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

[Chandler Carruth via cfe-commits]

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.


>
>
>> 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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>>>>
>>>
>>>
>>
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