On 12/26/2011 8:36 AM, Vincent Lefevre wrote:
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.
True, in the case where left-to-right evaluation is not over-ruled by
parens. However, certain compilers which ignore parens also have no
reliable left-to-right or right-to-left evaluation rules.
--
Tim Prince
