[LLVMdev] MMX/SSE subtarget feature in IR

[suyog sarda]

Thanks Kevin for the reply. I got the point now :)
On 10 Apr 2015 00:18, "Smith, Kevin B" <kevin.b.smith at intel.com> wrote:

>  For x86_64 ABI, a minimum feature set of SSE2 is required.
>
>
>
> Kevin
>
>
>
> *From:* llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] *On
> Behalf Of *suyog sarda
> *Sent:* Thursday, April 09, 2015 11:27 AM
> *To:* LLVM Developers Mailing List; David Majnemer; Sanjay Patel
> *Subject:* [LLVMdev] MMX/SSE subtarget feature in IR
>
>
>
> Hi all,
>
>
>
> I have a sample test case :
>
>
>
> $ cat 1.c
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>
>
> int foo(int x, int y){
>
>   int z = x + y;
>
>   return z/2;
>
> }
>
>
>
> I tried to get its IR form with clang providing subtarget feature as mmx
> for target x86_64
>
>
>
> $ clang -O3 -mmmx 1.c -S -emit-llvm
>
>
>
> in the IR generated i can see the subtarget-features as function attribute
> :
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>
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> "target-features"="+mmx"
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>
>
> In the SelectionDAG phase in file "X86ISelLowering.cpp", i checked in one
> of the function  what is the subtarget feature by calling few routines of
> Subtarget.
>
>
>
> Subtarget->hasMMX()  ------ true
>
> Subtarget->hasSSE1() ------ true
>
> Subtarget->hasSSE2() ------ true
>
>
>
> These functions just compare the X86SSELevel with subtarget enum values
> like MMX, SSE1, SSE2 etc.
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>
>
> hasMMX() { return X86SSELevel >= MMX}; // similar for others
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>
>
> Now, enum values start from  MMX and goes on increasing with SSE1, SSE2,
> etc.
>
>
>
> For the above test case, the X86SSELevel is set to SSE2, which is
> confusing, since i have explicitly specified the target feature as 'mmx'.
> Why the X86SSELevel is getting set to SSE2 despite providing target-feature
> as mmx? Is it something to do with default feature of x86_64?
>
>
>
> I wanted to distinguish my code generation based on subtarget feature for
> MMX and SSE, which i am unable to do so currently because of above scenario.
>
>
>
> I am sure i am missing something fundamental, but unable to exactly find
> out what.
>
>
>
> Can someone please help me out on above question?
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>
>
> Thanks.
>
>
>
> Regards,
>
> Suyog Sarda
>
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