| Issue |
169361
|
| Summary |
Behavior of lifetime intrinsics depends on code that is never executed
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
tmiasko
|
Behavior of lifetime intrinsics depends on instructions that are yet to be executed or are never executed, with calamitous consequences.
Consider allocations `a` and `b`. Their lifetimes are disjoint, except for a single code path where they do overlap. This code path is executed only when allocations have been observed to have the same address -- an impossibility. On the one hand this code path must be dead, on the other hand removing it might change the behavior because lifetimes are no longer overlapping.
An [Alive2](https://alive2.llvm.org/ce/z/xB9o52) example demonstrating surprising consequences. In the example, it is invalid to replace `%1` with `1` despite preceding assumption `call void @llvm.assume(i1 %1)`. The nature of this specific failure might be an implementation detail of Alive2, but I hope conveys the overall sense of the issue.
```llvm
define void @src() {
%a = alloca ptr
%b = alloca ptr
call void @llvm.lifetime.start.p0(ptr %a)
call void @llvm.lifetime.end.p0(ptr %a)
call void @llvm.lifetime.start.p0(ptr %b)
call void @llvm.lifetime.end.p0(ptr %b)
%1 = icmp ne ptr %a, %b
call void @llvm.assume(i1 %1)
br i1 %1, label %bb2, label %bb1
bb1:
call void @llvm.lifetime.start.p0(ptr %a)
call void @llvm.lifetime.start.p0(ptr %b)
call void @llvm.lifetime.end.p0(ptr %b)
call void @llvm.lifetime.end.p0(ptr %a)
br label %bb2
bb2:
ret void
}
define void @tgt() {
%a = alloca ptr
%b = alloca ptr
call void @llvm.lifetime.start.p0(ptr %a)
call void @llvm.lifetime.end.p0(ptr %a)
call void @llvm.lifetime.start.p0(ptr %b)
call void @llvm.lifetime.end.p0(ptr %b)
%1 = icmp ne ptr %a, %b
call void @llvm.assume(i1 %1)
br i1 1, label %bb2, label %bb1
bb1:
call void @llvm.lifetime.start.p0(ptr %a)
call void @llvm.lifetime.start.p0(ptr %b)
call void @llvm.lifetime.end.p0(ptr %b)
call void @llvm.lifetime.end.p0(ptr %a)
br label %bb2
bb2:
ret void
}
```
```
Transformation doesn't verify!
ERROR: Source is more defined than target
```
![]()
_______________________________________________
llvm-bugs mailing list
[email protected]
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
