AMD x86 manual Floating-Point Compare Instructions

AMD Athlon™ Processor x86 Code Optimization 22007E/0 — November 1999

Use FFREEP Macro to Pop One Register from the FPU Stack

In FPU intensive code, frequently accessed data is often pre-loaded at the bottom of the FPU stack before processing floating-point data. After completion of processing, it is desirable to remove the pre-loaded data from the FPU stack as quickly as possible. The classical way to clean up the FPU stack is to use either of the following instructions:

FSTP	ST(0)	;removes	one	register from stack
FCOMPP		;removes	two	registers from stack

On the AMD Athlon processor, a faster alternative is to use the FFREEP instruction below. Note that the FFREEP instruction, although insufficiently documented in the past, is supported by all 32-bit x86 processors. The opcode bytes for FFREEP ST(i) are listed in Table 22 on page 212.

FFREEP ST(0) ;removes one register from stack

FFREEP ST(i) works like FFREE ST(i) except that it increments the FPU top-of-stack after doing the FFREE work. In other words, FFREEP ST(i) marks ST(i) as empty, then increments the x87 stack pointer. On the AMD Athlon processor, the FFREEP instruction converts to an internal NOP, which can go down any pipe with no dependencies.

Many assemblers do not support the FFREEP instruction. In these cases, a simple text macro can be created to facilitate use of the FFREEP ST(0).

FFREEP_ST0 TEXTEQU <DB 0DFh, 0C0h>

Floating-Point Compare Instructions

For branches that are dependent on floating-point comparisons, use the following instructions:

■FCOMI

■FCOMIP

■FUCOMI

■FUCOMIP

98 Use FFREEP Macro to Pop One Register from the FPU