[llvm-commits] PATCH: Preserving the 'nsw' flag in the instruction combiner.

[Eli Friedman]

On Wed, Aug 10, 2011 at 12:57 PM, Nick Lewycky <nlewycky at google.com> wrote:
> On 10 August 2011 12:37, Pranav Bhandarkar <pranavb at codeaurora.org> wrote:
>>
>> Hi Nick,
>>
>> >>Thanks for working on this! Firstly, I have a high-level question: why
>> can't (X +nsw (C1 +nsw C2) always become (X +nsw C3)? Your patch spends a
>> lot of time verifying that >>overflow couldn't occur, but you're given an
>> assumption a priori that it can't because the nsw flag is present.
>>
>> The reason is that 127 (C1) + 2 (C2) = 129 (C3) is not strictly true
>> always.
>> For instance, when the type is i8, then C3 is -127.
>
> 127 + 2 = 129 remains true with i8 values, because i8 -127 = i8 129. They
> have the same bit pattern.
> Can you pick values for X, C1 and C2 which still satisfy the +nsw
> relationship between each other that can't be converted into X +nsw C3 where
> C3 = C1+C2 (no nsw requirement)? 127 and 2 don't work as counter-examples
> because that triggers undefined behaviour (it violates the 'nsw' bit present
> on the add).

X = -128, C1 = 127, C1 = 2?

-Eli