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Re: [MIPS] Add sbasic supoert ffor MSA (SIMD)

On 28/05/14 09:03, Matthew Fortune wrote:
>> You shouldn't need to declare __builtin_* functions anyway.  And if a
>> function can be represented directly with GNU C vector extensions, it's
>> preferred to implement it that way inline in the header rather than having
>> built-in functions duplicating existing GNU C functionality.  (Look at
>> what AArch64 arm_neon.h does where possible, and what ARM arm_neon.h has
>> been moved towards lately.  I don't now what the msa.h functions do, so I
>> don't know if this actually applies to any of them - but it's something to
>> consider, so that built-in functions are only defined where actually
>> needed.)
> In the aarch64 arm_neon.h header there are a decent number of inline asm
> implementations too instead of builtins. It is not immediately obvious to me
> as to what the deciding factor is between adding a builtin and using inline
> asm when vector extensions do not support the operation. Do you happen to
> know why inline asm is used in places?

Speed of implementation.  We're gradually replacing these with proper
builtins, but that takes a lot more work.

> This looks like a reasonable idea to use GNU extensions where available. The
> down-side to this approach is that it may be necessary to write quite
> dis-similar headers for LLVM vs GCC which I think is part of the reason why
> the header is written as it is. I don't know if that is a good reason to
> require builtins or not though.

I regard these headers as part of the compiler.  In an ideal world the
contents of arm_neon.h should be replacable with

#pragma GCC neon_intrinsics

which would make parsing pretty much instantaneous.

It's never that simple, though; sadly :-(


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