The first mention of contraction in the standard (I'm looking at N3220 draft that I have handy) is:
replyA 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.
It seems to me that either you want to allow for contraction everywhere, or not all. Allowing it only sometimes is worst of both worlds.
replyIf you allow contraction after inlining, whether or not an FMA will get contracted becomes subject to the vicissitudes of inlining and other compiler decisions that can be hard-to-predict. It turns out to be a lot harder of a problem to solve than it appears at first glance.
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