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.NET Forum / Languages / Managed C++ / September 2005Is this a VC7.1 optimizer bug?
I'm seeing a problem where with -O2 -Ob2 on, my compiled code loads the floating point value 1.0f on top of the floating point register stack (ST0), then calls some functions (which happens to call driver functions in the kernel), then later when it needs to assign the constant 1.0f to a variable, it tries to use the value stored in ST0. The only problem is that the 1.0f value is no longer there, the call into the kernel has set ST0 to float NaN! Is this a problem with the VC optimizer (incorrectly assuming calls won't modify the FP stack) or with the kernel call that modifies the FP stack? >I'm seeing a problem where with -O2 -Ob2 on, my compiled code loads the >floating point value 1.0f on top of the floating point register stack (ST0), >then calls some functions (which happens to call driver functions in the >kernel), then later when it needs to assign the constant 1.0f to a variable, >it tries to use the value stored in ST0. The only problem is that the 1.0f >value is no longer there, the call into the kernel has set ST0 to float NaN! James, Do you have a short example that anyone can use to reproduce the problem? Dave  SignatureMVP VC++ FAQ: http://www.mvps.org/vcfaq > >I'm seeing a problem where with -O2 -Ob2 on, my compiled code loads the > >floating point value 1.0f on top of the floating point register stack (ST0), [quoted text clipped - 7 lines] > Do you have a short example that anyone can use to reproduce the > problem? No, and it's unlikely I can make one since the kernel call in question is NtGdiDrawPrimitives2, which calls into graphics driver code which of course is video-card specific, and the problem only occurs with my rather large dataset, although it is 100% reproducible. I was just hoping some assembly/windows kernel guru would know if I should be pointing the finger of blame at the compiler or the kernel/driver. >> Do you have a short example that anyone can use to reproduce the >> problem? [quoted text clipped - 3 lines] >is video-card specific, and the problem only occurs with my rather large >dataset, although it is 100% reproducible. A quick google on that kernel call you mention shows up nothing, so I think the hard part is going to be finding someone who's familiar with that. I don't know where to suggest you find such a person. Dave SignatureMVP VC++ FAQ: http://www.mvps.org/vcfaq Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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