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

[ed at modk.it]

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
:

*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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