[LLVMdev] About Scalar Evolution Pass and SCEVAddRecExpr

[ihcinihsdk]

Hi, thank you for your suggestion. It works well now!


Hongbin Zheng wrote:
> 
> Hi,
> 
> You may try function "getSCEVAtScope" of the ScalarEvolution analysis:
> 
> /// getSCEVAtScope - Return a SCEV expression for the specified value
> /// at the specified scope in the program.  The L value specifies a loop
> /// nest to evaluate the expression at, where null is the top-level or a
> /// specified loop is immediately inside of the loop.
> ///
> /// This method can be used to compute the exit value for a variable
> defined
> /// in a loop by querying what the value will hold in the parent loop.
> ///
> /// In the case that a relevant loop exit value cannot be computed, the
> /// original value V is returned.
> 
> best regards
> ether
> 
> On Wed, Apr 25, 2012 at 5:16 PM, ihcinihsdk <ali28 at wisc.edu> wrote:
>>
>> Hi,
>> I wonder how many of you are familiar with scalar evolution pass. I met a
>> problem regarding to the SCEVAddRecExpr. Say for the code:
>> const int N = 100;
>> int a[N];
>> for(int i=0;i<N;i++) a[i] = 0;
>> For the access of a[i], the pass will transform this a[i] to a
>> SCEVAddRecExpr <@a, +, sizeof(int)><BB_Name>, which means the access of
>> the
>> array `a' starts from the address `a' and has an access stride of
>> `sizeof(int)'.
>> However, when this code is slightly modified:
>>
>> const int N=100;
>> int a[N];
>> int i;
>> for(i=0;i<N-1; i++) {}
>> a[i] = 0;
>>
>> So the loop is empty and there's an access of `a[i]' (actually a[99] in
>> this
>> case), this a[i] will still be transformed into the same SCEVAddRecExpr
>> as
>> above. This does not really seem to make sense to me because the access
>> pattern is not really so at all. Is is a small bug in this tool? Thank
>> you!
>> --
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>> http://old.nabble.com/About-Scalar-Evolution-Pass-and-SCEVAddRecExpr-tp33745284p33745284.html
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>>
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