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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - __int128 stack calling conventions are incorrect on x86-64"
href="https://bugs.llvm.org/show_bug.cgi?id=42439">42439</a>
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<th>Summary</th>
<td>__int128 stack calling conventions are incorrect on x86-64
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>srhines@google.com
</td>
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<div>
<pre>This is a bug report that was posted internally at Google, since the user can't
register here on bugs.llvm.org:
Clang violates x86-64 calling convention in the obscure case when __int128 is
passed on stack.
Conside the following two functions:
__int128 foo(__int128 x, __int128 y, __int128 z, uint64_t a, __int128 c) {
return x + y + z + a + c;
}
__int128 foo(__int128 x, __int128 y, __int128 z, uint64_t a, uint64_t b,
__int128 c) {
return x + y + z + a + c;
}
Gcc generates identical code for these because __int128 have to be stored
aligned on stack (psABI says:
Arguments of type __int128 offer the same operations as INTEGERs, yet they do
not fit into one general purpose register but require two registers.
For classification purposes __int128 is treated as if it were implemented as:
typedef struct {
long low, high;
} __int128;
with the exception that arguments of type __int128 that are stored in memory
must be aligned on a 16-byte boundary.
Clang generates different code for these two functions:
<a href="https://godbolt.org/z/ZKjb4Z">https://godbolt.org/z/ZKjb4Z</a>
Note that both GCC and Clang show the alignment of __int128 properly as 16
bytes, but this is only getting ignored when the variable is passed on the
stack.</pre>
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