[llvm-bugs] [Bug 159697] [LoopVectorizer] The vectorization condition for the "early exit loop" is too strict.

[LLVM Bugs via llvm-bugs]

Issue	159697
Summary	[LoopVectorizer] The vectorization condition for the "early exit loop" is too strict.
Labels	new issue
Assignees
Reporter	Guo-yyds

    For the new feature "early exit loop vectorization", the specific restrictions include that there must be **no potentially faulty loads**. 

I understand this is for safety considerations, such as in the provided test ll file.

```
define i64 @same_exit_block_pre_inc_use1() {
; CHECK-LABEL: LV: Checking a loop in 'same_exit_block_pre_inc_use1'
; CHECK:       LV: Found an early exit loop with symbolic max backedge taken count: 63
; CHECK-NEXT:  LV: We can vectorize this loop!
; CHECK-NOT:   LV: Not vectorizing
entry:
  %p1 = alloca [1024 x i8]
  %p2 = alloca [1024 x i8]
  call void @init_mem(ptr %p1, i64 1024)
  call void @init_mem(ptr %p2, i64 1024)
  br label %loop

loop:
  %index = phi i64 [ %index.next, %loop.inc ], [ 3, %entry ]
  %arrayidx = getelementptr inbounds i8, ptr %p1, i64 %index
  %ld1 = load i8, ptr %arrayidx, align 1
  %arrayidx1 = getelementptr inbounds i8, ptr %p2, i64 %index
  %ld2 = load i8, ptr %arrayidx1, align 1
  %cmp3 = icmp eq i8 %ld1, %ld2
  br i1 %cmp3, label %loop.inc, label %loop.end

loop.inc:
  %index.next = add i64 %index, 1
  %exitcond = icmp ne i64 %index.next, 67
  br i1 %exitcond, label %loop, label %loop.end

loop.end:
  %retval = phi i64 [ %index, %loop ], [ 67, %loop.inc ]
  ret i64 %retval
}
```

For this example, where p1 and p2 are allocated within the function, in many cases, the array is pre-allocated and then passed in as a parameter. 

I provide the following examples:

This does not meet the requirement of "no potentially faulty loads".
```
int find_v0(vector<int> &a, int target) {
    for (int i = 0; i < a.size(); i++) {
        if (a[i] == target) {
            return i;
 }
    }
    return -1;
}
```

It is clearly safe to read, but currently it cannot be vectorized.

I would like to know if there is a plan to complete this task by adding some annotations in a loop, so that it can be vectorized.

If not, I will try to do it.

_______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs