getSmallConstantTripCount function doesn't work for obvious cases

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#1

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

#2

It's usually better to give IR testcases, but I'll assume you ran the
equivalent of clang -O2 over it on some 64-bit platform.

It looks like a missed optimization is causing the tail of the loop to
end up in a strange form:

for.inc29: ; preds =
%for.inc26, %if.then4
  %exitcond38 = icmp eq i32 %l.036, 200
  br i1 %exitcond38, label %if.end32, label %for.inc29.for.body_crit_edge

for.inc29.for.body_crit_edge: ; preds = %for.inc29
  %phitmp = add i32 %l.036, 1
  br label %for.body

getSmallConstantTripCount doesn't know how to deal with this, so it
fails. Depending on what you're doing, I'd suggest using SCEV, which
is a bit more powerful and can partially analyze this loop.

-Eli

#3

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