[llvm-dev] How to prevent clang/llvm from generating floating-point instructions?

[Xiaochu Liu via llvm-dev]

I was wondering if there exists an LLVM pass that emulate floating
point operations with integers? Or simply just convert fp ops to int
ons in an approximate way... I found certain backend has software
float-point emulation support but not on IR-level.


On Wed, Mar 16, 2016 at 3:19 PM, Xiaochu Liu <xiaochu1122 at gmail.com> wrote:
> 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 outdated one) that I am working on is using gcc+dragonegg to generate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.