r331056 - [docs] add -ffp-cast-overflow-workaround to the release notes

[Richard Smith via cfe-commits]

On 27 April 2018 at 17:09, Chandler Carruth via cfe-commits <
cfe-commits at lists.llvm.org> wrote:

> On Fri, Apr 27, 2018 at 4:36 PM Richard Smith via cfe-commits <
> cfe-commits at lists.llvm.org> wrote:
>
>> On 27 April 2018 at 16:07, Sanjay Patel via cfe-commits <
>> cfe-commits at lists.llvm.org> wrote:
>>
>>> Missing dash corrected at r331057. I can improve the doc wording, but
>>> let's settle on the flag name first, and I'll try to get it all fixed up in
>>> one shot.
>>>
>>> So far we have these candidates:
>>> 1. -ffp-cast-overflow-workaround
>>> 2. -fstrict-fp-trunc-semantics
>>> 3. -fstrict-fp-cast-overflow
>>>
>>> I don't have a strong opinion here, but on 2nd reading, it does seem
>>> like a 'strict' flag fits better with existing options.
>>>
>>
>> The corresponding UBSan check is called -fsanitize=float-cast-overflow,
>> so maybe -fno-strict-float-cast-overflow would be the most consistent
>> name?
>>
>
> On this topic: we were hit by this on a *lot* of code. All of that code
> builds and passes tests with -fsanitize=float-cast-overflow. So I think
> we've been mistaken in assuming that this sanitizer catches all of the
> failure modes of the optimization. That at least impacts the sanitizer
> suggestion in the release notes. And probably impacts the flag name /
> attribuet name.
>

That's interesting, and definitely sounds like a bug (either the sanitizer
or LLVM is presumably getting the range check wrong). Can you point me at
an example?


> On Fri, Apr 27, 2018 at 4:41 PM, Richard Smith <richard at metafoo.co.uk>
>>> wrote:
>>>
>>>> On 27 April 2018 at 09:21, Sanjay Patel via cfe-commits <
>>>> cfe-commits at lists.llvm.org> wrote:
>>>>
>>>>> Author: spatel
>>>>> Date: Fri Apr 27 09:21:22 2018
>>>>> New Revision: 331056
>>>>>
>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=331056&view=rev
>>>>> Log:
>>>>> [docs] add -ffp-cast-overflow-workaround to the release notes
>>>>>
>>>>> This option was added with:
>>>>> D46135
>>>>> rL331041
>>>>> ...copying the text from UsersManual.rst for more exposure.
>>>>>
>>>>>
>>>>> Modified:
>>>>>     cfe/trunk/docs/ReleaseNotes.rst
>>>>>
>>>>> Modified: cfe/trunk/docs/ReleaseNotes.rst
>>>>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/
>>>>> ReleaseNotes.rst?rev=331056&r1=331055&r2=331056&view=diff
>>>>> ============================================================
>>>>> ==================
>>>>> --- cfe/trunk/docs/ReleaseNotes.rst (original)
>>>>> +++ cfe/trunk/docs/ReleaseNotes.rst Fri Apr 27 09:21:22 2018
>>>>> @@ -83,6 +83,15 @@ Non-comprehensive list of changes in thi
>>>>>  New Compiler Flags
>>>>>  ------------------
>>>>>
>>>>> +- :option:`-ffp-cast-overflow-workaround` and
>>>>> +  :option:`-fnofp-cast-overflow-workaround`
>>>>>
>>>>
>>>> Shouldn't this be -fno-fp-cast-overflow-workaround?
>>>>
>>>> Also, our convention for flags that define undefined behavior is
>>>> `-fno-strict-*`, so perhaps this should be `-fno-strict-fp-cast-overflow`
>>>> ?
>>>>
>>>>
>>>>> +  enable (disable) a workaround for code that casts floating-point
>>>>> values to
>>>>> +  integers and back to floating-point. If the floating-point value is
>>>>> not
>>>>> +  representable in the intermediate integer type, the code is
>>>>> incorrect
>>>>> +  according to the language standard.
>>>>
>>>>
>>>> I find this hard to read: I initially misread "the code is incorrect
>>>> according to the language standard" as meaning "Clang will generate code
>>>> that is incorrect according to the language standard". I think what you
>>>> mean here is "the code has undefined behavior according to the language
>>>> standard, and Clang will not guarantee any particular result. This flag
>>>> causes the behavior to be defined to match the overflowing behavior of the
>>>> target's native float-to-int conversion instructions."
>>>>
>>>>
>>>>> This flag will attempt to generate code
>>>>> +  as if the result of an overflowing conversion matches the
>>>>> overflowing behavior
>>>>> +  of a target's native float-to-int conversion instructions.
>>>>> +
>>>>>  - ...
>>>>>
>>>>>  Deprecated Compiler Flags
>>>>>
>>>>>
>>>>> _______________________________________________
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>>>>> cfe-commits at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
>>>>>
>>>>
>>>>
>>>
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