[llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")

[Hal Finkel via llvm-dev]

----- Original Message -----
> From: "Sanjoy Das" <sanjoy at playingwithpointers.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Chandler Carruth" <chandlerc at google.com>, "llvm-dev" <llvm-dev at lists.llvm.org>, "Philip Reames"
> <listmail at philipreames.com>, "Duncan P. N. Exon Smith" <dexonsmith at apple.com>
> Sent: Thursday, February 25, 2016 1:38:48 AM
> Subject: Re: [llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")
> 
> On Wed, Feb 24, 2016 at 11:18 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> > It might be much more challenging, but let's try. This is not an
> > issue we need to fix by the end of the month, and the potential
> > optimization regressions are significant. Our deductions of
> > readonly/readnone/nocapture/etc. are really important for enabling
> > other optimizations. Given that all of our C++ constructors, inline
> > functions, etc. end up in comdat sections, this is really
> > important.
> 
> The only optimizations I can think of that are okay are algebraic
> simplifications that don't exploit no-overflow, inbounds or exact

Why? Can you provide an example using nsw, inbounds, etc.?

 -Hal

> flags and CFG simplifications like loop unswitch and loop unroll
> (that
> don't actually change the trace that we'll see at runtime).
> 
> -- Sanjoy
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory