SSE instruction error

[inteleverywhere]

Could someone please point out what the problem is with the following snippet. An exception occurs in MSVC.

__m128i R0, R1;

__m128i *R0P1, *R0P2;

__m128i *R1P1, *R1P2;

char buf_chr[20] = { 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',

'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't'

};

R0 = _mm_loadu_si128((__m128i*) (buf_chr));

R1 = _mm_loadu_si128((__m128i*) (buf_int));

R0P1 = (__m128i *) (buf_chr);

R0P2 = (__m128i *) (buf_chr);

R0 = _mm_add_epi8(R0, R1);

*R0P1 = _mm_add_epi8((__m128i) *R0P1, (__m128i) *R0P2); // ---------> Exception occurs here.

Is there a way to do the addition if I have a register used as a pointer. Can de-referencing be done by using *? This question arises because the following statements pass without incident!

R0P1++;

R0P2++;

Thanks and Regards
Deepak

[Arthur_Moroz]

Check this: sizeof(__m128i) is 16 bytes. After first increment of R0P1 you are out of your fixed array boundary (20 bytes).

[inteleverywhere]

Hi, It happens (the exception) for the very first access. The size of the array is enough for the first access. the problem may be something else. I am interested in the meaning of *Register when Register is used as a pointer (__m128i*). Does *(__m128i*) mean the 16 bytes pointed by the regsiter? Thanks and Regards Deepak

[Arthur_Moroz]

I've just reproduced your result. It happens because char[] is generally not aligned to 16 byte boundaries.

[inteleverywhere]

My dear friend that is not a problemsince I have used an unaligned load instruction. Please assume everything is right with respect to the buffer. Please read my question with regard to the use of a register as a pointer. What is the possibility to access the content pointed to by the register. I am also working on it and shall get back to you soon.

Thanks and Regards
Deepak

[Arthur_Moroz]

My dear friend,

I know how to use debugger. And in debugger I see that your line

*R0P1 = _mm_add_epi8((__m128i) *R0P1, (__m128i) *R0P2);

is translated to

movdqa xmm0,xmmword ptr [eax]
[..skipped..]
paddb xmm1,xmm0

Guess where exception happens if eax is not 16B aligned?

[inteleverywhere]

Sorry and thanks mate. Well actually you are right. I had triedin vainand thought thesolution was this

__m128i* R0P1;

Register1 =_mm_loadu_si128(R0P1);

You are right *R0P1 works if the buffer pointed to by R0P1 is aligned to a 16 byte boundary.

Register1 = *R0P1;

Follow up question: What if it is not an aligned address?Can we use the *operator to get the contents.


Regards
Deepak

[Arthur_Moroz]

If it is not aligned it is fatser to use usual registers (eax, edx, etc) and don't bother with SSE optimization