False negative with loops + memory

[dtcxzyw]

False negative with loops + memory

[nunoplopes]

When you've loops you've to manually specify the unrolling factor. Since these loops iterate exactly 8 times, you need to pass the following options: -src-unroll=8 -tgt-unroll=8
Doing that, Alive2 can detect the problem: https://alive2.llvm.org/ce/z/qArern

We will implement automatic unroll factor detection in the future, but there's no ETA.

[dtcxzyw]

Reduced case:

define i32 @src() {
entry:
  br label %for.header

for.header:
  %ind1 = phi i64 [ %ind1.i, %for.latch ], [ 0, %entry ]
  %ind2 = phi i64 [ %ind2.i, %for.latch ], [ 5, %entry ]
  %rem = srem i64 %ind2, 4
  %cmp6 = icmp eq i64 %ind1, %rem
  br i1 %cmp6, label %exit1, label %for.latch

for.latch:
  %ind2.i = add i64 %ind2, 1
  %ind1.i = add i64 %ind1, 1
  %cond = icmp eq i64 %ind1.i, 8
  br i1 %cond, label %exit2, label %for.header

exit2:
  %cmp = icmp eq i64 %rem, 0
  %ret = zext i1 %cmp to i32
  ret i32 %ret

exit1:
  ret i32 0
}

define i32 @tgt() {
entry:
  br label %for.header

for.header:                                       ; preds = %for.latch, %entry
  %ind1 = phi i64 [ %ind1.i, %for.latch ], [ 0, %entry ]
  %ind2 = phi i64 [ %ind2.i, %for.latch ], [ 5, %entry ]
  %rem = srem i64 %ind2, 4
  %cmp6 = icmp eq i64 %ind1, %rem
  br i1 %cmp6, label %exit1, label %for.latch

for.latch:                                        ; preds = %for.header
  %ind2.i = add i64 %ind2, 1
  %ind1.i = add i64 %ind1, 1
  %cond = icmp eq i64 %ind1.i, 8
  br i1 %cond, label %exit2, label %for.header

exit2:                                            ; preds = %for.latch
  %ret = zext i1 false to i32
  ret i32 %ret

exit1:                                            ; preds = %for.header
  ret i32 0
}

Alive2: https://alive2.llvm.org/ce/z/7FhVgy

[dtcxzyw]

Thank you for explaining this!