AMD x86 manual Example 2 Potentially faster

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

FPU into truncating mode, and performing all of the conversions before restoring the original control word.

The speed of the above code is somewhat dependent on the nature of the code surrounding it. For applications in which the speed of floating-point-to-integer conversions is extremely critical for application performance, experiment with either of the following substitutions, which may or may not be faster than the code above.

The first substitution simulates a truncating floating-point to integer conversion provided that there are no NaNs, infinities, and overflows. This conversion is therefore not IEEE-754 compliant. This code works properly only if the current FPU rounding mode is round-to-nearest-even, which is usually the case.

Example 2 (Potentially faster).

FLD	QWORD PTR [X]		;load double to be converted
FST	DWORD PTR [TX]		;store X	because sign(X) is needed
FIST	DWORD PTR [I]		;store rndint(x) as default result
FISUB	DWORD PTR [I]		;compute	DIFF = X - rndint(X)
FSTP	DWORD PTR [DIFF]		;store DIFF as we need sign(DIFF)
MOV	EAX, [TX]		;X
MOV	EDX, [DIFF]		;DIFF
TEST	EDX, EDX		;DIFF ==	0 ?
JZ	$DONE		;default	result is OK, done
XOR	EDX, EAX	; need correction if sign(X) != sign(DIFF)
SAR	EAX, 31		;(X<0) ?	0xFFFFFFFF : 0
SAR	EDX, 31		; sign(X)!=sign(DIFF)?0xFFFFFFFF:0
LEA	EAX, [EAX+EAX+1]		;(X<0) ?	0xFFFFFFFF : 1
AND	EDX, EAX		;correction: -1, 0, 1
SUB	[I], EDX		;trunc(X)=rndint(X)-correction
$DONE:

The second substitution simulates a truncating floating-point to integer conversion using only integer instructions and therefore works correctly independent of the FPUs current rounding mode. It does not handle NaNs, infinities, and overflows according to the IEEE-754 standard. Note that the first instruction of this code may cause an STLF size mismatch resulting in performance degradation if the variable to be converted has been stored recently.

Minimize Floating-Point-to-Integer Conversions 101