| Originally posted by: nou i found a great feature which isnt make it clear in specification. in section 5.4.3.1 there are option -I. i call clBuildProgram(.., "-I.", ); and in kernel have #include "header.cl". and it work. |
That is awesome, I didn't know that, thanks!
| Originally posted by: nou another thing. in release notes is stated that every element of structure must be aligned to float4. but i write struct and i get sizeof() that structure is 324 byte. which is exactly the sum size of data types in the struct. i tested it only on CPU. is on GPU some restriction in alignment? |
The way I see it, you have to make sure that when using for example float4, you have to take care of it lying on a N*16 Byte address.
lets say we have a struct of three single floats, A, B, one float4 C and another float D.
The Memory for one instance is then (Memory offset in decimal notation)
00 A
04 B
08 C
12 C
16 C
20 C
24 D
sizeof() will tell you 28 Bytes, which is correct and expected, but OpenCL will not be able to access C at adress 8, because it can not load from there since it is not aligned to 16 Bytes. This is why you need to introduce 8 bytes of padding between B and C.
00 A
04 B
08 P
12 P
16 C
20 C
24 C
28 C
32 D
But now, when two structs are in an array, the alignment is shifted again!
00 A
04 B
08 P
12 P
16 C
20 C
24 C
28 C
32 D
36 A
40 B
44 P
48 P
52 C
56 C
60 C
64 C
68 D
OpenCL will not be able to access [1].C either, as it is on address 52, and thus not on the alignment boundary. So you need to add yet another 12 Bytes of padding at the end of the struct (in this example.. in reality you would just exchange D and C and remove one of the paddings)
00 A
04 B
08 P
12 P
16 C
20 C
24 C
28 C
32 D
36 P
40 P
44 P
sizeof is now 48 Bytes, which is a multiple of the largest required alignment 16, so all alignment issues should now be resolved
00 A
04 B
08 P
12 P
16 C
20 C
24 C
28 C
32 D
36 P
40 P
44 P
48 A
52 B
56 P
60 P
64 C
68 C
72 C
76 C
80 D
84 P
88 P
92 P
et voila: [1].C lies on address 64 – perfectly adressable by openCL!
struct { cl_float A; cl_float B; cl_float4 C; cl_float D; }; // with one padding struct { cl_float A; cl_float B; cl_char[8] P; cl_float4 C; cl_float D; }; // fully padded struct { cl_float A; cl_float B; cl_char[8] Padding; cl_float4 C; cl_float D; cl_char[12] morePadding; };
