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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW " title="NEW --- - AnalyzeBranch() fails on non-equality comparison between a float value and zero on X86." href="https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_bugs_show-5Fbug.cgi-3Fid-3D23875&d=AwMBaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=pF93YEPyB-J_PERP4DUZOJDzFVX5ZQ57vQk33wu0vio&m=OiMR4kj55ME6PhLGpXFAB1ezQLBo6Dd0jkgJUbRbzcY&s=WPip-kw0X6yZKGmvw98RuOU29q02tYIW_CY1i6G8Kfw&e=">23875</a>
</td>
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<th>Summary</th>
<td>AnalyzeBranch() fails on non-equality comparison between a float value and zero on X86.
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<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>Linux
</td>
</tr>
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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>
</tr>
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<th>Component</th>
<td>Backend: X86
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>congh@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>In LLVM and on X86, AnalyzeBranch() returns true (which means failure) on the
branch in the following function:
void foo(float f) {
if (f != 0)
printf("*");
}
The generated machine code for this function is:
BB#0: derived from LLVM BB %entry
Live Ins: %XMM0
%XMM1<def,tied1> = XORPSrr %XMM1<undef,tied0>, %XMM1<undef>
UCOMISSrr %XMM0<kill>, %XMM1<kill>, %EFLAGS<imp-def>
JNE_1 <BB#1>, %EFLAGS<imp-use>
JNP_1 <BB#2>, %EFLAGS<imp-use>
Successors according to CFG: BB#1(1) BB#2(101)
BB#1: derived from LLVM BB %if.then
Predecessors according to CFG: BB#0
%EDI<def> = MOV32ri 42
TAILJMPd64 <ga:@putchar>, <regmask>, %RSP<imp-use>, %RSP<imp-use>,
%EDI<imp-use,kill>
BB#2: derived from LLVM BB %if.end
Predecessors according to CFG: BB#0
RETQ
To reproduce this problem, compile the test case shown in the bottom with the
command:
clang++ -O2 test.C -c -mllvm -debug 2>&1 | grep "unanalyzable fallthrough"
You will see the message:
Pre-merging due to unanalyzable fallthrough: BB#0 (derived from LLVM BB
'entry') -> BB#1 (derived from LLVM BB 'if.then')
which is emitted from lib/CodeGen/MachineBlockPlacement.cpp.
The test case:
#include <stdio.h>
void foo(float f) {
if (f != 0)
printf("*");
}</pre>
</div>
</p>
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