llvm dev - Mar 2016 - How to prevent clang/llvm from generating floating-point instructions?

Dear there,

I was trying to compile a code with only integer type variables and
integer operations. Clang/llvm kept showing me llvm code with
floating-point instructions (fmul, fadd, fptosi, etc.). Is there a way
in Clang or llvm to stop the compiler from doing that? My experiment
does not allow floating-point operations...

Thanks,
Xiaochu

Hi Xiaochu,

On 16 March 2016 at 11:49, Xiaochu Liu via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> I was trying to compile a code with only integer type variables and
> integer operations. Clang/llvm kept showing me llvm code with
> floating-point instructions (fmul, fadd, fptosi, etc.). Is there a way
> in Clang or llvm to stop the compiler from doing that? My experiment
> does not allow floating-point operations...
I think Clang's "-mno-implicit-float" is probably the option you
want.

I'm very surprised you're getting real floating operations like fadd
if your source really contains no floats though (it's mostly only used
implicitly for things like memcpy). So if that doesn't work, you
should probably investigate where they're coming from more deeply.
Maybe it's some external header you've included?

Cheers.

Tim.

Hi Tim,

Thanks for your message! It turns out that the infrastructure (an
outdated one) that I am working on is using gcc+dragonegg to generate
llvm code:

gcc -m32 -S -c -O0 -fplugin=$(DRAGONEGG_SO)
-fplugin-arg-dragonegg-emit-ir $< -o $@.tmp

It directly generates llvm code with fadd, etc. I'm not familiar with
dragonegg plugin...

Thanks,
XIaochu




On Wed, Mar 16, 2016 at 12:00 PM, Tim Northover <t.p.northover at
gmail.com> wrote:
> Hi Xiaochu,
>
> On 16 March 2016 at 11:49, Xiaochu Liu via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> I was trying to compile a code with only integer type variables and
>> integer operations. Clang/llvm kept showing me llvm code with
>> floating-point instructions (fmul, fadd, fptosi, etc.). Is there a way
>> in Clang or llvm to stop the compiler from doing that? My experiment
>> does not allow floating-point operations...
>
> I think Clang's "-mno-implicit-float" is probably the option
you want.
>
> I'm very surprised you're getting real floating operations like
fadd
> if your source really contains no floats though (it's mostly only used
> implicitly for things like memcpy). So if that doesn't work, you
> should probably investigate where they're coming from more deeply.
> Maybe it's some external header you've included?
>
> Cheers.
>
> Tim.