[llvm-dev] Question about LLVM region analysis

[Anshunkang Zhou via llvm-dev]

Hi, developers,

    Recently, I am trying to use the LLVM Region analysis, the LLVM
version is 10. I use opt to print out the region analysis result using
"opt -view-regions-only region.ll". However, I am confused about the
result produced by LLVM, I found that LLVM seems to skip loop
preheader when grouping blocks into regions. Here is a simple example
code:

```region.c
void split(int a, int b) {

    int aaa = a * b;

    if (aaa > 100) {

        for (int i = 0; i < 100; i++) {

            printf("%d\n", i);
            if (i == 10)
                printf("asdsad\n");
        }

    } else if (aaa < 100) {

        printf("elif\n");
    }
}
```

Running "opt -regions -analyze region.ll" I got:

```
Printing analysis 'Detect single entry single exit regions' for
function 'split':
Region tree:
[0] entry => <Function Return>
  [1] entry => if.end9
    [2] for.cond => for.end
      [3] for.body => if.end
    [2] if.else => if.end8
End region tree
```

LLVM uses "for.cond" as the entry and "for.end" as the exit, but there
is a loop preheader block before "for.cond" named "if.then", using
"if.then" as the entry can still get a legal region. So I wonder why
LLVM does not contain the loop preheader into the region. The dot file
is in the mail attachment.

Thanks!
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