tue, 21 May 2013 13:07:11 -0700
- Subject: Re: [PATCH v2][BZ #12515] Improve precision of clock function
- References: <20130521145611 dot GM8927 at spoyarek dot pnq dot redhat dot com> <20130521151839 dot GA18430 at domone dot kolej dot mff dot cuni dot cz> <20130521153442 dot GO8927 at spoyarek dot pnq dot redhat dot com> <519B9A09 dot 6030305 at cs dot ucla dot edu> <20130521161441 dot GQ8927 at spoyarek dot pnq dot redhat dot com> <1369155615 dot 30724 dot 111 dot camel at spokane1 dot rchland dot ibm dot com>
On 05/21/13 10:00, Steven Munroe wrote:
>>> > > return (ts.tv_sec * CLOCKS_PER_SEC
>>> > > + ts.tv_nsec / (1000000000 / CLOCKS_PER_SEC));
> That will cause its own round error problems, so need to cast to long
> long for multiply and divide to avoid this.
I don't see any rounding error here -- can you give an example?
Note that 1000000000 % CLOCKS_PER_SEC is guaranteed to be zero,
which means it's safe to rewrite (x*1000000000)/CLOCKS_PER_SEC
to x/(1000000000/CLOCKS_PER_SEC).
If that isn't clear, perhaps we can prepend a line like the
following, to make it clear:
static_assert (1000000000 % CLOCKS_PER_SEC == 0);