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0 pointsvardump7y ago0 comments

> High-locality of reference can be achieved in any language that supports unboxed types, it doesn't require C (even a very high-level language like Haskell supports this).

You often also need correct alignment. Cache-line or page. Your unboxed access across two pages can cause two TLB misses, L1 misses etc. Not to mention two page faults.

Sometimes you need to ensure two (or more) buffers are NOT aligned in a particular way to avoid interfering with CPU caching mechanisms.

0 comments

willtim7y ago

The only support C is giving you for this is that it has sized unboxed types (and raw pointer access). Even then, you'd have to trust the compiler and take measurements to be sure.

unwind7y ago

That's not true, since C11 we have:

    #include <stdlib.h>

    void *aligned_alloc(size_t alignment, size_t size);

which works like malloc() but lets you specify the required alignment.

willtim7y ago

Could I not just call such a function from my alternative language?

rurban7y ago

Only for sizes <4KB

vardumpOP7y ago

That's not true. You can also control data alignment in various ways. For example, you don't need to write to the beginning of an allocated buffer, but skip to the point where low order address bits are what you want.

Something like this for example:

  char* aligned_buf;
  char* buf;
  size_t max_align_offset = (1<<align) - 1;
  buf = malloc(length_needed + max_align_offset);
  aligned_buf = (buf + max_align_offset) & ~max_align_offset;

In the example, if align==8, you have 256 byte alignment. If it's 12, 4kB alignment.

pjmlp7y ago

Good luck ensuring that doesn't trigger UB across all target architectures and compilers being used.

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willtim7y ago

This example looks like it should be turned into a malloc variant, i.e. needs only to be written once. Raw pointer access is also available in other languages of course, albeit it is usually made much more difficult.

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0 pointsvardump7y ago0 comments

> High-locality of reference can be achieved in any language that supports unboxed types, it doesn't require C (even a very high-level language like Haskell supports this).

You often also need correct alignment. Cache-line or page. Your unboxed access across two pages can cause two TLB misses, L1 misses etc. Not to mention two page faults.

Sometimes you need to ensure two (or more) buffers are NOT aligned in a particular way to avoid interfering with CPU caching mechanisms.

0 comments

willtim7y ago

The only support C is giving you for this is that it has sized unboxed types (and raw pointer access). Even then, you'd have to trust the compiler and take measurements to be sure.

unwind7y ago

That's not true, since C11 we have:

    #include <stdlib.h>

    void *aligned_alloc(size_t alignment, size_t size);

which works like malloc() but lets you specify the required alignment.

willtim7y ago

Could I not just call such a function from my alternative language?

rurban7y ago

Only for sizes <4KB

vardumpOP7y ago

Something like this for example:

  char* aligned_buf;
  char* buf;
  size_t max_align_offset = (1<<align) - 1;
  buf = malloc(length_needed + max_align_offset);
  aligned_buf = (buf + max_align_offset) & ~max_align_offset;

In the example, if align==8, you have 256 byte alignment. If it's 12, 4kB alignment.

pjmlp7y ago

Good luck ensuring that doesn't trigger UB across all target architectures and compilers being used.

1 more reply

willtim7y ago

j / k navigate · click thread line to collapse