[LLVMdev] About Scalar Evolution Pass and SCEVAddRecExpr

[Hongbin Zheng]

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!
> --
> View this message in context: http://old.nabble.com/About-Scalar-Evolution-Pass-and-SCEVAddRecExpr-tp33745284p33745284.html
> Sent from the LLVM - Dev mailing list archive at Nabble.com.
>
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