Dealing with Floating-point Exceptions in MSVC7\8 : Page 4

The Visual Studio Compiler Options
For both Debug and Release configurations, Visual Studio offers the "Code Generation" property page in "C/C++" property folder. Here, you can select the desired option for the "Enable Enhanced Instruction Set" property. The available options are:

1. "Not Set"
2. "Streaming SIMD Extensions (/arch:SSE):" The new set of instructions and registers on the Pentium III processor allows Single-Instruction Multiple-Data (SIMD) computations to be made on single-precision floating-point numbers. You may see the assembler code for both options above in a disassembly window during debugging that looks like:

   
       c = a/b;  //inside a __try block
            100114A8  fld         qword ptr [a] 
            100114AB  fdiv        qword ptr [b] 
            100114AE  fstp        qword ptr [c] 
            100114B1  wait             
       }
       100114B2  wait             
       100114B3  mov         dword ptr [ebp-4],0FFFFFFFEh 
       100114BA  jmp         $LN6+12h (100114EBh) 
       _except(EvaluateException(GetExceptionCode()))
   

   Because the assembly code is the same for both options, no changes are needed for the source code in order to trap floating-point exceptions when the option is set to /arch:SSE.
3. "Streaming SIMD Extensions 2 (/arch:SSE2)": The new set of instructions on the Pentium 4 and Intel Xeon processors operates on the XXM and MXCSR registers and performs SIMD computations on double-precision floating-point and integer numbers. The assembler code for this option looks like:

   
       c = a/b;  //inside a __try block 
            1001325A  movsd       xmm0,mmword ptr [a] 
            1001325F  divsd       xmm0,mmword ptr [b] 
            10013264  movsd       mmword ptr [c],xmm0 
            10013269  wait             
            1001326A  mov         dword ptr [ebp-4],0FFFFFFFFh 
            10013271  jmp         $L18835+12h (100132A2h) 
       }
       _except(EvaluateException(GetExceptionCode()))
   

The SSE and SSE2 instructions can generate the same floating-point exceptions (mentioned above) that are generated with x87 floating-point instructions and are defined in IEEE Standard 754. There is also a Denormal operand exception that is not defined in the standard. Each of the exception conditions has a corresponding flag and mask in the MXCSR register. Control register CR4's OSXMMEXCEPT flag provides additional control over generation of SIMD floating-point exceptions.

Comparing VS 2005 with VS 2003
VS 2005 introduced some improvements in dealing with floating-point exceptions. To compare with VS 2003:

1. The options for the Code Generation \Enable C++ Exceptions property in VS2003 are:
   • No: Structured and C++ exceptions are captured, but C++ objects that go out –f-scope as a result of exception will not be destroyed. This is an equivalent to /EHsc-.
   • Yes (/EHsc): Only C++ exceptions will be captured. This means that catch(. . .) will not catch C exceptions. The C++ objects that should be destroyed will be destroyed. During optimization, the Release configuration compiler may optimize away the catch clause if it does not see a throw nside the try block.

2. Changes for VS2005:
   • Yes (/EHsc): If a C exception is generated, the C++ objects that should be destroyed will not be destroyed. This means that C++ exception handling works correctly only for C++ exceptions; in other words, catch (. . .) does not catch C exceptions.
   • Yes with SHE Exceptions (/EHa) catch (. . .) will catch C and C++ exceptions. Also, if you install a translator function, this option should be selected.

3. The options for Code Generation\Enable Floating Point Exceptions property (Enable Floating Point Exception Semantics) in VS 2005 are:
   • No: Turned off.
   • Yes (/fp:except): The compiler introduces a WAIT instruction after each floating-point instruction. This forces the processor to synchronize with the state of the FPU and handle any pending exceptions. Also, this prevents the compiler from illegally optimizing floating-point code.
   • In addition to this option, which is used on a file-by-file basis, you can control the same floating-point semantic on a function-by-function basis by using the float_control pragma directive. For example:

     
         #pragma   float_control (except, off | on)
     

   • This option is not compatible with /arch:SSE.
   • The options for “Optimization\Floating-Point Consistency” property in VS2003 are:

   • Default Consistency: The compiler uses processor's 80 bit registers to hold the intermediate results of floating-point calculations.
   • Improved Consistency (/Op): The compiler loads data from memory prior to each floating-point operation (optimization is disabled).

4. Changes for VS2005:
   • This property has been removed. The new property is Code Generation\Floating Point Model with these options:
     • Precise (fp:precise): Replaces the /Ops option.
     • Strict (/fp:strict): Creates the strictest code.
     • Fast (/fp:fast): The compiler creates the fastest code in the majority of cases.
     • If you use _controlfp() in code compiled with VS 2005, you will get a C4996 warning that this function has been deprecated. Use _controlfp_s() instead.

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