r254574 - PR17381: Treat undefined behavior during expression evaluation as an unmodeled

[Richard Smith via cfe-commits]

On Tue, Dec 8, 2015 at 11:18 AM, Richard Smith <richard at metafoo.co.uk>
wrote:

> On Tue, Dec 8, 2015 at 10:59 AM, Robinson, Paul <
> Paul_Robinson at playstation.sony.com> wrote:
>
>> Okay, I'll bite:  so what *does* UINT128_MAX actually convert to?
>>
>
> $ echo 'unsigned __int128 max = -1; float f = max;' |
> ~/clang-8/build/bin/clang -x c++ - -emit-llvm -S -o - -O3 | grep @f
> @f = global float undef, align 4
>

And at runtime, on some targets, we use this:


https://llvm.org/svn/llvm-project/compiler-rt/trunk/lib/builtins/floatuntisf.c

... which gives a NaN in this case.

*From:* cfe-commits [mailto:cfe-commits-bounces at lists.llvm.org] *On Behalf
>> Of *Richard Smith via cfe-commits
>> *Sent:* Tuesday, December 08, 2015 10:52 AM
>> *To:* Joerg Sonnenberger; cfe-commits
>> *Subject:* Re: r254574 - PR17381: Treat undefined behavior during
>> expression evaluation as an unmodeled
>>
>>
>>
>> On Tue, Dec 8, 2015 at 2:13 AM, Joerg Sonnenberger via cfe-commits <
>> cfe-commits at lists.llvm.org> wrote:
>>
>> On Mon, Dec 07, 2015 at 01:32:14PM -0800, Richard Smith via cfe-commits
>> wrote:
>> > C11 6.3.1.5/1: "If the value being converted is outside the range of
>> values
>> > that can be represented, the behavior is undefined."
>>
>> The value of 1e100 can be represented as +inf, even if not precisely.
>>
>>
>>
>> Only if +inf is in the range of representable values, which, as already
>> noted, is problematic.
>>
>>
>>
>> This is a bit different from non-IEEE math like VAX, that doesn't have
>> infinities.
>>
>>
>> Joerg
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>>
>>
>>
>
>
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