Hello again :)
On 06/12/2013 10:37 PM, Andy Wingo wrote:
On Wed 12 Jun 2013 15:37, Jan Schukat <[email protected]> writes:
If you want to access native uniform vectors from c, sometimes you
really want guarantees about the alignment.
16 bytes I guess? Guile's uniforms are 8-byte-aligned by default, as
you probably know.
Yes, 16 bytes. But more could be useful too. And as I have stated in my
previous mail, the first element of the vector is guaranteed to be 4
byte aligned on 32 bit machines, because it starts directly after the 3
word header, which is allocated at 8 byte boundaries. And yes, I have
tested this, but should be obvious from the code too.
Just wondering if there is a better default.
+#ifdef SCM_VECTOR_ALIGN
+ contents = scm_gc_malloc_pointerless (SCM_BYTEVECTOR_HEADER_BYTES
+ c_len + SCM_VECTOR_ALIGN,
+ SCM_GC_BYTEVECTOR);
+ ret = PTR2SCM (contents);
+ contents += SCM_BYTEVECTOR_HEADER_BYTES;
+ contents += (addr + (SCM_VECTOR_ALIGN - 1)) & -SCM_VECTOR_ALIGN;
+#else
contents = scm_gc_malloc_pointerless (SCM_BYTEVECTOR_HEADER_BYTES
+ c_len,
SCM_GC_BYTEVECTOR);
ret = PTR2SCM (contents);
contents += SCM_BYTEVECTOR_HEADER_BYTES;
+#endif
This is somewhat dangerous, as you could lose the pointer to the start,
and then the contents get collected.
I guess this can be fixed in master, if you set the "holder" field on a
bytevector to the actual memory that you allocate.
Don't really understand the danger here, isn't this allocated as a whole
block and only collected as a whole block too? What am I missing?
Having the arrays aligned according to their type by default could be a
nice option, i.e. a word of padding for long and doubles on 32 bit
machines, and then also introducing a new 16byte simd128 and 32 byte
simd256 type id and their respective creation functions.
Regards
Jan Schukat
