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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - __attribute__((no_caller_saved_registers)) ignored for struct members and typedefs"
href="https://bugs.llvm.org/show_bug.cgi?id=50121">50121</a>
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<th>Summary</th>
<td>__attribute__((no_caller_saved_registers)) ignored for struct members and typedefs
</td>
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<th>Product</th>
<td>clang
</td>
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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>All
</td>
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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>-New Bugs
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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</tr>
<tr>
<th>Reporter</th>
<td>jhaberman@gmail.com
</td>
</tr>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>Take the following program that uses no_caller_saved_registers:
__attribute__((no_caller_saved_registers)) void g();
int f(int x) {
g();
return x;
}
This produces the expected output, edi is preserved across the call:
0000000000000000 <f>:
0: 50 push rax
1: 31 c0 xor eax,eax
3: e8 00 00 00 00 call 8 <f+0x8>
4: R_X86_64_PLT32 g-0x4
8: 89 f8 mov eax,edi
a: 59 pop rcx
b: c3 ret
However the following program does not generate the same output:
struct {
__attribute__((no_caller_saved_registers)) void (*g)();
} st;
int f(int x) {
st.g();
return x;
}
In this case the attribute is ignored, and edi is spilled to a normal
callee-save register:
0000000000000000 <f>:
0: 53 push rbx
1: 89 fb mov ebx,edi
3: 31 c0 xor eax,eax
5: ff 15 00 00 00 00 call QWORD PTR [rip+0x0] # b <f+0xb>
7: R_X86_64_PC32 st-0x4
b: 89 d8 mov eax,ebx
d: 5b pop rbx
e: c3 ret
If we copy into a local variable with the attribute, it works:
struct {
__attribute__((no_caller_saved_registers)) void (*g)();
} st;
int f(int x) {
__attribute__((no_caller_saved_registers)) void (*local)() = st.g;
local();
return x;
}
However it does not work if the attribute is on a typedef:
typedef __attribute__((no_caller_saved_registers)) void g_type();
g_type *g;
int f(int x) {
g();
return x;
}
This bug appears to be similar to <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED FIXED - __attribute__((__format__(printf ignored for function pointer members of struct"
href="show_bug.cgi?id=21082">https://bugs.llvm.org/show_bug.cgi?id=21082</a></pre>
</div>
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