Assembly Language for Beginners

1.31. WORKING WITH FLOATING POINT NUMBERS USING SIMD

Listing 1.404: Optimizing MSVC 2012 x64

v$ = 8

calculate_machine_epsilon PROC

movsdx QWORD PTR v$[rsp], xmm0

movaps xmm1, xmm0

inc QWORD PTR v$[rsp]

movsdx xmm0, QWORD PTR v$[rsp]

subsd xmm0, xmm1

ret 0

calculate_machine_epsilon ENDP

There is no way to add 1 to a value in 128-bit XMM register, so it must be placed into memory.

There is, however, theADDSDinstruction (Add Scalar Double-Precision Floating-Point Values) which can

add a value to the lowest 64-bit half of a XMM register while ignoring the higher one, but MSVC 2012

probably is not that good yet^193.

Nevertheless, the value is then reloaded to a XMM register and subtraction occurs. SUBSDis “Subtract

Scalar Double-Precision Floating-Point Values”, i.e., it operates on the lower 64-bit part of 128-bit XMM

register. The result is returned in the XMM0 register.

1.31.5 Pseudo-random number generator example revisited

Let’s revisit “pseudo-random number generator example” example listing.1.25.1.

If we compile this in MSVC 2012, it will use the SIMD instructions for the FPU.

Listing 1.405: Optimizing MSVC 2012

__real@3f800000 DD 03f800000r ; 1

tv128 = -4

_tmp$ = -4

?float_rand@@YAMXZ PROC

push ecx

call ?my_rand@@YAIXZ

; EAX=pseudorandom value

and eax, 8388607 ; 007fffffH

or eax, 1065353216 ; 3f800000H

; EAX=pseudorandom value & 0x007fffff | 0x3f800000

; store it into local stack:

mov DWORD PTR _tmp$[esp+4], eax

; reload it as float point number:

movss xmm0, DWORD PTR _tmp$[esp+4]

; subtract 1.0:

subss xmm0, DWORD PTR __real@3f800000

; move value to ST0 by placing it in temporary variable...

movss DWORD PTR tv128[esp+4], xmm0

; ... and reloading it into ST0:

fld DWORD PTR tv128[esp+4]

pop ecx

ret 0

?float_rand@@YAMXZ ENDP

All instructions have the -SS suffix, which stands for “Scalar Single”.

“Scalar” implies that only one value is stored in the register.

“Single”^194 stands forfloatdata type.

1.31.6 Summary.

Only the lower half of XMM registers is used in all examples here, to store number in IEEE 754 format.

(^193) As an exercise, you may try to rework this code to eliminate the usage of the local stack.
(^194) I.e., single precision.