Fused multiply add and default rounding modes

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It doesn’t violate IEEE-754, because IEEE-754 defers to languages on this point:

A language standard should also define, and require implementations to provide, attributes that allow and disallow value-changing optimizations, separately or collectively, for a block. These optimizations might include, but are not limited to:

…

― Synthesis of a fusedMultiplyAdd operation from a multiplication and an addition.

In standard C, the STDC FP_CONTRACT pragma provides the means to control this value-changing optimization. So GCC is licensed to perform the fusion by default, so long as it allows you to disable the optimization by setting STDC FP_CONTRACT OFF. Not supporting that means not adhering to the C standard.

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