[llvm] r237593 - Exploit dereferenceable_or_null attribute in LICM pass

[Philip Reames]

Sanjoy is out on vacation, but I'm going to look into this.  I'll be 
offline for about an hour, but should be able to start investigating by 
around 7pm.  On the surface, it looks like it should be an easy fix.  
Hopefully, I'll have this resolved within a couple of hours. Sorry for 
the breakage.

Philip

On 05/21/2015 04:38 PM, Nick Lewycky wrote:
> On 18 May 2015 at 11:07, Sanjoy Das <sanjoy at playingwithpointers.com 
> <mailto:sanjoy at playingwithpointers.com>> wrote:
>
>     Author: sanjoy
>     Date: Mon May 18 13:07:00 2015
>     New Revision: 237593
>
>     URL: http://llvm.org/viewvc/llvm-project?rev=237593&view=rev
>     <https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_viewvc_llvm-2Dproject-3Frev-3D237593-26view-3Drev&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=mQ4LZ2PUj9hpadE3cDHZnIdEwhEBrbAstXeMaFoB9tg&m=EZRvoJai0V1ZEULFLl3UFBFw3GpoHtruFDOok84hYrI&s=4QqYr7GqSrZjVxTSY4zWHbjeCHYmt-2bkaDpOtpbsM8&e=>
>     Log:
>     Exploit dereferenceable_or_null attribute in LICM pass
>
>     Summary:
>     Allow hoisting of loads from values marked with
>     dereferenceable_or_null
>     attribute. For values marked with the attribute perform
>     context-sensitive analysis to determine whether it's known-non-null or
>     not.
>
>
> This commit caused PR23608, see the testcase in that PR.
>
>     -/// Only sink or hoist an instruction if it is not a trapping
>     instruction
>     +/// Only sink or hoist an instruction if it is not a trapping
>     instruction,
>     +/// or if the instruction is known not to trap when moved to the
>     preheader.
>      /// or if it is a trapping instruction and is guaranteed to execute.
>     -///
>     -static bool isSafeToExecuteUnconditionally(const Instruction &Inst,
>     +static bool isSafeToExecuteUnconditionally(const Instruction &Inst,
>                                                 const DominatorTree *DT,
>     +                                           const
>     TargetLibraryInfo *TLI,
>                                                 const Loop *CurLoop,
>                                                 const LICMSafetyInfo
>     *SafetyInfo) {
>     -  // If it is not a trapping instruction, it is always safe to hoist.
>     -  if (isSafeToSpeculativelyExecute(&Inst))
>     +  const Instruction *CtxI =
>     CurLoop->getLoopPreheader()->getTerminator();
>
>
> A Loop* is not guaranteed to have a preheader, in which case 
> CurLoop->getLoopPreheader() will return null.
>
> Nick
>
>     + if (isSafeToSpeculativelyExecute(&Inst, CtxI, DT, TLI))
>          return true;
>
>        return isGuaranteedToExecute(Inst, DT, CurLoop, SafetyInfo);
>
>
>
>
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