llvm dev - Apr 2015 - [LLVMdev] inconsistent wording in the LangRef regarding "shl nsw"

> I wouldn't be surprised if InstSimplify were relying on (1) to
implement
> some of its optimizations:
>
http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InstructionSimplify.cpp?revision=233938&view=markup#l2298
Yup!  It looks like LLVM really implements (1) and (2) is just a
misleading anecdote.  So I'll change my vote from "remove (1)" to
"remove (2)".  :)

-- Sanjoy

On Mon, Apr 6, 2015 at 11:26 AM, Sanjoy Das <sanjoy at
playingwithpointers.com>
wrote:
> > I wouldn't be surprised if InstSimplify were relying on (1) to
implement
> > some of its optimizations:
> >
>
http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InstructionSimplify.cpp?revision=233938&view=markup#l2298
>
> Yup!  It looks like LLVM really implements (1) and (2) is just a
> misleading anecdote.  So I'll change my vote from "remove
(1)" to
> "remove (2)".  :)
>
I wrote those optimizations with an understanding that (1) was the one true
way.  Would you be apposed to switching to (2)? I have examples where
implementing (1) creates real regressions where (2) does not.

>
> -- Sanjoy
>
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> I wrote those optimizations with an understanding that (1) was the one true
> way.  Would you be apposed to switching to (2)? I have examples where
> implementing (1) creates real regressions where (2) does not.
I don't have fundamental reasons to favor (1) over (2).  I assumed
finding and fixing all the places within LLVM that implement and
assume (1) will be difficult and bug-prone, and therefore sticking
with (1) is the more pragmatic option.  I do think (2) is easier to
reason in, so if you think switching LLVM to consistently use (2) is
doable then I too will prefer (2).

In any case we should let this thread linger for a while before
changing anything, in case there are others with differing opinions.

-- Sanjoy