Note: I decided to repost from this thread http://www.vgchartz.com/forum/thread.php?id=32540&page=25 as it was already 25 pages deep.
@Everyone who is talking about SPE's not liking branchy code ... and actually knows how to code (Deneidez)
The issue is data parallelism (SIMD) not instruction parallelism (threads)
The reason people have difficultly getting decent performance out of the CBE is that compilers usually just give up when they are presented with a branch in an inner loop. Eg. the compiler has no problem vectorizing this
float a[N], b[N], c[N];
for (i = 0; i < N; i++)
a = b + c;
into something like this (going to use SSE intrinsics because I know most here -- that are actually programmers -- are more familiar with x86, but altivec works the same way)
__m128 *av, *bv, *cv;
av = (__m128*)a; // assume 16-byte aligned, but all u need to do is allocate with declspec or aligned malloc
bv = (__m128*)b;
cv = (__m128*)c;
for (i = 0; i < N/4; i++)
av = _mm_add_ps(bv, cv);
But, every compiler I know of will just give up on this
for (i = 0; i < N; i++)
if (a > 0)
a = b / c;
However, if the programmer knows what he is doing he can eliminate the branch by via logical operations:
__m128 *av = (__m128*)a;
__m128 *bv = (__m128*)b;
__m128 *cv = (__m128*)c;
__m128 zeros = _mm_setzero_ps();
for (i = 0; i < N/4; i++)
{
__m128 x = _mm_div_ps(bv, cv);
__m128 g = _mm_cmplt_ps(av, zeros);
__m128 y = _mm_andnot_ps(g, av);
__m128 z = _mm_and_ps(g, x);
av = _mm_or_ps(y, z);
}
Now, on most intel machines you are highly constrained by register pressure because the machines only have 8 simd registers making vectorization impractical for large loops without heavy amounts of loop fission which may not be possible. The SPEs on the other hand have 128 SIMD registers per-core which for the applications I've developed, register pressure was a non-factor for traditional loop vectorization techniques. And the stuff I've been working on atm is quite branchy.
(PS there may be some errors as I did this quickly, but the general principle is correct)
| Forum > Gaming Discussion > Xenon vs Cell Which one really is better ?? | ||||||
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NNN2004  Message Add FriendPosts: 1,906 vg$ 797.41 Jul 09, 2008 19:49 GMT |
thnx dpot .. this is the most useful post so far .. now from what i read i found that the 3 cores for xenon is PPE & for Cell is 8 PPE/SPE cores ... also some developers found the SPEs utterly useless for executing anything .. but the SPEs is far better in accelerate physics .. this a little from what i read if u want see the link and post ur comment ... maybe i post something wrong .. who knows.
____- PLAYSTATION®3 is the future.....NOW.......B_E_L_I_E_V_E its not BAN THE FAN
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Deneidez |
