Vector Class Discussion

Fallbacks for gatherXX()? - Chris Newbold - 2017-07-10

Fallbacks for gatherXX()? - Agner - 2017-07-10

Fallbacks for gatherXX()? - Chris Newbold - 2017-07-11

Fallbacks for gatherXX()? - Agner - 2017-07-11

Fallbacks for gatherXX()? - Chris Newbold - 2017-07-11

Fallbacks for gatherXX()? - Agner - 2017-07-11

Fallbacks for gatherXX()?

Author:

Date: 2017-07-10 13:58

I've been playing around with some toy examples coded with VCL and exploring the potential for write-once compile-many code to target multiple CPU architectures at runtime. I coded my first examples with AVX2 in mind, and wound up using gather4d() and gather8f() quite often. When I then tried compiling for lesser architectures, I discovered that these functions are undefined. Is this an oversight or intended? If intended, is there a recommended alternate coding pattern that works across multiple architectures? Thanks.

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Fallbacks for gatherXX()?

Author: Agner	Date: 2017-07-10 23:35
You may use the lookup functions. Gathering non-contiguous data is very inefficient anyway on CPUs that don't have these instructions. Sometimes you can avoid the problem by reorganizing the data or by using permute functions.

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Fallbacks for gatherXX()?

Author:

Date: 2017-07-11 07:25

Thanks. In my case I'm actually trying to write the routines that will reorganize data into a more friendly layout... Here's another one that looks more like an error; in this case I'm trying to compile blend4d for plain AVX:

In file included from vectorclass/vectorclass.h:51:
vectorclass/vectorf256.h:2485:17: error: no case matching constant switch condition '17' [-Werror]
switch (s2) {
^~
vectorclass/vectorf256.h:2556:16: note: in instantiation of function template specialization 'permute4d<1, 3, 5, 7>' requested here
return permute4d<i0, i1, i2, i3> (a);
^
foo.cpp:421:32: note: in instantiation of function template specialization 'blend4d<1, 3, 5, 7>' requested here
const Vec4d out1 = blend4d<1, 3, 5, 7>(in1, in2);

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Fallbacks for gatherXX()?

Author: Agner	Date: 2017-07-11 10:12
I don't know why you get this message. Which compiler are you using? You can ignore this error because the case s2 = 17 is covered in line 2444 above so it never gets to line 2485. BTW, the indexes i2, i3 are out of range, but that is not the cause of the message.

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Fallbacks for gatherXX()?

Author:

Date: 2017-07-11 11:19

The error manifests with Apple LLVM version 8.0.0 (clang-800.0.42.1) which is Xcode 8.1; same code compiles without incident on GCC 4.9. Agree with your assessment; seems like the warning from clang is too aggressive.

Regarding your comment on the out-of-range indexes, perhaps I'm mis-understanding how to use blend. With two Vec4d inputs, isn't this legal: blend4d<1, 3, 5, 7>(i1, i2) ? This seemed consistent with the documentation and examples for blend4d.

Thanks!

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Fallbacks for gatherXX()?

Author: Agner	Date: 2017-07-11 13:29
Sorry, the indexes would be out of range with permute4d, not with blend4d

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