4 ms·

The first mention of contraction in the standard (I'm looking at N3220 draft that I have handy) is: A floating expression may be contracted, that is, evaluated

The first mention of contraction in the standard (I'm looking at N3220 draft that I have handy) is: A floating expression may be contracted, that is, evaluated as though it were a single opera- tion, thereby omitting rounding errors implied by the source code and the expression evalua- tion method.86) The FP_CONTRACT pragma in <math.h> provides a way to disallow contracted expressions. Otherwise, whether and how expressions are contracted is implementation-defined. If you're making a language that generates C, it's probably a good idea to pin down which C compilers are supported, and control the options passed to them. Then you can more or less maintain the upper hand on issues like this.

20k 9mo ago
The magic here with the word-ese is that a floating point expression may be contracted. In C, you write things like: float a = /*expression*/ float b = /*expression*/ float c = a * b; Contraction can only happen within the expressions on the right hand side, not mutually between a and b. This makes inlining, and even variable substitution not legally permitted by the C spec to be equivalent to the actually written out inlined version. Ie you can't transform: float a = v1 * v2; float b = v3 * v4; float c = a + b; Into float c = (v1 * v2) + (v3 * v4); As they aren't semantically equivalent, and compilers do follow these rules. The same applies for inline functions. Its one of the reasons why macros can sometimes perform better than force-inline functions In practice, the big 3 compilers all perform floating point contraction according to the spec, and this is a major issue if you want to get good gpgpu performance because this extends to OpenCL and C-style languages in general (like CUDA)