Re: Should -ffp-contract=off the default on GCC?

[Jeff Law <jeffreyalaw@gmail.com>]

On 3/21/23 11:00, Qing Zhao via Gcc-patches wrote:
> 
> 
>> On Mar 21, 2023, at 12:56 PM, Paul Koning <paulkoning@comcast.net> wrote:
>>
>>
>>
>>> On Mar 21, 2023, at 11:01 AM, Qing Zhao via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
>>>
>>> ...
>>> Most of the compiler users are not familiar with language standards, or no access to language standards. Without clearly documenting such warnings along with the option explicitly, the users have not way to know such potential impact.
>>
>> With modern highly optimized languages, not knowing the standard is going to get you in trouble.  There was a wonderful paper from MIT a few years ago describing all the many ways C can bite you if you don't know the rules.
> 
> Yes, it’s better to know the details of languages standard. -:)
> However, I don’t think that this is a realistic expectation to the compiler users:  to know all the details of a language standard.
Umm, they really do need to know that stuff.

If the developer fails to understand the language standard, then they're 
likely going to write code that is ultimately undefined or doesn't 
behave in they expect.  How is the compiler supposed to guess what the 
developer originally intended?  How should the compiler handle the case 
when two developers have different understandings of how a particular 
piece of code should work?  In the end it's the language standard that 
defines how all this stuff should work.

Failure to understand the language is a common problem and we do try to 
emit various diagnostics to help developers avoid writing non-conformant 
code.  But ultimately if a developer fails to understand the language 
standard, then they're going to be surprised by the behavior of their code.

Jeff