[LLVMdev] [Clang] Reasons for lack of -fsingle-precision-constant support? Alternatives?

[Reid Kleckner]

No real reason, I'm sure, other than a general desire to limit the size of
the zoo of flags that need maintenance.

On Wed, Jul 15, 2015 at 5:01 AM, ed at modk.it <ed at modk.it> wrote:

> Hi All,
>
> Clang lacks support for the -fsingle-precision-constant flag.  Are there
> specific reasons for this or is it just waiting to be implemented?
>
> This flag is especially important in the embedded world.  From
> http://processors.wiki.ti.com/index.php/Floating_Point_Optimization#float_vs._double_vs._long_double
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__processors.wiki.ti.com_index.php_Floating-5FPoint-5FOptimization-23float-5Fvs.-5Fdouble-5Fvs.-5Flong-5Fdouble&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=SziAjiSP8N_rg2b1QO6yqe6DPgAQ-gYuJ_OnsXjdwQo&s=q4wtrH7nx201oaODowm17ir7UAGlp2RfLweJDCQuXLA&e=>
> :
>
> *Once all of your data is defined as float, there are still cases where
> you may unknowingly cause the compiler to generate double precision
> operations. The most common issue is when floating point literals such as
> 3.14159 are used. In C these literals are of type double, and if they are
> used in an expression consisting of single precision operands, the
> operations will be promoted to double precision. The proper way to specify
> a single precision literal is to use an 'f' suffix, 3.14159f.*
>
> While defining literals without the 'f' suffix may point to performance
> issues, in our case the target crashes when the 'f' is omitted.  This is an
> end user programmable system where we can't expect users to add the 'f'.  I
> don't fully understand why this occurs (this is a bare metal system where
> user code is run by a quasi-OS which is maintained by a different team and
> itself is compiled with a different compiler so it's hard to track down) we
> remedied this with the -fsingle-precision-constant in gcc.  Now that we're
> moving to clang/llvm we need a solution and expecting the user to add 'f'
> is not going to work.
>
> Any thoughts?
>
> Thanks,
> Ed
>
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