On 2011-12-26 08:22:11 -0500, Tim Prince wrote: > On 12/26/2011 7:59 AM, Vincent Lefevre wrote: > >On 2011-12-26 12:37:27 +0100, David Brown wrote: > >>If it matters that "a + b - c" be calculated "(a + b) - c" or "a + (b - c)", > >>then use brackets. > > > >but brackets shouldn't change anything with -fassociative-math. > > > >In C, brackets are purely syntactic, i.e. a + b - c is equivalent > >to (a + b) - c. > This was so prior to 1989, but the rules changed with the advent of ISO ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > standards. Even where compilers support algebraic simplification across > parentheses in violation of the standards, the results are unreliable as > well as non-portable. Wrong! The ISO C standard even gives an example (5.1.2.3p14) saying that an expression like a + b - c is equivalent to (a + b) - c. -- Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <http://www.vinc17.net/> 100% accessible validated (X)HTML - Blog: <http://www.vinc17.net/blog/> Work: CR INRIA - computer arithmetic / Arénaire project (LIP, ENS-Lyon)
