Understanding unbounded optimums

[Harish]

Hi All,

I have a function with two pointer arguments, as shown in LLVM IR. When running a custom pass, I am getting an unbounded optimum error.
I want to understand why am I getting bounds for MemRef9 but not MemRef8? Please let me know if you need additional details.

LLVM IR:

declare void @test(<4 x float>* nocapture %0, <4 x float>* nocapture %1)

Custom Pass:

scop.Arrays()

<4 x float> MemRef8[*]; // Element size 16
<4 x float> MemRef9[*]; // Element size 16 

Code:
  for (const ScopStmt &Stmt : scop) {
      isl::set domain = Stmt.getDomain();
      for (MemoryAccess *MA : Stmt) {
        if (!MA->isScalarKind()) {
        isl::set accessed;
        accessed = MA->getAccessRelation().intersect_domain(domain).range();
        accessed.dump()
        accessed.lexmin_pw_multi_aff().dump()
        }
      }
    }

Dump:

[p_0, p_1] -> { MemRef9[i0] : i0 <= 7 + 8p_0 + 8p_1 and i0 >= 8p_0 + 8p_1 }
[p_0, p_1] -> { MemRef9[(8p_0 + 8p_1)] }
[p_0, p_1] -> { MemRef8[i0] }
/llvm-project/polly/lib/External/isl/isl_tab_pip.c:703: unbounded optimum

Thanks,
Harish C

[Michael Kruse]

The set { [p_0, p_1] -> { MemRef8[i0] } has no (lexical) minimum: i0 can be chosen arbitrarily small. If anything, it would approach negative infinity.

In contrast to that, in MemRef9[i0], has a lower bound `i0 >= 8p_0 + 8p_1`, hence that value is the minimum value of i0.

Michael

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