From af0266b904973eddbacd3462bad78dab13fb7e4e Mon Sep 17 00:00:00 2001 From: Junyan He Date: Tue, 17 Mar 2015 18:08:40 +0800 Subject: 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 Reviewed-by: Zhigang Gong --- backend/src/llvm/llvm_printf_parser.cpp | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'backend') diff --git a/backend/src/llvm/llvm_printf_parser.cpp b/backend/src/llvm/llvm_printf_parser.cpp index 8e662b37..ab8e1a0a 100644 --- a/backend/src/llvm/llvm_printf_parser.cpp +++ b/backend/src/llvm/llvm_printf_parser.cpp @@ -822,6 +822,12 @@ error: case Type::DoubleTyID: case Type::FloatTyID: { + /* llvm 3.6 will give a undef value for NAN. */ + if (dyn_cast(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. */ -- cgit v1.2.3