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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 --- - The direction flag in the FLAGS register may be wrong in interrupt handler"
href="https://llvm.org/bugs/show_bug.cgi?id=26406">26406</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>The direction flag in the FLAGS register may be wrong in interrupt handler
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<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>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>hjl.tools@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>a.bataev@hotmail.com, amjad.aboud@intel.com, llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The x86 psABI says
The direction flag DF in the FLAGS register must be clear (set to “forward”
direction) on function entry and return. Other user flags have no specified
role in the standard calling sequence and are not preserved across calls
Since the direction flag (DF) in the FLAGS register is undetermined in
interrupt handler, the result will be wrong if DF isn't cleared:
[hjl@gnu-6 bin]$ cat /tmp/x.i
extern void *a;
extern int b;
__attribute__ ((interrupt))
void
foo (void *frame)
{
__builtin_memset (a, b, 40);
}
[hjl@gnu-6 bin]$ ./clang -S -Os /tmp/x.i -m32 -mno-sse
[hjl@gnu-6 bin]$ cat x.s
.text
.file "/tmp/x.i"
.globl foo
.type foo,@function
foo: # @foo
# BB#0: # %entry
pushl %esp
pushl %edx
pushl %ecx
pushl %eax
subl $12, %esp
subl $4, %esp
pushl $40
pushl b
pushl a
calll memset
addl $16, %esp
addl $12, %esp
popl %eax
popl %ecx
popl %edx
popl %esp
iretl
.Lfunc_end0:
.size foo, .Lfunc_end0-foo
.ident "clang version 3.9.0 (<a href="http://llvm.org/git/clang.git">http://llvm.org/git/clang.git</a>
1f64ddbc4c5d1036b68ec896765a7535537ded85) (<a href="http://llvm.org/git/llvm.git">http://llvm.org/git/llvm.git</a>
43b517fe4e0a181b1cf20f36fd9eb92f7b32946c)"
.section ".note.GNU-stack","",@progbits
[hjl@gnu-6 bin]$
The current interrupt spec:
<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66960#c4">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66960#c4</a>
has
Since the direction flag in the FLAGS register in interrupt (exception)
handlers is undetermined, cld instruction must be emitted in function
prologue if rep string instructions are used in interrupt (exception)
handler or interrupt (exception) handler isn't a leaf function.</pre>
</div>
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