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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 --- - [ARMv6] Optimization doesn't consider FPSCR as a register that may get clobbered by function calls"
href="https://llvm.org/bugs/show_bug.cgi?id=30439">30439</a>
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<th>Summary</th>
<td>[ARMv6] Optimization doesn't consider FPSCR as a register that may get clobbered by function calls
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>3.9
</td>
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<th>Hardware</th>
<td>Other
</td>
</tr>
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<th>OS</th>
<td>other
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<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>-New Bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>ed@80386.nl
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Consider the following piece of code:
---
extern void bar(void);
int foo(void) {
int x = __builtin_flt_rounds();
bar();
return x + __builtin_flt_rounds();
}
---
When built for ARMv6 with Clang 3.9.0, we get the following code:
---
00000000 <foo>:
0: e92d4c10 push {r4, sl, fp, lr}
4: e28db008 add fp, sp, #8
8: eef10a10 vmrs r0, fpscr
c: e3a01003 mov r1, #3
10: e2800501 add r0, r0, #4194304 ; 0x400000
14: e0010b20 and r0, r1, r0, lsr #22
18: e0804000 add r4, r0, r0
1c: ebfffffe bl 0 <bar>
20: e1a00004 mov r0, r4
24: e8bd8c10 pop {r4, sl, fp, pc}
---
Notice how FPSCR is only loaded once and multiplied by two, instead of being
loaded separately. This means that if bar() is implemented as a function
calling fesetround(), the result of this function would be incorrect.</pre>
</div>
</p>
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