| Issue |
142242
|
| Summary |
Empty basic blocks with unconditional branches aren't eliminated
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
ashivaram23
|
The following function calculates `a && b`:
```llvm
define i1 @a_and_b(i1 %a) {
start:
br i1 %a, label %a_is_true, label %a_or_b_is_false
a_is_true:
%b = call i1 @get_b()
br i1 %b, label %end, label %a_or_b_is_false
a_or_b_is_false:
br label %end
end:
%retval = phi i1 [ true, %a_is_true ], [ false, %a_or_b_is_false ]
ret i1 %retval
}
declare i1 @get_b()
```
No `-O3` pass is able to remove the `a_or_b_is_false` block, resulting in more assembly instructions than needed:
```nasm
_a_and_b:
push rbx
test dil, 1
je LBB0_2
mov bl, 1
call _get_b
test al, 1
jne LBB0_3
LBB0_2:
xor ebx, ebx
LBB0_3:
mov eax, ebx
pop rbx
ret
```
Instead, the function should become something like
```llvm
define i1 @a_and_b(i1 %a) {
start:
br i1 %a, label %a_is_true, label %end
a_is_true:
%b = tail call i1 @get_b()
br label %end
end:
%retval = phi i1 [ false, %start ], [ %b, %a_is_true ]
ret i1 %retval
}
```
and generate x86 instructions like this:
```nasm
_a_and_b:
test dil, 1
jne _get_b
xor eax, eax
ret
```
That should mostly happen in SimplifyCFGPass. I think the fix could be as simple as removing the check for `BB->phis().empty()` in `CanRedirectPredsOfEmptyBBToSucc` in Transforms/Utils/Local.cpp. A comment there says "There must be phis in BB, otherwise BB will be merged into Succ directly," but that's not always true.
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