[LLVMdev] getSmallConstantTripCount function doesn't work for obvious cases

[Andrew Trick]

On Jan 17, 2012, at 6:30 PM, Bo Wu <wubousc at gmail.com> wrote:

> Hi,
> 
> My pass heavily relies on llvm::Loop's getSmallConstantTripCount method. However, I found that it doesn't work for some simple cases.
> 
> In the following example, I can get the constant trip count of the outermost loop if statement "a[l] = a[l] + 1" is there. After commenting out this line, the returned constant trip count for that loop is 0. In my pass, I traverse the nested loops starting from statement "a[i] = b[i] * b[i+1]; ", and use a for loop to traverse each parent loop.
> 
> void f(int *a, int b[], double *c, int n)
> {
>   int i,j,k,l;
>   if(n>20) 
>     a[j] = 4;
>   else {
>     for(l=0; l<200; ++l) {
>       a[l] = a[l] + 1; //if remove this line, the above loop's trip count can not be obtained.
>       if(b[1]>30) {
>         c[k] = c[k] * c[k+1];
>       }
>       else {
>         for(k=0; k<400;++k) {
>           for(i=0; i<400; ++i) {
>             a[i] = b[i] * b[i+1]; // nested loop traversal starts here.
>           }
>         }
>       }
>     }
>   }
> }
> 
> As long as statement "a[l] = a[l] + 1;" is in the outermost loop body, the trip count can be obtained. Is there a way to get the constant trip count of a loop in similar cases, even if the induction variable is not used in the loop body?
> 
> Thanks,
> Bo

Using llvm trunk, SCEV is able to analyze the trip count of all three loops:

/extra/llvm/build/Release+Asserts/bin/clang -O2 t.c -mllvm -debug-only=loop-unroll -c
Loop Unroll: F[f] Loop %for.body22
  Loop Size = 10
  Too large to fully unroll with count: 400 because size: 4000>150
  will not try to unroll partially because -unroll-allow-partial not given
Loop Unroll: F[f] Loop %for.cond20.preheader
  Loop Size = 14
  Too large to fully unroll with count: 400 because size: 5600>150
  will not try to unroll partially because -unroll-allow-partial not given
Loop Unroll: F[f] Loop %for.body
  Loop Size = 36
  Too large to fully unroll with count: 200 because size: 7200>150
  will not try to unroll partially because -unroll-allow-partial not given

...maybe you're using an older llvmm before we move to SCEV-based trip counts.

-Andy