[llvm-commits] [PATCH] Fix various instances of undefined behavior

[Eli Friedman]

On Wed, Oct 12, 2011 at 4:37 PM, Ahmed Charles <ahmedcharles at gmail.com> wrote:
> FDIS 5.6/4 suggests that floating point isn't special:
>
> The binary / operator yields the quotient, and the binary % operator
> yields the remainder from the division of the first expression by the
> second. If the second operand of / or % is zero the behavior is
> undefined. For integral operands the / operator yields the algebraic
> quotient with any fractional part discarded;[81] if the quotient a/b
> is representable in the type of the result, (a/b)*b + a%b is equal to
> a.
>
> 81) This is often called truncation towards zero.
>
> Is there somewhere else that gives floating point different semantics?

The IEEE floating-point standard explicitly defines the result of
division by zero.

-Eli

> On Wed, Oct 12, 2011 at 4:28 PM, Ahmed Charles <ahmedcharles at gmail.com> wrote:
>> I'll do that later after merging your changes and doing another round of tests.
>>
>> On Wed, Oct 12, 2011 at 4:24 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>>> On Wed, Oct 12, 2011 at 1:50 AM, Ahmed Charles <ahmedcharles at gmail.com> wrote:
>>>> Similar patch with stuff found compiling boost. The one in APFloat
>>>> seems like an actual bug. "make check-all" passes for all of these
>>>> changes.
>>>>
>>>> On Tue, Oct 11, 2011 at 9:17 PM, Ahmed Charles <ahmedcharles at gmail.com> wrote:
>>>>> I'm not sure if each instance is a bug, other than simply executing
>>>>> undefined behavior (which meets my personal bar), but some of them
>>>>> actually seem like logic bugs or typos. Please review and commit some
>>>>> or all of the changes.
>>>
>>> I committed changes to APFloat, EarlyCSE, DenseMapInfo.h, and
>>> SelectionDAGBuilder.cpp.
>>>
>>> For the LSR ones, I'm not entirely sure what the intent is; the math
>>> seems a bit suspicious.
>>>
>>> For the change to lib/Analysis/ProfileEstimatorPass.cpp, as far as I
>>> know, a floating-point divide-by-zero is not undefined behavior.
>>>
>>> For everything else, would you mind consolidating the remaining
>>> changes into a single patch?
>>>
>>> -Eli
>>>
>>
>>
>>
>> --
>> Ahmed Charles
>>
>
>
>
> --
> Ahmed Charles
>