From: Dan Gohman
The portion of the instruction definition in bold indicates the pattern used to match the instruction. The DAG operators (like fmul/fadd) are defined in the lib/Target/TargetSelectionDAG.td file. -"F4RC" is the register class of the input and result values.
+"F4RC" is the register class of the input and result values.
The TableGen DAG instruction selector generator reads the instruction patterns in the .td file and automatically builds parts of the pattern @@ -1556,7 +1556,7 @@ instance, a sequence of instructions such as:
-can be safely substituted by the single instruction: +
can be safely substituted by the single instruction: