| Issue |
132085
|
| Summary |
[StackColoring] Incorrect slot merging due to stackcoloring-lifetime-start-on-first-use
|
| Labels |
llvm:codegen,
miscompilation
|
| Assignees |
|
| Reporter |
tmiasko
|
In the program below, the allocation `a` and allocation `b` have overlapping
live ranges, so it should be impossible to observe them having the same
address. Nevertheless StackColoring merges stack slots for `a` and `b`. This
is incorrect since addresses of those allocations might be captured by `g`.
```llvm
define void @f() {
start:
%a = alloca [1000 x i8], align 1
%b = alloca [1000 x i8], align 1
call void @llvm.lifetime.start.p0(i64 1000, ptr %a)
call void @llvm.lifetime.start.p0(i64 1000, ptr %b)
call void @g(ptr %a)
call void @llvm.lifetime.end.p0(i64 1000, ptr %a)
call void @g(ptr %b)
call void @llvm.lifetime.end.p0(i64 1000, ptr %b)
ret void
}
declare void @g(ptr %n)
```
```console
$ llc-21 a.ll -print-before=stack-coloring -print-after=stack-coloring
# Machine code for function f: IsSSA, TracksLiveness
Frame Objects:
fi#0: size=1000, align=1, at location [SP+8]
fi#1: size=1000, align=1, at location [SP+8]
<snip>
# *** IR Dump After Merge disjoint stack slots (stack-coloring) ***:
# Machine code for function f: IsSSA, TracksLiveness
Frame Objects:
fi#0: size=1000, align=1, at location [SP+8]
fi#1: dead
<snip>
```
This is caused by stackcoloring-lifetime-start-on-first-use (the default) where
stack coloring shrinks live range towards first use.
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