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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 - Redundant Restore of $x0 when memcpy always returns the first argument."
href="https://bugs.llvm.org/show_bug.cgi?id=38849">38849</a>
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<th>Summary</th>
<td>Redundant Restore of $x0 when memcpy always returns the first argument.
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>7.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>Other
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>hiraditya@msn.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
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<div>
<pre>$ cat test.cpp
struct BigStruct {
int x[64];
};
void structByValue(BigStruct s);
void callStructByValue(int unused, int unused2, BigStruct s) {
structByValue(s);
}
$ ./bin/clang++ -O3 -arch arm64 redundant-reg.cpp -S -o -
.section __TEXT,__text,regular,pure_instructions
.ios_version_min 5, 0
.globl __Z17callStructByValueii9BigStruct ; -- Begin function
_Z17callStructByValueii9BigStruct
.p2align 2
__Z17callStructByValueii9BigStruct: ; @_Z17callStructByValueii9BigStruct
.cfi_startproc
; %bb.0: ; %entry
sub sp, sp, #288 ; =288
stp x28, x27, [sp, #256] ; 8-byte Folded Spill
stp x29, x30, [sp, #272] ; 8-byte Folded Spill
add x29, sp, #272 ; =272
.cfi_def_cfa w29, 16
.cfi_offset w30, -8
.cfi_offset w29, -16
.cfi_offset w27, -24
.cfi_offset w28, -32
mov x8, x2
mov x0, sp <<<<------------------- Copy
orr w2, wzr, #0x100
mov x1, x8
bl _memcpy
mov x0, sp <<<<------------------- Copy
bl __Z13structByValue9BigStruct
ldp x29, x30, [sp, #272] ; 8-byte Folded Reload
ldp x28, x27, [sp, #256] ; 8-byte Folded Reload
add sp, sp, #288 ; =288
ret
.cfi_endproc
; -- End function
.subsections_via_symbols
We could just do remove the second 'mov x0, sp' as memcpy is guaranteed to
return the pointer to desination.
<a href="http://man7.org/linux/man-pages/man3/memcpy.3.html">http://man7.org/linux/man-pages/man3/memcpy.3.html</a></pre>
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