-polyhedral-info -polly-check-parallel analyzes non parallel loop as parallel.

[Quuxplusone]

Bugzilla Link	PR36388
Status	NEW
Importance	P normal
Reported by	Masaaki Takiuchi (takiuchi@oscartech.jp)
Reported on	2018-02-14 21:40:17 -0800
Last modified on	2019-11-15 05:45:04 -0800
Version	unspecified
Hardware	PC Linux
CC	bin.narwal@gmail.com, llvm-bugs@lists.llvm.org
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The outer loop of the following program is analyzed as parallel by -polyhedral-
info -polly-check-parallel. But the outerloop has loop-carried dependence and
is analyzed as not parallel by -polly-parallel analysis.

float a[256],b[256];
float aa[256][256],bb[256][256],cc[256][256];
int main () {
  for (int i = 1; i < 256;i++) {
    a[i] = a[i-1]+b[i] ;
    for (int j=0;j<256;j++ ) {
       aa[i][j] = aa[i][j]+cc[i][j];
    }
  }
  return 0;
}

clang -O1 -S -emit-llvm -o loop.s loop.c
opt -S -polly-canonicalize loop.s > loop.preopt.ll
opt -polly-parallel -polly-ast -polyhedral-info -polly-check-parallel -analyze -
q loop.preopt.ll -polly-process-unprofitable

Printing analysis 'Polly - Generate an AST from the SCoP (isl)' for region:
'for.body8 => for.cond.cleanup7' in function 'main':
Printing analysis 'Polly - Generate an AST from the SCoP (isl)' for region:
'for.body => for.cond.cleanup' in function 'main':
:: isl ast :: main :: %for.body---%for.cond.cleanup

if (1)

    #pragma minimal dependence distance: 1
    for (int c0 = 0; c0 <= 254; c0 += 1) {
      Stmt0(c0);
      #pragma simd
      #pragma known-parallel
      for (int c1 = 0; c1 <= 255; c1 += 1)
        Stmt1(c0, c1);
    }

else
    {  /* original code */ }

Printing analysis 'Polly - Generate an AST from the SCoP (isl)' for region:
'entry => <Function Return>' in function 'main':
Printing analysis 'Polly - Interface to polyhedral analysis engine' for
function 'main':
  for.body:     Loop is parallel.
  for.body8:    Loop is parallel.

[Quuxplusone]

I think PolyhedralInfo::getScheduleForLoop is not working as expected for parallelism checking.