[llvm] r242281 - [PM/AA] Fix *numerous* serious bugs in GlobalsModRef found by

[Sean Silva]

Btw, the way I originally found this was compiling with `-fno-inline -flto`
(I don't remember why). Kind of a crazy combination, but it seemed to smoke
out this issue (and I think there was at least one other one that this
revealed, but I didn't dig into it).

-- Sean Silva

On Wed, Jul 15, 2015 at 2:18 PM, Chandler Carruth <chandlerc at gmail.com>
wrote:

> Awesome! Do you want to land that test case? If not, I will, but you may
> get to it sooner at this point...
>
> On Wed, Jul 15, 2015 at 2:08 PM Sean Silva <chisophugis at gmail.com> wrote:
>
>> Thanks! I think this fixed PR23345; I didn't know about
>> I.mayWriteToMemory()
>>
>> -- Sean Silva
>>
>> On Wed, Jul 15, 2015 at 1:53 AM, Chandler Carruth <chandlerc at gmail.com>
>> wrote:
>>
>>> Author: chandlerc
>>> Date: Wed Jul 15 03:53:29 2015
>>> New Revision: 242281
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=242281&view=rev
>>> <https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_viewvc_llvm-2Dproject-3Frev-3D242281-26view-3Drev&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=mQ4LZ2PUj9hpadE3cDHZnIdEwhEBrbAstXeMaFoB9tg&m=v_2U7OQd83CL4gchkkSZVU3fb0tMVRRG0j7MxIm2l1Q&s=FIjAFu91y9yQQlNOxBR3MNPtBoxckgBquFpcazgFFcc&e=>
>>
>>
>>> Log:
>>> [PM/AA] Fix *numerous* serious bugs in GlobalsModRef found by
>>> inspection.
>>>
>>> While we want to handle calls specially in this code because they should
>>> have been modeled by the call graph analysis that precedes it, we should
>>> *not* be re-implementing the predicates for whether an instruction reads
>>> or writes memory. Those are well defined already. Notably, at least the
>>> following issues seem to be clearly missed before:
>>> - Ordered atomic loads can "write" to memory by causing writes from other
>>>   threads to become visible. Similarly for ordered atomic stores.
>>> - AtomicRMW instructions quite obviously both read and write to memory.
>>> - AtomicCmpXchg instructions also read and write to memory.
>>> - Fences read and write to memory.
>>> - Invokes of intrinsics or memory allocation functions.
>>>
>>> I don't have any test cases, and I suspect this has never really come up
>>> in the real world. But there is no reason why it wouldn't, and it makes
>>> the code simpler to do this the right way.
>>>
>>> While here, I've tried to make the loops significantly simpler as well
>>> and added helpful comments as to what is going on.
>>>
>>> Modified:
>>>     llvm/trunk/lib/Analysis/IPA/GlobalsModRef.cpp
>>>
>>> Modified: llvm/trunk/lib/Analysis/IPA/GlobalsModRef.cpp
>>>
>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/IPA/GlobalsModRef.cpp?rev=242281&r1=242280&r2=242281&view=diff
>>> <https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_viewvc_llvm-2Dproject_llvm_trunk_lib_Analysis_IPA_GlobalsModRef.cpp-3Frev-3D242281-26r1-3D242280-26r2-3D242281-26view-3Ddiff&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=mQ4LZ2PUj9hpadE3cDHZnIdEwhEBrbAstXeMaFoB9tg&m=v_2U7OQd83CL4gchkkSZVU3fb0tMVRRG0j7MxIm2l1Q&s=Q0Y9JUhRV2iEM7NdBl52tiz7eVvdznT8n2zfuYufLF0&e=>
>>
>>
>>>
>>> ==============================================================================
>>> --- llvm/trunk/lib/Analysis/IPA/GlobalsModRef.cpp (original)
>>> +++ llvm/trunk/lib/Analysis/IPA/GlobalsModRef.cpp Wed Jul 15 03:53:29
>>> 2015
>>> @@ -439,30 +439,39 @@ void GlobalsModRef::AnalyzeCallGraph(Cal
>>>      }
>>>
>>>      // Scan the function bodies for explicit loads or stores.
>>> -    for (unsigned i = 0, e = SCC.size(); i != e && FunctionEffect !=
>>> ModRef;
>>> -         ++i)
>>> -      for (inst_iterator II = inst_begin(SCC[i]->getFunction()),
>>> -                         E = inst_end(SCC[i]->getFunction());
>>> -           II != E && FunctionEffect != ModRef; ++II)
>>> -        if (LoadInst *LI = dyn_cast<LoadInst>(&*II)) {
>>> +    for (auto *Node : SCC) {
>>> +      if (FunctionEffect == ModRef)
>>> +        break; // The mod/ref lattice saturates here.
>>> +      for (Instruction &I : inst_range(Node->getFunction())) {
>>> +        if (FunctionEffect == ModRef)
>>> +          break; // The mod/ref lattice saturates here.
>>> +
>>> +        // We handle calls specially because the graph-relevant aspects
>>> are
>>> +        // handled above.
>>> +        if (auto CS = CallSite(&I)) {
>>> +          if (isAllocationFn(&I, TLI) || isFreeCall(&I, TLI)) {
>>> +            // FIXME: It is completely unclear why this is necessary
>>> and not
>>> +            // handled by the above graph code.
>>> +            FunctionEffect |= ModRef;
>>> +          } else if (Function *Callee = CS.getCalledFunction()) {
>>> +            // The callgraph doesn't include intrinsic calls.
>>> +            if (Callee->isIntrinsic()) {
>>> +              ModRefBehavior Behaviour =
>>> +                  AliasAnalysis::getModRefBehavior(Callee);
>>> +              FunctionEffect |= (Behaviour & ModRef);
>>> +            }
>>> +          }
>>> +          continue;
>>> +        }
>>> +
>>> +        // All non-call instructions we use the primary predicates for
>>> whether
>>> +        // thay read or write memory.
>>> +        if (I.mayReadFromMemory())
>>>            FunctionEffect |= Ref;
>>> -          if (LI->isVolatile())
>>> -            // Volatile loads may have side-effects, so mark them as
>>> writing
>>> -            // memory (for example, a flag inside the processor).
>>> -            FunctionEffect |= Mod;
>>> -        } else if (StoreInst *SI = dyn_cast<StoreInst>(&*II)) {
>>> +        if (I.mayWriteToMemory())
>>>            FunctionEffect |= Mod;
>>> -          if (SI->isVolatile())
>>> -            // Treat volatile stores as reading memory somewhere.
>>> -            FunctionEffect |= Ref;
>>> -        } else if (isAllocationFn(&*II, TLI) || isFreeCall(&*II, TLI)) {
>>> -          FunctionEffect |= ModRef;
>>> -        } else if (IntrinsicInst *Intrinsic =
>>> dyn_cast<IntrinsicInst>(&*II)) {
>>> -          // The callgraph doesn't include intrinsic calls.
>>> -          Function *Callee = Intrinsic->getCalledFunction();
>>> -          ModRefBehavior Behaviour =
>>> AliasAnalysis::getModRefBehavior(Callee);
>>> -          FunctionEffect |= (Behaviour & ModRef);
>>> -        }
>>> +      }
>>> +    }
>>>
>>>      if ((FunctionEffect & Mod) == 0)
>>>        ++NumReadMemFunctions;
>>>
>>>
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