[LLVMdev] [RFC] New function attributes for errno-setting functions

[Dan Gohman]

It was pointed out to me that I misunderstood what problem you were trying
to solve. Sorry for the noise.

Dan


On Fri, Sep 13, 2013 at 2:02 PM, Dan Gohman <dan433584 at gmail.com> wrote:

> Food for thought: If you have a codebase which can't use -fno-math-errno,
> because it relies on errno values from math functions, it may be a more
> effective long-term strategy to work on modernizing your codebase,
> eliminating the dependencies on errno, rather than going through the
> trouble of adding even more complexity to compilers to keep errno support
> limping along.
>
> Of course, whether this actually makes sense for you in your situation
> depends on many factors.
>
> Dan
>
>
> On Thu, Sep 12, 2013 at 5:44 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>
>> Hello,
>>
>> Our current handling of -fno-math-errno (and this -ffast-math) in Clang
>> is broken on systems for which libm functions do actually set errno. This
>> is because, when -fno-math-errno is in effect, libm functions like sqrt,
>> sin, cos, etc. are marked as readnone. As a result, these calls can be
>> reordered with respect to other calls that set errno, and can clobber errno
>> before it can be read. For example:
>>
>> int fd = open(...);
>> if (fd == -1) {
>>   perror("oops");
>> }
>> double f = sqrt(a);
>>
>> If we're on a system (like Linux, for example), where sqrt might set
>> errno to EDOM when a is negative, we can have a problem if some
>> optimization rearranges the code to something like this:
>>
>> int fd = open(...);
>> double f = sqrt(a);
>> if (fd == -1) {
>>   perror("oops");
>> }
>>
>> if the open fails, and a is -2.0, then perror will print the wrong error
>> string (thus confusing the user). This is not really a problem with the
>> optimization because sqrt was marked as readnone. On the other hand, we
>> don't want to tag sqrt and writing to some arbitrary external state
>> variable, because this will prevent autovectorization, will prevent CSE
>> from collecting duplicate calls to sqrt, and a host of other important
>> optimizations.
>>
>> To fix this problem, I think that we need to stop treating errno as some
>> arbitrary external state, and model is explicitly. Functions need to be
>> able carry two additional attributes:
>>
>>   1. 'writes-only-errno' - An attribute to indicate that the function may
>> set errno, and that is the only external state to which it might write.
>> This is useful because getModRefInfo queries can return more accurate
>> answers when comparing to pointers that we know cannot point to errno (such
>> as alloca'd memory and functions) [is this allowed at all?].
>>
>>   2. 'errno-ignored' - Set when -fno-math-errno is in effect, indicating
>> that it is safe to assume that the user does not care about any value of
>> errno set by this function. This will allow CSE and autovectorization to
>> happen (by modifying those passes to specifically query this attribute).
>>
>> With these new attributes, the functions are, on systems for which libm
>> functions might set errno, not marked readnone or readonly. This will
>> prevent unwanted reordering. Thoughts?
>>
>> Thanks again,
>> Hal
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>
>
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