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Nicolas Capens

2008-May-08 15:24 UTC

[LLVMdev] Vector code

Hi all,

 

I'm trying to use LLVM to generate SIMD code at runtime (in particular Intel
SSE). But I'm having a bit of trouble understanding how to create even the
simplest function; adding two vectors of four single-precision
floating-point elements. I can get it to add the elements one at a time but
not using one vector instruction.

 

All help much appreciated!

 

Nicolas Capens

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Nicolas Capens

2008-May-08 17:09 UTC

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[LLVMdev] Vector code

Hi Chris,

Thanks for the advise, but I'm actually not trying to compile code from
text. For now I'm just trying to construct the function directly. Think of
it as the vector equivalent of the HowToUseJIT.cpp example.

Cheers,

-Nicolas


-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Chris Lattner
Sent: Thursday, 08 May, 2008 19:14
To: LLVM Developers Mailing List
Subject: Re: [LLVMdev] Vector code

On Thu, 8 May 2008, Nicolas Capens wrote:> I'm trying to use LLVM to generate SIMD code at runtime (in particular
Intel> SSE). But I'm having a bit of trouble understanding how to create even
the
> simplest function; adding two vectors of four single-precision
> floating-point elements. I can get it to add the elements one at a time
but> not using one vector instruction.
I'd suggest writing code in C and seeing what llvm-gcc does with it. 
You can also look at (for example) llvm/test/CodeGen/X86/*.ll for many 
examples.

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/
_______________________________________________
LLVM Developers mailing list
LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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Chris Lattner

2008-May-08 17:14 UTC

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[LLVMdev] Vector code

On Thu, 8 May 2008, Nicolas Capens wrote:> I'm trying to use LLVM to generate SIMD code at runtime (in particular
Intel
> SSE). But I'm having a bit of trouble understanding how to create even
the
> simplest function; adding two vectors of four single-precision
> floating-point elements. I can get it to add the elements one at a time but
> not using one vector instruction.
I'd suggest writing code in C and seeing what llvm-gcc does with it. 
You can also look at (for example) llvm/test/CodeGen/X86/*.ll for many 
examples.

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/

Dan Gohman

2008-May-08 17:47 UTC

head link

[LLVMdev] Vector code

On Thu, May 8, 2008 8:24 am, Nicolas Capens wrote:> Hi all,
>
>
>
> I'm trying to use LLVM to generate SIMD code at runtime (in particular
> Intel
> SSE). But I'm having a bit of trouble understanding how to create even
the
> simplest function; adding two vectors of four single-precision
> floating-point elements. I can get it to add the elements one at a time
> but
> not using one vector instruction.
What is your target set to? If LLVM thinks it's targeting a processor
that doesn't have SIMD instructions, it'll split vectors into
scalars like this.

Dan

Anton Korobeynikov

2008-May-08 17:58 UTC

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[LLVMdev] Vector code

Nicolas,
> Thanks for the advise, but I'm actually not trying to compile code from
> text. For now I'm just trying to construct the function directly. Think
of
> it as the vector equivalent of the HowToUseJIT.cpp example.llvm2cpp is your friend then. It's now a separate 'target' in llc.
It
will generate C++ code, which will construct provided IR.

-- 
With best regards, Anton Korobeynikov.

Faculty of Mathematics & Mechanics, Saint Petersburg State University.

Nicolas Capens

2008-May-08 18:37 UTC

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[LLVMdev] Vector code

Hi Dan,

My CPU supports up to SSSE3, and I assume LLVM uses that as a target by
default? I don't think that's the problem really, I'm just
struggling to
find the right functions/classes to create and manipulate vectors...

Thank you,

-Nicolas


-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Dan Gohman
Sent: Thursday, 08 May, 2008 19:47
To: LLVM Developers Mailing List
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Vector code

On Thu, May 8, 2008 8:24 am, Nicolas Capens wrote:> Hi all,
>
>
>
> I'm trying to use LLVM to generate SIMD code at runtime (in particular
> Intel
> SSE). But I'm having a bit of trouble understanding how to create even
the
> simplest function; adding two vectors of four single-precision
> floating-point elements. I can get it to add the elements one at a time
> but
> not using one vector instruction.
What is your target set to? If LLVM thinks it's targeting a processor
that doesn't have SIMD instructions, it'll split vectors into
scalars like this.

Dan


_______________________________________________
LLVM Developers mailing list
LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Nicolas Capens

2008-May-08 18:40 UTC

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[LLVMdev] Vector code

Hi Anton,

I assume that's the same as the online demo's "Show LLVM C++ API
code"
option (http://llvm.org/demo/)? I've tried that with a structure containing
four floating-point components but it also appears to add them individually
using extract/insert. Maybe I have to try an array of floats...

Thanks,

Anton


-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Anton Korobeynikov
Sent: Thursday, 08 May, 2008 19:59
To: LLVM Developers Mailing List
Subject: Re: [LLVMdev] Vector code

Nicolas,
> Thanks for the advise, but I'm actually not trying to compile code from
> text. For now I'm just trying to construct the function directly. Think
of
> it as the vector equivalent of the HowToUseJIT.cpp example.llvm2cpp is your friend then. It's now a separate 'target' in llc.
It
will generate C++ code, which will construct provided IR.

-- 
With best regards, Anton Korobeynikov.

Faculty of Mathematics & Mechanics, Saint Petersburg State University.


_______________________________________________
LLVM Developers mailing list
LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

B. Scott Michel

2008-May-14 22:58 UTC

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[LLVMdev] Vector code

From the cellspu's test harnesses:


define <4 x float> @fp_add_vec(<4 x float> %arg1, <4 x float>
%arg2) {
%A = add <4 x float> %arg1, %arg2
ret <4 x float> %A
}

It's a simple vector add for two v4f32 quantities. There are things you 
need to do in order to make the optimizer not optimize, e.g.:

v4f32 fp_add_vec(v4f32 arg1, v4f32 arg2) {
return arg1 + arg2;
}

Hope this helps.


-scooter

Nicolas Capens wrote:>
> Hi all,
>
> I’m trying to use LLVM to generate SIMD code at runtime (in particular 
> Intel SSE). But I’m having a bit of trouble understanding how to 
> create even the simplest function; adding two vectors of four 
> single-precision floating-point elements. I can get it to add the 
> elements one at a time but not using one vector instruction.
>
> All help much appreciated!
>
> Nicolas Capens
>
> ------------------------------------------------------------------------
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>

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