Generate NAN for UNDEF value in printf parser.

llvm 3.6 will give a UNDEF value for NAN. The will cause
the store instruction for UNDEF to be ignored. We need
to modify it to NAN here.

Comments from Zhigang:
"
The related commit of why LLVM won't just simply return NaN for such
case is at:

    Make the sqrt intrinsic return undef for a negative input.

    As 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

    The sqrt of a negative number when using the llvm intrinsic is undefined.
    We should return undef rather than 0.0 to match the definition in the LLVM IR lang ref.

    This change should not affect any code that isn't using "no-nans-fp-math";
    ie, no-nans is a requirement for generating the llvm intrinsic in place of a sqrt function call.

    Unfortunately, the behavior introduced by this patch will not match current gcc, xlc, icc, and
    possibly other compilers. The current clang/llvm behavior of returning 0.0 doesn't either.
    We knowingly approve of this difference with the other compilers in an attempt to flag code
    that is invoking undefined behavior.

    A front-end warning should also try to convince the user that the program will fail:
    http://llvm.org/bugs/show_bug.cgi?id=21093

    Differential Revision: http://reviews.llvm.org/D5527

This patch is a workaround for the following scenario:
printf("%f \n", sqrt(-1.0f));
"

Signed-off-by: Junyan He <junyan.he@linux.intel.com>
Reviewed-by: Zhigang Gong <zhigang.gong@linux.intel.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/backend/src/llvm/llvm_printf_parser.cpp b/backend/src/llvm/llvm_printf_parser.cpp
index 7800c019..2f854430 100644
--- a/backend/src/llvm/llvm_printf_parser.cpp
+++ b/backend/src/llvm/llvm_printf_parser.cpp
@@ -831,6 +831,12 @@ error:
 
       case Type::DoubleTyID:
       case Type::FloatTyID: {
+        /* llvm 3.6 will give a undef value for NAN. */
+        if (dyn_cast<llvm::UndefValue>(arg)) {
+          APFloat nan = APFloat::getNaN(APFloat::IEEEsingle, false);
+          arg = ConstantFP::get(module->getContext(), nan);
+        }
+
         /* Because the printf is a variable parameter function, it does not have the
            function prototype, so the compiler will always promote the arg to the
            longest precise type for float. So here, we can always find it is double. */