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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - bad and questionable codegen for inline asm returning a value in a fixed register"
href="https://bugs.llvm.org/show_bug.cgi?id=47447">47447</a>
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<th>Summary</th>
<td>bad and questionable codegen for inline asm returning a value in a fixed register
</td>
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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>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: RISC-V
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>sorear@fastmail.com
</td>
</tr>
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<th>CC</th>
<td>asb@lowrisc.org, llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>__builtin_thread_pointer() is not supported on released versions of risc-v gcc
or clang, so I looked for alternatives that would generate the same code. I
tried:
void *get_tp() {
register void *tp asm("tp");
asm("" : "=r"(tp));
return tp;
}
define i8* @get_tp() {
%1 = tail call i8* asm "", "={x4}"()
ret i8* %1
}
(<a href="https://gcc.godbolt.org/z/GP5ab4">https://gcc.godbolt.org/z/GP5ab4</a>)
This works fine on released gcc (can even fold into subsequent memory accesses
which become tp-relative) but llvm generates a superfluous stack frame as
though tp were a call-saved register. Since tp is a fixed register, I don't
think llvm is _allowed_ to insert saves and restores for it; the asm might be
erroneous, I am not sure of the intended semantics, but if it is accepted it
should be accepted without generating a stack frame.</pre>
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</p>
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