[Openmcl-devel] Floating point exceptions on Windows

[Martin]

(I had sent this reply earlier, but it seems to have been copied only to 
R. Matthew Emerson. Apologies if it appears multiple times in the list.)

R. Matthew Emerson wrote:
> On May 3, 2009, at 11:18 AM, Martin wrote:
> 
>> Hi!
>>
>> I am using CCL 1.3-r11949 on Windows XP SP2 and have a problem with
>> floating point exception propagation.
>>
>> Now, I have a file app1.c with a function with the following code  
>> extract:
>>
>>
>> __declspec(dllexport) double fn1 ()
>> {
>>   double zero = 0;
>>   return 1.0/zero;
>> }
>>
>> I compiled it into a DLL (Using MS Visual C++ 2005 Express Edition)  
>> with
>> the following command:
>>
>> cl /LD app1.c
>>
>> to yield app1.dll.
>>
>> Using it in CCL:
>>
>> Welcome to Clozure Common Lisp Version 1.3-r11949M  (WindowsX8632)!
>> ? (open-shared-library "D:/studies/floats/app1/app1.dll")
>> #<SHLIB app1.dll #x89DED5E>
>> ? (ff-call (%reference-external-entry-point (external  
>> "fn1")) :double-float)
>> 1.0000000000000002D0
>> ? (log 10)
>>> Error: DIVISION-BY-ZERO detected
>>>       performing LOG on (10.0)
>>> While executing: CCL::%FP-ERROR-FROM-STATUS, in process listener(1).
>>> Type :POP to abort, :R for a list of available restarts.
>>> Type :? for other options.
>> 1 >
> 
> This is kind of a long shot, but...if you disassemble fn1(), does it  
> use x87 or SSE2 instructions?
> 

fn1() seems to be using x87 instructions.

(I don't know much assembly myself---actually trying to learn at least 
for to be able to read compiler sources.)

Regardless:

Compiling app1.c with the assembly output option, _fn1() compiles to:


_fn1    PROC
     push    ebp
     mov    ebp, esp
     sub    esp, 8
     fldz
     fstp    QWORD PTR _zero$[ebp]
     fld1
     fdiv    QWORD PTR _zero$[ebp]
     mov    esp, ebp
     pop    ebp
     ret    0
_fn1    ENDP

(A disassembly of fn1() in the compiled DLL shows the same code
sequence.)

(
Compiling with the /arch:SSE2 flag added yields:

_fn1    PROC
     push    ebp
     mov    ebp, esp
     sub    esp, 8
     movsd    xmm0, QWORD PTR __real at 0000000000000000
     movsd    QWORD PTR _zero$[ebp], xmm0
     fld1
     fdiv    QWORD PTR _zero$[ebp]
     mov    esp, ebp
     pop    ebp
     ret    0
_fn1    ENDP

So, I suspect my conclusion is correct.
)

> On non-Darwin x8632 systems (including Windows), we assume that  
> foreign code will use the x87 for floating-point operations.  After  
> the ff-call, we therefore will look at the x87 state to see if the  
> foreign code signaled any fp exceptions.   If the foreign code uses  
> SS2E, we won't notice any exceptiopns (because we're looking for them  
> in the wrong place: the x87 state, and not the mxcsr).
> 
>>  2. How can I disable floating point exceptions from within CCL?
> 
> The manual documents SET-FPU-MODE and GET-FPU-MODE, which will let you  
> do this.
> 
> http://ccl.clozure.com/ccl-documentation.html#floating-point


Thanks. I hadn't thought to search the manual and/or sources. Just tried 
can now disable signaling.

Regards,
Martin.