https://bugs.llvm.org/show_bug.cgi?id=43802
Bug ID: 43802
Summary: [LVI] Miscompile by CorrelatedValuePropagation: LVI
doesn't take nsw into account
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Consider following code:
define i32 @foo(i32 %arg) {
bb:
%tmp = sub nsw i32 0, %arg
%tmp1 = icmp eq i32 %arg, -2147483648
br i1 %tmp1, label %bb2, label %bb3
bb2: ; preds = %bb
br label %bb3
bb3: ; preds = %bb2, %bb
%tmp4 = phi i32 [ -2147483648, %bb2 ], [ %tmp, %bb ]
ret i32 %tmp4
}
This cmd:
opt -correlated-propagation -S repro.ll
Produces following result:
define i32 @foo(i32 %arg) {
bb:
%tmp = sub nsw i32 0, %arg
%tmp1 = icmp eq i32 %arg, -2147483648
br i1 %tmp1, label %bb2, label %bb3
bb2: ; preds = %bb
br label %bb3
bb3: ; preds = %bb2, %bb
ret i32 %tmp
}
Before the transformation result of `foo` was defined for all `%arg` values,
after the transformation result of `foo` when `%arg == MIN_INT` is `undef`.
This regression was introduced by following commit:
commit 3b6d46761f139e7ee3057758698efa4487323829
Author: Sanjay Patel <[email protected]>
Date: Tue Apr 10 20:42:39 2018 +0000
[CVP] simplify phi with constant incoming values that match common variable
edge values
This is based on an example that was recently posted on llvm-dev:
void *propagate_null(void* b, int* g) {
if (!b) {
return 0;
}
(*g)++;
return b;
}
https://godbolt.org/g/xYk3qG
The original code or constant propagation in other passes has obscured the
fact
that the phi can be removed completely.
Differential Revision: https://reviews.llvm.org/D45448
llvm-svn: 329755
The transformation introduced by the commit replaces phi when all incoming
values except one are constants and that one non-constant incoming value is
equal to every constant when coming over the corresponding edge. To check this
condition it queries LazyValueInfo::getConstantOnEdge for the non-constant
value and the incoming edge and compares the result with the corresponding
incoming constant. It appears that LazyValueInfo::getConstantOnEdge doesn't
take the `nsw` flag into account.
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