https://bugs.llvm.org/show_bug.cgi?id=43188
Bug ID: 43188
Summary: Incorrect fold of uadd.with.overflow with undef
Product: libraries
Version: trunk
Hardware: All
OS: All
Status: NEW
Keywords: miscompilation
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected]
Alive2 complains about a transformation in
Transforms/ConstProp/overflow-ops.ll:
define {i8, i1} @uadd_undef() {
%0:
%t = uadd_overflow i8 142, undef
ret {i8, i1} %t
}
=>
define {i8, i1} @uadd_undef() {
%0:
ret {i8, i1} undef
}
Transformation doesn't verify!
ERROR: Value mismatch
Example:
Source:
{i8, i1} %t = { #x8e (142, -114), #x0 (0) } [based on undef value]
Target:
Source value: { #x8e (142, -114), #x0 (0) } [based on undef value]
Target value: { #x00 (0), #x0 (0) }
In summary, there's no value in the source that the undef can take that allows
uadd to return {0, 0}. To return 0, it has to overflow (unsigned), and hence
the 2nd value would be 1.
Two valid return values I can think off: {%a, 0}; {undef, 1}.
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