float v = (float) ((float) a) * ((float) b) + c;
float v = ((float) a) * ((float) b) + c;
Not necessarily! Floating-point contraction is allowable essentially within statements but not across them. By assigning the result of a * b into a value, you prohibit contraction from being able to contract with the addition into an FMA.
In practice, every compiler has fast-math flags which says stuff it and allows all of these optimizations to occur across statements and even across inline boundaries.
(Then there's also the issue of FLT_EVAL_METHOD, another area where what the standard says and what compilers actually do are fairly diametrically opposed.)
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.