[llvm-commits] [llvm] r147560 - in /llvm/trunk: include/llvm/Analysis/ValueTracking.h lib/Analysis/ValueTracking.cpp

[Chris Lattner]

On Jan 5, 2012, at 12:25 AM, Duncan Sands wrote:

> Hi Dan,
> 
>> Generalize isSafeToSpeculativelyExecute to work on arbitrary
>> Values, rather than just Instructions, since it's interesting
>> for ConstantExprs too.
> 
> since constants are not executed, what does this mean?  I notice that all
> constants results in 'false', i.e. not safe to speculatively execute.  Why
> is that?

Constant expressions can potentially trap (e.g. divide with a RHS that turns out to be zero).  This is why they have the canTrap() method.  This is annoying and should be fixed someday, but until then this is a reasonable thing to do.  The dyncast to Operator handles both instructions and constantexprs.

-Chris

> 
> Ciao, Duncan.
> 
>> 
>> Modified:
>>     llvm/trunk/include/llvm/Analysis/ValueTracking.h
>>     llvm/trunk/lib/Analysis/ValueTracking.cpp
>> 
>> Modified: llvm/trunk/include/llvm/Analysis/ValueTracking.h
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/ValueTracking.h?rev=147560&r1=147559&r2=147560&view=diff
>> ==============================================================================
>> --- llvm/trunk/include/llvm/Analysis/ValueTracking.h (original)
>> +++ llvm/trunk/include/llvm/Analysis/ValueTracking.h Wed Jan  4 17:01:09 2012
>> @@ -174,7 +174,7 @@
>>    /// the correct dominance relationships for the operands and users hold.
>>    /// However, this method can return true for instructions that read memory;
>>    /// for such instructions, moving them may change the resulting value.
>> -  bool isSafeToSpeculativelyExecute(const Instruction *Inst,
>> +  bool isSafeToSpeculativelyExecute(const Value *V,
>>                                      const TargetData *TD = 0);
>> 
>>  } // end namespace llvm
>> 
>> Modified: llvm/trunk/lib/Analysis/ValueTracking.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=147560&r1=147559&r2=147560&view=diff
>> ==============================================================================
>> --- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
>> +++ llvm/trunk/lib/Analysis/ValueTracking.cpp Wed Jan  4 17:01:09 2012
>> @@ -1879,8 +1879,12 @@
>>    return true;
>>  }
>> 
>> -bool llvm::isSafeToSpeculativelyExecute(const Instruction *Inst,
>> +bool llvm::isSafeToSpeculativelyExecute(const Value *V,
>>                                          const TargetData *TD) {
>> +  const Operator *Inst = dyn_cast<Operator>(V);
>> +  if (!Inst)
>> +    return false;
>> +
>>    for (unsigned i = 0, e = Inst->getNumOperands(); i != e; ++i)
>>      if (Constant *C = dyn_cast<Constant>(Inst->getOperand(i)))
>>        if (C->canTrap())
>> 
>> 
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