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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - ARM: Incorrect COFF relocation type for thumb bl instruction"
href="https://bugs.llvm.org/show_bug.cgi?id=38620">38620</a>
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<th>Summary</th>
<td>ARM: Incorrect COFF relocation type for thumb bl instruction
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>6.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
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<th>Status</th>
<td>NEW
</td>
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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>jordanrh@microsoft.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>LLVM generates incorrect relocation entry for thumb BL instruction, which
causes the instruction to be overwritten by a BLX instruction at load time.
In lib/Target/ARM/MCTargetDesc/ARMWinCOFFObjectWriter.cpp :
ARMWinCOFFObjectWriter::getRelocType(),
case ARM::fixup_t2_condbranch:
return COFF::IMAGE_REL_ARM_BRANCH20T;
case ARM::fixup_t2_uncondbranch:
return COFF::IMAGE_REL_ARM_BRANCH24T;
case ARM::fixup_arm_thumb_bl: <--- BLX23T is wrong relocation type for this
instruction
case ARM::fixup_arm_thumb_blx:
return COFF::IMAGE_REL_ARM_BLX23T;
fixup_arm_thumb_bl should return COFF::IMAGE_REL_ARM_BRANCH24T.
Here are the descriptions of the relocation types from
<a href="https://docs.microsoft.com/en-us/windows/desktop/debug/pe-format#arm-processors">https://docs.microsoft.com/en-us/windows/desktop/debug/pe-format#arm-processors</a>.
IMAGE_REL_THUMB_BRANCH24
0x0014
The instruction is fixed up with the 25-bit relative displacement to the 2-byte
aligned target. The least significant bit of the displacement is zero and is
not stored.This relocation corresponds to a Thumb-2 B instruction.
IMAGE_REL_THUMB_BLX23
0x0015
The instruction is fixed up with the 25-bit relative displacement to the 4-byte
aligned target. The low 2 bits of the displacement are zero and are not stored.
This relocation corresponds to a Thumb-2 BLX instruction.
This bug manifested when porting the rust compiler to thumbv7a-pc-windows-msvc.</pre>
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