[LLVMdev] Integer divide by zero

[Owen Anderson]

On Apr 5, 2013, at 1:42 PM, Cameron McInally <cameron.mcinally at nyu.edu> wrote:

> On Fri, Apr 5, 2013 at 2:40 PM, Joshua Cranmer 🐧 <Pidgeot18 at gmail.com> wrote:
> ... 
> Per C and C++, integer division by 0 is undefined. That means, if it happens, the compiler is free to do whatever it wants. It is perfectly legal for LLVM to define r to be, say, 42 in this code; it is not required to preserve the fact that the idiv instruction on x86 and x86-64 will trap.
> 
> This is quite a conundrum to me. Yes, I agree with you on the C/C++ Standards interpretation. However, the x86-64 expectations are orthogonal. I find that other compilers, including GCC, will trap by default at high optimization levels on x86-64 for this test case. Hardly scientific, but every other compiler on our machines issues a trap.
> 
> We take safety seriously in our compiler. Our other components go to great lengths not to constant fold an integer division by zero. We would also like LLVM to do the same. Are there others in the community who feel this way?
> 
> I can envision an option which preserves faults during constant folding. E.g. an integer version of -enable-unsafe-fp-math or gfortran's -ffpe-trap. More accurately, this hypothetical option would suppress the folding of unsafe integer expressions altogether. Would an option such as this benefit the community at large?

Clang's -fsanitize=integer-divide-by-zero option seems to provide what you want.  See the Clang User Manual: http://clang.llvm.org/docs/UsersManual.html#controlling-code-generation

--Owen

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