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

[Pranav Bhandarkar]

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. Then we cannot guarantee
the nsw flag for the values of x that we could before this combination (e.g
x=-10). Adding the 'nsw' flag in this would mean that the semantics are not
preserved in the strictest sense of the term. Does this address your
question?

>>Sure it can overflow the unsigned wrap point (crossing between -1 to 0)
but when merging two adds or two subs, does that ever matter?

I do not think unsigned wrap matters.

Thanks for your comments, I will incorporate your suggestions on the patch.
Do let me know your thoughts on the reasoning above.

Pranav

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