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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [missed optimisation] Reading a callee-saved register writes to the stack"
href="http://llvm.org/bugs/show_bug.cgi?id=22735">22735</a>
</td>
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<tr>
<th>Summary</th>
<td>[missed optimisation] Reading a callee-saved register writes to the stack
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</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>Linux
</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>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>adam@consulting.net.nz
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>There is no way to inform the compiler one is reading a callee-saved register
without the compiler also writing the register to the stack:
read_callee_saved_register.c:
#include <stdint.h>
uint64_t read_rbx(void) {
register uint64_t rbx asm ("rbx");
uint64_t value;
asm ("mov %[rbx], %[value]" : [value] "=r" (value) : [rbx] "r" (rbx));
return value;
}
int main(void) {
return 0;
}
The code above explicitly tells the complier that local register variable rbx
is only being read. Here is the output of gcc and clang respectively:
$ gcc-snapshot.sh -O3 read_callee_saved_register.c && objdump -d -m i386:x86-64
a.out|less
00000000004004c0 <read_rbx>:
4004c0: 48 89 d8 mov %rbx,%rax
4004c3: 53 push %rbx
4004c4: 5b pop %rbx
4004c5: c3 retq
4004c6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
4004cd: 00 00 00
$ clang-3.7 -O3 read_callee_saved_register.c && objdump -d -m i386:x86-64
a.out|less
00000000004004e0 <read_rbx>:
4004e0: 53 push %rbx
4004e1: 48 89 d8 mov %rbx,%rax
4004e4: 5b pop %rbx
4004e5: c3 retq
4004e6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
4004ed: 00 00 00
Neither gcc nor clang should be pushing and popping rbx.
Note: gcc already trusts that rbx is merely being read since the assembly is
inserted before rbx is pushed and popped off the stack.</pre>
</div>
</p>
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