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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 - false positive: MD_misexpect is not updated after the branch condition is flipped"
href="https://bugs.llvm.org/show_bug.cgi?id=45073">45073</a>
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<th>Summary</th>
<td>false positive: MD_misexpect is not updated after the branch condition is flipped
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Transformation Utilities
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>wmi@google.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
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<pre>We turned on -Wmisexpect and found some false positive warning. Here is a
testcase to show the problem. The cause is that MD_misexpect is not updated
after the branch condition is flipped. This looks like a general issue of
MD_misexpect.
---------------- 1.ll --------------------
declare void @goo()
declare i64 @llvm.expect.i64(i64, i64)
define void @foo(i1 %t0) {
%t1 = xor i1 %t0, true
%t2 = zext i1 %t1 to i64
%t4 = call i64 @llvm.expect.i64(i64 %t2, i64 0)
%t3 = icmp ne i64 %t2, 0
br i1 %t3, label %true, label %false, !prof !0, !misexpect !1
true:
call void @goo()
br label %false
false:
ret void
}
!0 = !{!"branch_weights", i32 1, i32 2000}
!1 = !{!"misexpect", i64 1, i64 2000, i64 1} ====> misexpect predicts else
branch is hot, which matches with !prof !0.
------------------------------------------
~/workarea/llvm/build/d20200121/bin/opt -instcombine -S 1.ll
Note that the branch condition is flipped after instcombine.
; ModuleID = '1.ll'
source_filename = "1.ll"
declare void @goo()
; Function Attrs: nounwind readnone willreturn
declare i64 @llvm.expect.i64(i64, i64) #0
define void @foo(i1 %t0) {
br i1 %t0, label %false, label %true, !prof !0, !misexpect !1
true: ; preds = %0
call void @goo()
br label %false
false: ; preds = %0, %true
ret void
}
attributes #0 = { nounwind readnone willreturn }
!0 = !{!"branch_weights", i32 2000, i32 1}
!1 = !{!"misexpect", i64 1, i64 2000, i64 1} ====> misexpect still predicts
else branch is hot, which contradicts with !prof !0.</pre>
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