[LLVMbugs] [Bug 21093] New: add domain warnings for math functions

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=21093

            Bug ID: 21093
           Summary: add domain warnings for math functions
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

When using '-ffinite-math-only' (or any other flag that implicitly turns on
"no-nans-fp-math"), the following code causes undefined behavior that is
detectable at compile-time, so clang should emit a warning:

$ cat bad_math.c 
#include <math.h>
double bad_sqrt() { return sqrt(-2.0); }

$ ./clang bad_math.c -O1 -ffinite-math-only -c

The square root of a negative number is implementation-defined behavior for
C/C++. Most hardware and software (IEEE-754, POSIX, etc.) implementations
return NaN for this case. But with -ffinite-math-only, the user has guaranteed
not to use NaNs...so UB.

For completeness, domain checking should be added for several of the standard
math calls, not just 'sqrt'.

This was discussed here:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140609/220598.html

And again here:
http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-September/077168.html

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