[llvm-commits] [llvm] r155884 - in /llvm/trunk: lib/Transforms/Scalar/LICM.cpp test/Transforms/LICM/speculate.ll

[Evan Cheng]

Has this been fixed?

Evan

On Apr 30, 2012, at 10:37 PM, Nick Lewycky <nicholas at mxc.ca> wrote:

> Eli Friedman wrote:
>> On Mon, Apr 30, 2012 at 9:03 PM, Nick Lewycky<nicholas at mxc.ca>  wrote:
>>> Author: nicholas
>>> Date: Mon Apr 30 23:03:01 2012
>>> New Revision: 155884
>>> 
>>> URL: http://llvm.org/viewvc/llvm-project?rev=155884&view=rev
>>> Log:
>>> An instruction in a loop is not guaranteed to be executed just because the loop
>>> has no exit blocks. Fixes PR12706!
>>> 
>>> Modified:
>>>    llvm/trunk/lib/Transforms/Scalar/LICM.cpp
>>>    llvm/trunk/test/Transforms/LICM/speculate.ll
>>> 
>>> Modified: llvm/trunk/lib/Transforms/Scalar/LICM.cpp
>>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LICM.cpp?rev=155884&r1=155883&r2=155884&view=diff
>>> ==============================================================================
>>> --- llvm/trunk/lib/Transforms/Scalar/LICM.cpp (original)
>>> +++ llvm/trunk/lib/Transforms/Scalar/LICM.cpp Mon Apr 30 23:03:01 2012
>>> @@ -618,6 +618,11 @@
>>>     if (!DT->dominates(Inst.getParent(), ExitBlocks[i]))
>>>       return false;
>>> 
>>> +  // As a degenerate case, if the loop is statically infinite then we haven't
>>> +  // proven anything since there are no exit blocks.
>>> +  if (ExitBlocks.empty())
>>> +    return false;
>> 
>> Even if there are exit blocks, you still haven't proven anything; take
>> something like the following, assuming the compiler doesn't know
>> anything about exit():
>> 
>> int f(int x, int y) {
>>   int n = 0;
>>   for (int i = 0; i<  10; i++) {
>>     exit(0);
>>     n += x / (y+i);
>>   }
>>   return n;
>> }
> 
> Great point Eli. I was struggling with a different aspect of its design 
> (why use exit blocks vs. exiting blocks) but missed this bug. I don't 
> have the time to fix it, so I'd appreciate it if you want to take over!
> 
> Nick
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