[llvm-bugs] [Bug 32997] New: [Polly - AST Generation] Use != for upper loop bound

via llvm-bugs llvm-bugs at lists.llvm.org
Wed May 10 23:32:31 PDT 2017


            Bug ID: 32997
           Summary: [Polly - AST Generation] Use != for upper loop bound
           Product: Polly
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: isl
          Assignee: polly-dev at googlegroups.com
          Reporter: tobias at grosser.es
                CC: llvm-bugs at lists.llvm.org

Currently the isl AST generator derives

for (long i = LB; i <= UB; i++)

for most loops. An alternative would be to generate

for (long i = LB; i != UB + 1; i++)

In the second version a value of LB that is strictly larger than UB + 1 will
cause i to eventually overflow. As we know this is not valid (and we annotate i
in Polly with NSW), the IR carries the information that such an overflow can
never happen
and consequently that LB must be smaller or equal to UB + 1.

LLVM and its -indvars pass prefers the second alternative precisely for this

We should consider to generate IR with the above type.

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