[LLVMdev] Support for Soft-float

[Eric Cheng]

Hi,

I'm trying to generate some SPARCv8 assembly for a sparc target that doesn't have an FPU. I'm unable to get the flow to generate calls to a soft-float library. Since I wasn't able to find a definitive answer, I was hoping someone might be able to offer some pointers or shed some light.

Running "clang -c -emit-llvm -msoft-float test.c -o test.bc" doesn't generate IR with soft-float library calls (perhaps understandably) - only hard float instructions. I would expect the frontend to at least be able to generate the library calls even though I'm not asking it to link to any library yet.

Running "llc -march=sparc -soft-float test.bc -o test.s" next still does not replace the hard float instructions with soft-float calls. Is this a known issue, am I doing something wrong, etc.?

Just as a test, I see that running "llc -march=arm -soft-float test.bc -o test.s" will indeed generate the appropriate soft-float library call. I checked using mips, x86, ppc32, etc. and it seems that only the arm target actually generates the soft-float call.

Is there a reason many of the backend targets will not generate the library call? Is there some way to fix this?

Thanks,
Eric

[Asiri Rathnayake]

Hi Eric,

> Running "clang -c -emit-llvm -msoft-float test.c -o test.bc" doesn't generate IR with soft-float library calls (perhaps understandably) - only hard float instructions. I would expect the frontend to at least be able to generate the library calls even though I'm not asking it to link to any library yet.

I think you have to explicitly pass the target. For an example:

./clang -c -emit-llvm -msoft-float test.c -o test.bc -###

Does not show -msoft-float passed into the backend. Whereas the following invocation does:

./clang -target sparc-none-linux-gnu -c -emit-llvm -msoft-float test.c -o test.bc -###

May be give this a try and see if it produces the correct lib calls?

I happened to be working on something related (for ARM).

- Asiri

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[Asiri Rathnayake]

And, Apologies for the footer. Wrong account.

Best,

- Asiri

[Eric Cheng]

Hi Asiri,

Unfortunately, I had also tried using the -target switch previously ("-target sparc", "-target arm", etc.). However, since it ultimately made difference, I left it out of my example for brevity.

Thanks,
Eric

On Thu, Sep 25, 2014 at 3:02 AM, Asiri Rathnayake <asiri.ra...@arm.com> wrote:

And, Apologies for the footer. Wrong account.

Best,

- Asiri

-----Original Message-----
From: llvmdev...@cs.uiuc.edu [mailto:llvmdev...@cs.uiuc.edu] On
Behalf Of Asiri Rathnayake
Sent: 25 September 2014 10:48
To: Eric Cheng; llv...@cs.uiuc.edu
Subject: Re: [LLVMdev] Support for Soft-float

[Renato Golin]

On 24 September 2014 07:02, Eric Cheng <ecc...@stanford.edu> wrote:
> Running "clang -c -emit-llvm -msoft-float test.c -o test.bc" doesn't
> generate IR with soft-float library calls (perhaps understandably) - only
> hard float instructions. I would expect the frontend to at least be able to
> generate the library calls even though I'm not asking it to link to any
> library yet.

Hi Eric,

If you mean "IR floating point instructions", than this is correct.
The front-end doesn't know what the back-end is capable of (hard
multiply, divide etc), so it emits "fp instructions" and the back-end
will then generate the correct call/instruction for each separate
case.

> Running "llc -march=sparc -soft-float test.bc -o test.s" next still does not
> replace the hard float instructions with soft-float calls. Is this a known
> issue, am I doing something wrong, etc.?

Maybe the Sparc back-end is not able (or willing) to transform the
instructions into library calls. The ARM back-end certainly does.

> Is there a reason many of the backend targets will not generate the library
> call? Is there some way to fix this?

It may be that they never found it relevant? Or that your options is
not correctly propagated (as Asiri commented).

Looking at X86ISelLowering.cpp, there aren't that many calls to
setLibcallName(), which there are zillions for ARM. The Sparc back-end
has more than the X86, but mostly to round numbers and convert from FP
to Int and vice-versa, so I doubt it'll ever call a library function
to multiply or divide floating points.

If that's the only problem, the way to fix this is to repeat the
pattern in the ARM back-end, and add *all* library call hooks and make
sure they're called when -msoft-float is specified. Also, if you do
so, please add as many tests as necessary to make sure both soft and
hard float are covered correctly.

cheers,
--renato