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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 --- - AArch64 does not implement FLT_ROUNDS"
href="https://llvm.org/bugs/show_bug.cgi?id=25191">25191</a>
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</tr>
<tr>
<th>Summary</th>
<td>AArch64 does not implement FLT_ROUNDS
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: AArch64
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@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>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The C99 FLT_ROUNDS macro is supposed to return the current rounding mode of the
processor's floating point unit. Clang's __builtin_flt_rounds() can be used to
implement the FLT_ROUNDS macro.
Consider the following piece of code:
int foo(void) {
return __builtin_flt_rounds();
}
For x86-64 we build the following piece of code:
0000000000000000 <foo>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: d9 7d f0 fnstcw -0x10(%rbp)
7: 0f b7 45 f0 movzwl -0x10(%rbp),%eax
b: 89 c1 mov %eax,%ecx
d: 81 e1 00 04 00 00 and $0x400,%ecx
13: c1 e9 09 shr $0x9,%ecx
16: 25 00 08 00 00 and $0x800,%eax
1b: c1 e8 0b shr $0xb,%eax
1e: 09 c8 or %ecx,%eax
20: ff c0 inc %eax
22: 83 e0 03 and $0x3,%eax
25: 5d pop %rbp
26: c3 retq
Which is correct. Now if we build exactly the same code with Clang targeted
against aarch64, we obtain:
0000000000000000 <foo>:
0: 320003e0 orr w0, wzr, #0x1
4: d65f03c0 ret
This is obviously not correct. It should return a value based on bits 22:23 of
FPCR.
I tried patching up Clang myself to support this, but I have to confess that I
don't know what I'm doing. I started out copying
ARMTargetLowering::LowerFLT_ROUNDS_() to
AArch64TargetLowering::LowerFLT_ROUNDS_(), but then I have no idea how to fetch
the FPCR register.</pre>
</div>
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