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From: | Jan Schukat |
Subject: | bug#14599: An option to make vector allocation aligned |
Date: | Thu, 13 Jun 2013 09:07:41 +0200 |
User-agent: | Mozilla/5.0 (X11; Linux i686; rv:17.0) Gecko/20130510 Thunderbird/17.0.6 |
Hello again :) On 06/12/2013 10:37 PM, Andy Wingo wrote:
On Wed 12 Jun 2013 15:37, Jan Schukat <address@hidden> 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; +#endifThis 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
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