[llvm-dev] '__builtin_nanl' and soft-FP64 support

Martin J. O'Riordan via llvm-dev llvm-dev at lists.llvm.org
Mon Sep 25 11:40:56 PDT 2017

Thanks Eli,


Hadn't thought of that.  Yes, it us present in the IR emitted from CLang, so
I'll have to see if I find where it changes by dumping the IR between


All the best,




From: Friedman, Eli [mailto:efriedma at codeaurora.org] 
Sent: 25 September 2017 18:47
To: Martin J. O'Riordan <martin.oriordan at movidius.com>; 'LLVM Developers'
<llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] '__builtin_nanl' and soft-FP64 support


On 9/25/2017 5:35 AM, Martin J. O'Riordan via llvm-dev wrote:

I am seeing failures in two tests after migrating to v5.0 final, these are:




However, these are new tests and it turns out that the underlying problem is
that the builtin '__builtin_nanl("")' is returning the value
0x0000000000000000.  I tested this builtin with our v4.0 compiler and it has
the same problem, so this is not a regression but rather an existing bug
exposed by the new LibC++ implementation of '<limits>'.


But our implementation of FP64 is a software only implementation using the
FP64 support functions in 'compiler-rt' and we have no lowering actions of
our own.


Where should I be looking to find out how to fix this?  Is it a CLang issue
of an LLVM issue?


__builtin_nanl("") gets constant-folded by clang; if you look at the LLVM
IR, you should see something like "double 0x7FF8000000000000".  Where
exactly is 0x0000000000000000 showing up?


Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
Foundation Collaborative Project
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