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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Miscompilation with `-jump-threading -correlated-propagation` and llvm.assume"
href="https://bugs.llvm.org/show_bug.cgi?id=33917">33917</a>
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<th>Summary</th>
<td>Miscompilation with `-jump-threading -correlated-propagation` and llvm.assume
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>5.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>alex@crichton.co
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<pre>In upgrading rust-lang/rust to LLVM 5.0 we've discovered that the compiler
itself will segfault when compiled with LLVM 5.0. This looks like it's a
misoptimization during one of the `-jump-threading -correlated-propagation`
passes.
We used `bugpoint` to shorten the program from ~1 million lines of IR to just a
few:
; ModuleID = 'small.ll'
source_filename = "bugpoint-output-834ffe1.bc"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
declare i8* @foo()
declare i32 @rust_eh_personality() unnamed_addr
; Function Attrs: nounwind
declare void @llvm.assume(i1) #0
define void @bar() personality i32 ()* @rust_eh_personality {
bb9:
%t9 = invoke i8* @foo()
to label %good unwind label %bad
bad: ; preds = %bb9
%t10 = landingpad { i8*, i32 }
cleanup
resume { i8*, i32 } %t10
good: ; preds = %bb9
%t11 = icmp ne i8* %t9, null
%t12 = zext i1 %t11 to i64
%cond = icmp eq i64 %t12, 1
br i1 %cond, label %if_true, label %done
if_true: ; preds = %good
call void @llvm.assume(i1 %t11)
br label %done
done: ; preds = %if_true, %good
ret void
}
attributes #0 = { nounwind }
This was tested out locally via `opt -S -jump-threading -correlated-propagation
foo.ll`, and then the results from before/after were diffed, yielding:
--- before.ll 2017-07-24 15:52:48.364787248 -0700
+++ after.ll 2017-07-24 15:53:34.701537275 -0700
@@ -21,13 +21,12 @@
resume { i8*, i32 } %t10
good: ; preds = %bb9
- %t11 = icmp ne i8* %t9, null
- %t12 = zext i1 %t11 to i64
+ %t12 = zext i1 true to i64
%cond = icmp eq i64 %t12, 1
br i1 %cond, label %if_true, label %done
if_true: ; preds = %good
- call void @llvm.assume(i1 %t11)
+ call void @llvm.assume(i1 true)
br label %done
done: ; preds = %if_true, %good
I think that the transformation here may be invalid? The `%t11` temporary is
inferred to being always `true` but that's not always the case coming out of
`@foo`</pre>
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