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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 - [InstCombine] GEP is wrongly optimized for 32-bit targets"
href="https://bugs.llvm.org/show_bug.cgi?id=39908">39908</a>
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<th>Summary</th>
<td>[InstCombine] GEP is wrongly optimized for 32-bit targets
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>mati865@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>Forwarding <a href="https://github.com/rust-lang/rust/issues/52026">https://github.com/rust-lang/rust/issues/52026</a>
Minimal example by neivv and rkruppe:
```
target datalayout = "p:32:32"
%S = type { [2 x i32] }
define i1 @foo([0 x %S]* %p, i32 %n) {
%start.cast = bitcast [0 x %S]* %p to %S*
%end = getelementptr inbounds [0 x %S], [0 x %S]* %p, i32 0, i32 %n, i32 0,
i32 0
%end.cast = bitcast i32* %end to %S*
%last = getelementptr inbounds %S, %S* %end.cast, i32 -1
%cmp = icmp eq %S* %last, %start.cast
ret i1 %cmp
}
```
GodBolt link: <a href="https://godbolt.org/z/C84Axg">https://godbolt.org/z/C84Axg</a>
For 32 bit targets it turns to:
define i1 @foo([0 x %S]* %p, i32 %n) {
%cmp = icmp eq i32 %n, -536870911
ret i1 %cmp
}
While 64 bit targets are optimized correctly:
define i1 @foo([0 x %S]* %p, i32 %n) {
%cmp = icmp eq i32 %n, 1
ret i1 %cmp
}
Explanation of the issue from
<a href="https://github.com/rust-lang/rust/issues/52026#issuecomment-403628144">https://github.com/rust-lang/rust/issues/52026#issuecomment-403628144</a>:
In this issue, the code is comparing roughly (S *)slice_start == (S
*)slice_start + slice_len - 1.
This function is extracting the constant byte offset, -8, and dividing it by 8,
sizeof(S). However, before that, compiling to a 32-bit target, the values are
masked by PtrSizeMask 0xFFFFFFFF, which discards the sign bit of int64 and
gives +4294967288, which divided by 8 is the same 536870911 which ends up being
in the result.</pre>
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</p>
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