AMD x86 manual Top

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

Use Load-Execute Floating-Point Instructions with Floating-Point Operands

	When operating on single-precision or double-precision
	floating-point data, wherever possible use floating-point
TOP	load-execute instructions to increase code density.
✩	Note: This optimization applies only to floating-point instructions
	with floating-point operands and not with integer operands,
	as described in the next optimization.
	This coding style helps in two ways. First, denser code allows
	more work to be held in the instruction cache. Second, the
	denser code generates fewer internal OPs and, therefore, the
	FPU scheduler holds more work, which increases the chances of
	extracting parallelism from the code.

Example 1 (Avoid):

FLD	QWORD	PTR [TEST1]
FLD	QWORD	PTR	[TEST2]
FMUL	ST, ST(1)

Example 2 (Preferred):

FLD	QWORD	PTR	[TEST1]
FMUL	QWORD	PTR	[TEST2]

Avoid Load-Execute Floating-Point Instructions with Integer Operands

✩operands: FIADD, FISUB, FISUBR, FIMUL, FIDIV, FIDIVR,

TOPFICOM, and FICOMP. Remember th at floatin g -p ointDo not use load-execute floating-point instructions with integerinstructions can have integer operands while integerinstruction cannot have floating-point operands.

Floating-point computations involving integer-memory operands should use separate FILD and arithmetic instructions. This optimization has the potential to increase decode bandwidth and OP density in the FPU scheduler. The floating- point load-execute instructions with integer operands are VectorPath and generate two OPs in a cycle, while the discrete equivalent enables a third DirectPath instruction to be decoded in the same cycle. In some situations this optimizations can also reduce execution time if the FILD can be scheduled several instructions ahead of the arithmetic instruction in order to cover the FILD latency.

Load-Execute Instruction Usage 35