Daryl McDaniel [mailto:edk2-li...@mc2research.org] wrote:
]Sent: Saturday, December 05, 2015 11:00 AM
]To: 'Shubha Ramani' <shubharam...@yahoo.com>; edk2-de...@ml01.01.org
]Subject: Re: [edk2] Double or floating point on edk2 ?
]
]Hello,
]
]Doing floating point operations within the UEFI environment
]is not recommended. There are many reasons, but the main one
]is that floating point exceptions are unlikely to be handled.
I think the need to enable and trap floating point exceptions
is rare. For most applications, the normal masked response
works well. Here is Intel's official position:
"Exception-handling software is often difficult to write,
and the masked responses have been tailored to deliver the
most reasonable result for each condition. For the majority
of applications, masking all exceptions yields satisfactory
results with the least programming effort."
]The default FPU mode (rounding, precision, ...) may also not
]be what you would want.
I think the default FPU mode meets the requirements of the C
programming language. But if not, it could be changed.
]There is also a chance that one or more of the UEFI functions
]may destroy a floating point or vector register that you are using.
Is this really true? I think the compiler will preserve the registers
when needed for C code. ASM code could certainly destroy floating
point registers. A problem would occur if SMI or the timer interrupt
code were to use floating point registers. Could it be possible
for timer callback code to use floating point and destroy registers
of the interrupted application?
]That said, there is nothing that would prevent you from using
]floating point types and operations in your Shell application.
]If you do, it is your responsibility to ensure that the environment
]is properly set up for your needs. The UEFI specification details
]what registers are preserved, or not.
]
]Using StdLib does not alleviate this responsibility. The only thing
]StdLib does is ensure that the correct rounding, floor, and NAN modes
]are set in the FPU in order to conform to the ISO/IEC 9899 Addendum-1
](C95) specification. The sole purpose of StdLib's floating point
]support is to facilitate porting existing C applications to the UEFI
]environment.
]
]Unfortunately, using floating point is entirely at your own risk.
The good thing is the problem with unresolved external symbol __dtoui3
is gone with the current EDK2. This problem used to occur when building
AppPkg for IA32 using VS2013 or newer. It no longer occurs because of
"BaseTools: Add /arch:IA32 option in VS2012 and VS2013". Though that
change was made to prevent the compiler from generating the cmov
instruction, it also has the side effect of turning off SSE2 math.
Thanks,
Scott
]Sincerely,
]Daryl McDaniel
> -----Original Message-----
> From: edk2-devel [mailto:edk2-devel-boun...@lists.01.org] On Behalf Of Shubha
> Ramani
> Sent: Friday, December 04, 2015 4:39 PM
> To: edk2-devel@lists.01.org
> Subject: [edk2] Double or floating point on edk2 ?
>
> I know this question has been answered before. Do I need to convert my
> application to a UEFI StdLib applicationto do double or
> floating point math ? Can't do it in a regular Shell App ?
> Please illuminate.
> Thanks,
> Shubha Shubha D. ramanishubharam...@gmail.com
> shubharam...@yahoo.com
> _______________________________________________
> edk2-devel mailing list
> edk2-devel@lists.01.org
> https://lists.01.org/mailman/listinfo/edk2-devel
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