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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [ARM] llvm-objdump can't disassemble multiply instructions on pre-armv6"
href="https://bugs.llvm.org/show_bug.cgi?id=43274">43274</a>
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<th>Summary</th>
<td>[ARM] llvm-objdump can't disassemble multiply instructions on pre-armv6
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>8.0
</td>
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<th>Hardware</th>
<td>Other
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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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: ARM
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
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<th>Reporter</th>
<td>husseydevin@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, peter.smith@linaro.org, Ties.Stuij@arm.com
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<p>
<div>
<pre>Apparently, LLVM attempts to put an <unknown> if there is an illegal register
layout for the mul family of instructions on pre-ARMv6.
However, for whatever reason, instead, ALL mul instructions (even valid ones)
are disassembled as an <unknown>:
clang -O3 -c --target=arm-none-eabi -march=armv4t -fomit-frame-pointer mul.c
#include <stdint.h>
uint64_t mul(uint64_t a, uint64_t b)
{
return a * b;
}
llvm-objdump -d mul.o
mul.o: file format ELF32-arm-little
Disassembly of section .text:
0000000000000000 mul:
0: 10 40 2d e9 push {r4, lr}
4: 92 c0 8e e0 <unknown>
8: 92 e1 24 e0 <unknown>
c: 93 40 21 e0 <unknown>
10: 0c 00 a0 e1 mov r0, r12
14: 10 40 bd e8 pop {r4, lr}
18: 1e ff 2f e1 bx lr
arm-none-eabi-objdump -d mul.o
mul.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <mul>:
0: e92d4010 push {r4, lr}
4: e08ec092 umull ip, lr, r2, r0
8: e024e192 mla r4, r2, r1, lr
c: e0214093 mla r1, r3, r0, r4
10: e1a0000c mov r0, ip
14: e8bd4010 pop {r4, lr}
18: e12fff1e bx lr
I can confirm that this occurs on MUL, UMULL, MLA, and UMLAL. I haven't tested
other variants.
This does not occur if the binary targets ARMv6+.
This was tested on Clang 8.0.1, but I presume it also affects trunk.</pre>
</div>
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