The SSE-ops lack a huge amount of very usefull functions. Basically every single mathematical relevant function is absent, worst, trying to re-create them and reach the same precision as the FPU-routines results in magnitude slower code. Sometimes one can find a usefull short-cut, but not very often.
Here I tried to make a cbrt(x), the cubic [...]
padddq (128bit + 64bit = 128bit):
// carry emulation via pcmpgtq equivalent
movdqa xmmx, xmma
movdqa xmmy, xmmb
movdqa xmmz, xmma
paddq xmma, xmmb
pcmpgtd xmmx, xmmb
pcmpgtd xmmy, xmma
pcmpeqd xmmz, xmmb
pshufd xmmx, xmmx, 1|1|1|1
pshufd xmmz, xmmz, 1|1|1|1
pand xmmz, xmmy
por xmmz, xmmx
punpcklqdq xmmz, xmmz
pslldq xmmz, 8 or pshufd
psubq xmma, xmmz
When you don’t have SSE4 available you have to compensate for the lack of ome essencial ops. In this series I’m going to write down equivalents of either missing or (currently) non-AMD ops.
pcmpeqq (64bit integer compare):
// equality == (hi == hi) && (lo == lo)
movdqa xmm?3, xmma
pcmpeqd xmm?3, xmmb
pshufd xmm?1, xmm?3, 3|3|1|1 (b == a)
pshufd xmm?2, xmm?3, 2|2|0|0 (B == A)
pand xmm?1, [...]