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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - miscompiling sub without nsw"
href="https://bugs.llvm.org/show_bug.cgi?id=47830">47830</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>miscompiling sub without nsw
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>11.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>Other
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: PowerPC
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>programmerjake@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, nemanja.i.ibm@gmail.com
</td>
</tr></table>
<p>
<div>
<pre>From <a href="https://github.com/rust-lang/rust/issues/72023#issuecomment-707901014">https://github.com/rust-lang/rust/issues/72023#issuecomment-707901014</a>
I didn't actually run it on a Power processor to test, but AFAICT the bug
reduces to this:
```rust
pub fn f(a: i64, b: i64) -> u64 {
if a < b {
0
} else {
a.wrapping_sub(b) as u64
}
}
```
generated IR:
<a href="https://rust.godbolt.org/z/GYT7oG">https://rust.godbolt.org/z/GYT7oG</a>
target datalayout = "e-m:e-i64:64-n32:64"
target triple = "powerpc64le-unknown-linux-gnu"
define i64 @_ZN7example1f17h0a621b2f201f6c8fE(i64 %a, i64 %b) unnamed_addr #0
!dbg !6 {
%_3 = icmp slt i64 %a, %b, !dbg !10
%0 = sub i64 %a, %b
%spec.select = select i1 %_3, i64 0, i64 %0, !dbg !11
ret i64 %spec.select, !dbg !12
}
attributes #0 = { norecurse nounwind nonlazybind readnone uwtable
"target-cpu"="ppc64le" }
<snip>
LLVM miscompiles it assuming that `a.wrapping_sub(b)` won't have signed
overflow.
<a href="https://rust.godbolt.org/z/49fncx">https://rust.godbolt.org/z/49fncx</a>
Output assembly:
```
example::f:
sub. 3, 3, 4
isellt 3, 0, 3
blr
.long 0
.quad 0
```
the `sub.` sets the condition register `CR0` based on sign of wrapped result,
not on if `r3` is less than, equal to, or greater than `r4`.</pre>
</div>
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