What is "fcmov"

FCMOV is a floating point conditional move opcode of the Intel x86 architecture, first introduced in Pentium Pro processors. It copies the contents of one of the floating point stack register, depending on the contents of EFLAGS integer flag register, to the ST(0) (top of stack) register. There are 8 variants of the instruction selected by the condition codes that need be set for the instruction to perform the move.

Similar to the CMOV instruction, FCMOV allows some conditional operations to be performed without the usual branching overhead. However, it has a higher latency than conditional branch instructions. Therefore, it is most useful for simple yet unpredictable comparison or conditional operations, where it can provide substantial performance gains.

The instruction is usually used with the FCOMI instruction or the FCOM- FSTSW- SAHF idiom to set the relevant condition codes based on the result of a floating point comparison.