<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - unexpected failure for -mno-sse2"
href="https://bugs.llvm.org/show_bug.cgi?id=43334">43334</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>unexpected failure for -mno-sse2
</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>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>ndesaulniers@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com, srhines@google.com
</td>
</tr>
<tr>
<th>Blocks</th>
<td>4068
</td>
</tr></table>
<p>
<div>
<pre>Consider:
$ cat foo.c
// double __floatunsidf (unsigned int i)
double foo(unsigned int i) {
return (double) i;
}
$ clang -O2 -mno-sse2 foo.c
error: SSE2 register return with SSE2 disabled
return (double) i;
^
$ gcc -O2 -mno-sse2 foo.c && llvm-objdump -d foo.o
foo:
movl %edi, %edi
movq %rdi, -8(%rsp)
fildq -8(%rsp)
fstpl -8(%rsp)
movlps -8(%rsp), %xmm0
ret
Adding -mno-80387 to clang seems to then complicate things further: clang
generates:
foo: # @foo
pushq %rax
callq __floatunsidf
popq %rcx
retq
This is important because:
1. the kernel builds with -mno-see2 -mno-80387 (and a few other related but
less relevant flags:
<a href="https://github.com/ClangBuiltLinux/linux/blob/52a5525214d0d612160154d902956eca0558b7c0/arch/x86/Makefile#L61-L62">https://github.com/ClangBuiltLinux/linux/blob/52a5525214d0d612160154d902956eca0558b7c0/arch/x86/Makefile#L61-L62</a>)
since saving/restoring fp register state gets complicated/expensive for all
registers added via ISA extensions).
2. the kernel does not define soft float routines like __floatunsidf.
-mhard-float and -ffreestanding don't prevent clang from generating references
to these soft float routines.</pre>
</div>
</p>
<div id="referenced">
<hr style="border: 1px dashed #969696">
<b>Referenced Bugs:</b>
<ul>
<li>
[<a class="bz_bug_link
bz_status_CONFIRMED "
title="CONFIRMED - [Meta] Compiling the Linux kernel with clang"
href="https://bugs.llvm.org/show_bug.cgi?id=4068">Bug 4068</a>] [Meta] Compiling the Linux kernel with clang
</li>
</ul>
</div>
<br>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>