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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 --- - Fail to generate indirect jmp with memory operand (for tail call)"
href="http://llvm.org/bugs/show_bug.cgi?id=16010">16010</a>
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<th>Summary</th>
<td>Fail to generate indirect jmp with memory operand (for tail call)
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Backend: X86
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>silvas@purdue.edu
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>We could fold a load + indirect jump through reg into just an indirect jump
through memory.
typedef void fn(int reg, int *pc);
fn *dispatch_tab[0xFF];
void test_no_mem_indirect_jmp_tail(int reg, int *pc) {
(*dispatch_tab[pc[0] & 0xFF])(reg, pc);
}
0000000000000000 <test_no_mem_indirect_jmp_tail>:
0: 8b 06 mov eax,DWORD PTR [rsi]
2: 0f b6 c0 movzx eax,al
5: 48 8b 04 c5 00 00 00 00 mov rax,QWORD PTR [rax*8+0x0]
d: ff e0 jmp rax
We could replace:
5: 48 8b 04 c5 00 00 00 00 mov rax,QWORD PTR [rax*8+0x0]
d: ff e0 jmp rax
With:
ff 24 c5 00 00 00 00 jmp QWORD PTR [rax*8+0x0]
Interestingly enough, GCC (4.7.3) fails to form the indirect-through-memory jmp
as well.
Also, we form an indirect-through-memory call in the same situation when there
is no tail call (GCC does too):
extern void use(int*);
typedef void fn(int reg, int *pc);
fn *dispatch_tab[0xFF];
void test_no_mem_indirect_jmp_notail(int reg, int *pc) {
(*dispatch_tab[pc[0] & 0xFF])(reg, pc);
use(pc);
}
0000000000000000 <test_no_mem_indirect_jmp_notail>:
0: 53 push rbx
1: 48 89 f3 mov rbx,rsi
4: 8b 03 mov eax,DWORD PTR [rbx]
6: 0f b6 c0 movzx eax,al
9: ff 14 c5 00 00 00 00 call QWORD PTR [rax*8+0x0] <----
10: 48 89 df mov rdi,rbx
13: 5b pop rbx
14: e9 00 00 00 00 jmp 19
<test_no_mem_indirect_jmp_notail+0x19>
Tested with clang built from trunk on Apr 20, 2013. (this is all with -O2
and -O3)</pre>
</div>
</p>
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