[llvm-bugs] [Bug 28871] New: Scalar dependence on induction variable when lower bound is not synthesizable

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=28871

            Bug ID: 28871
           Summary: Scalar dependence on induction variable when lower
                    bound is not synthesizable
           Product: Polly
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Optimizer
          Assignee: polly-dev at googlegroups.com
          Reporter: llvm at meinersbur.de
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

This inner loops is not detected as parallel (-mllvm
-polly-process-unprofitable -mllvm -polly-parallel):

for (int j = 1; j <= lastrow - firstrow + 1; j++) {
    int e = rowstr[j + 1];
    int b = rowstr[j];
     for (int k = b; k < e; k++) {
         colidx[k] = colidx[k] - firstcol + 1;
     }
}


AST output:
      Stmt_for_body();
      if (p_1_loaded_from_rowstr >= p_0_loaded_from_rowstr + 1)
        Stmt_for_body7_lr_ph();
      #pragma minimal dependence distance: 1
      for (int c0 = 0; c0 < -p_0_loaded_from_rowstr + p_1_loaded_from_rowstr;
c0 += 1)
        Stmt_for_body7(c0);




Whereas this one is:

for (int j = 1; j <= lastrow - firstrow + 1; j++) {
    int e = rowstr[j + 1];
    int b = rowstr[j];
     for (int k = 0; k < e-b; k++)
     {
         colidx[k+b] = colidx[k+b] - firstcol + 1;
     }
}


AST output:
      Stmt_for_body();
      if (p_1_loaded_from_rowstr >= p_0_loaded_from_rowstr + 1)
        Stmt_for_body8_lr_ph();
      #pragma simd
      #pragma known-parallel
      for (int c0 = 0; c0 < -p_0_loaded_from_rowstr + p_1_loaded_from_rowstr;
c0 += 1)
        Stmt_for_body8(c0);



This is cause by the lower bound ('b') appearing in the lower bound of the SCEV
for the induction variable. It is not considered synthesizable albeit
load-hoisted. Thus causing scalar dependences created for its PHINode.

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