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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [arm assembler]: Incorrect ARM alignment NOPs in mixed ARM and Thumb assembly"
href="https://bugs.llvm.org/show_bug.cgi?id=48512">48512</a>
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<th>Summary</th>
<td>[arm assembler]: Incorrect ARM alignment NOPs in mixed ARM and Thumb assembly
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<th>Product</th>
<td>libraries
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<th>Version</th>
<td>10.0
</td>
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<th>Hardware</th>
<td>Other
</td>
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
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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: ARM
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>husseydevin@gmail.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, smithp352@googlemail.com, Ties.Stuij@arm.com
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<pre>Seems to be related to <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED FIXED - ARM integrated assembler generates incorrect nop opcode when switching from arm to thumb mode"
href="show_bug.cgi?id=18019">bug 18019</a>.
In a mixed ARM and Thumb file, two Thumb NOPs will be emitted in a manual
alignment, even in ARM mode.
This rarely causes an issue in practice, as most uses of 8-byte alignment are
for constant pools which will be branched over anyways, but it is an issue
nonetheless.
.syntax unified
.text
.globl arm
.type arm, %function
.p2align 3, 0 @ 8-byte align
arm:
nop @ normal nop, now we are not 8-byte aligned
.p2align 3, 0 @ align again
bx lr
.thumb @ we just need this directive in the file to trigger the bug, no
Thumb code is required
$ clang --target=arm-none-eabi -march=armv4t -c test.s -o test.o
$ arm-none-eabi-objdump -d test.o
test.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <arm>:
0: e1a00000 nop ; (mov r0, r0)
4: 46c046c0 strbmi r4, [r0], r0, asr #13
8: e12fff1e bx lr
While the manual nop is correct, the implicit alignment nop is two Thumb nops.</pre>
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