Auto-Vectorization and array/vector operations

[Iain Buclaw via Digitalmars-d]

On 16 July 2015 at 00:42, Steven via Digitalmars-d
<digitalmars-d at puremagic.com> wrote:
> I was trying to show someone how awesome Dlang was earlier, and how the
> vector operations are expected to take advantage of the CPU vector
> instructions, and was dumbstruck when dmd and gdc both failed to
> auto-vectorize a simple case.  I've stripped it down to the bare minimum and
> loaded the example on the interactive compiler:
> http://asm.dlang.org/#%7B%22version%22%3A3%2C%22filterAsm%22%3A%7B%7D%2C%22compilers%22%3A%5B%7B%22sourcez%22%3A%22JYWwDg9gTgLgBAY2gUwHQGdQBMDcAoPAMwBsIBDGAbQCYBWANgF05kAPM8Y5AQQAoTyVOkzhkANHAEUaDZgCMAlHgDeeOJNLThzBPnUB6fXCjJ0AV2Ix0cYADs45uenRrElZgF5R7uAFo4cu56xsgwZlD2ungAvgRSQrIs7JzIAEL8mgki4hqCMiKKKq7xAByUAMzUjABuZHBeCGToMBmCZZWMCmTBpRVV1XL1iE0tvR0Kcj2Z7f1R6q6GIeaW1nYOZnJgLurVCD5etT7%2BA0EE6iZhEcPNrVqyCrv40UAAA%3D%22%2C%22compiler%22%3A%22dmd2067%22%2C%22options%22%3A%22-O%20-release%20-inline%20-boundscheck%3Doff%22%7D%5D%7D
>
> The reference documentation for arrays says:
> Implementation note: many of the more common vector operations are expected
> to take advantage of any vector math instructions available on the target
> computer.
>

DMD makes leverage of vector operations in the library, rather than in
the generated code.  So as long as you are doing array operations
using any of the supported types...


> Does this mean that while compilers are expected to take advantage of them,
> they currently do not, even when they have proper alignment?  I haven't
> tried LDC yet, so maybe LDC does perform auto-vectorization and I should
> attempt to use LDC if I plan on using vector ops a lot?
>

Auto-vectorization is deliberately strict in what triggers it to occur.

It is possible to give the compiler hints, however I'm not sure that
this should be done by the code generator.

See, for example: http://goo.gl/iMBbRs

Regards
Iain