[llvm-bugs] [Bug 36589] New: [LoopVersioningLICM] wrong code

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36589

            Bug ID: 36589
           Summary: [LoopVersioningLICM]  wrong code
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: paulsson at linux.vnet.ibm.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 19994
  --> https://bugs.llvm.org/attachment.cgi?id=19994&action=edit
reduced test case

It seems that passing -loop-versioning-licm produces wrong result with the
following reduced csmith program:

a[6];
int b = 5, c = 0, d = 0, f = 0, j = 0;
short e = 0, i = 0;
g[50];
*h = &f;
main() {
  for (; c < 6; c++)
    a[5] = c;
  for (; e >= 0; e--) {
    *h = 1;
    d = 0;
    for (; d <= 4; d++) {
      g[e] = i;
      g[d] = 80;
    }
  }
  b = a[b ^ g[j]];
  printf("checksum = %X\n", b);
}

The checksum becomes 0 instead of 5 (per 8 other compilations).

I added some extra printf statements like:
...
    for (; d <= 4; d++) {
      g[e] = i;
      g[d] = 80;
    }
  }
  printf("b = %d, g[0]=%d, ", b, g[0]);
  int L = b ^ g[j];
  printf ("L = %d ", L);
  b = a[L];
  printf("b = %d, j = %d ", b, j);
  printf("checksum = %X\n", b);
}

and got
...
b = 5, g[0]=0, L = 5 b = 5, j = 0 checksum = 5
...
b = 5, g[0]=80, L = 85 b = 0, j = 0 checksum = 0

, which shows that g[0] is 80, and not 0 as it should be, since i and e are
constant 0.

A minimal opt line, and debug output:
bin/opt -mtriple=s390x-linux-gnu -mcpu=z13 tc_lv_licm.bc -o tc_lv_licm.opt.ll
-S -tbaa -scoped-noalias -loop-rotate -licm -loop-unroll -gvn -sccp -instcombin
e -globalopt -loop-versioning-licm -licm -debug-only=loop-versioning-licm       

Loop: Loop at depth 1 containing:
%for.body4<header>,%for.body8.lr.ph,%for.body8.prol.preheader,%for.body8.prol,%for.body8.prol.loopexit.unr-lcssa,%for.body8.prol.loopexit,%for.body8.lr.ph.ne
w,%for.body8,%for.cond5.for.end14_crit_edge.unr-lcssa,%for.cond5.for.end14_crit_edge,%for.end14<latch><exiting>
    Loop Versioning found to be beneficial

Loop: Loop at depth 1 containing: %for.body<header><latch><exiting>
    LAA: Runtime check not found !!
    Loop instructions not suitable for LoopVersioningLICM

Loop: Loop at depth 1 containing: %for.body.prol<header><latch><exiting>
    Parallel loop is not worth versioning
    Loop structure not suitable for LoopVersioningLICM

It seems then that it is only one loop that is handled.

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