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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Wrong offsets calculation when compiling embedded arm read/write pic code (-frwpi)"
href="https://bugs.llvm.org/show_bug.cgi?id=51481">51481</a>
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<th>Summary</th>
<td>Wrong offsets calculation when compiling embedded arm read/write pic code (-frwpi)
</td>
</tr>
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<th>Product</th>
<td>lld
</td>
</tr>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>ELF
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>davidstdio@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, smithp352@googlemail.com
</td>
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<p>
<div>
<pre>I'm compiling with -frwpi flag (for STM32L486 mcu), which generates static base
relative read-write data access. By default, lld generated for me separate
segments for .data and .bss sections. So offsets in .data section were
calculated relatively to ".data" segment base, and offsets in .bss section were
calculated relatively to .bss segment base.
Section to Segment mapping:
...
04 .data
05 .bss ._user_heap_stack
So for example for following function
uint32_t uwTick;
uint32_t uwTickFreq = 1U; /* 1KHz */
void HAL_IncTick(void)
{
uwTick += (uint32_t)uwTickFreq;
}
Following assembly was generated:
0800a1ee <HAL_IncTick>:
800a1ee: 40 f2 20 00 movw r0, #0x20
800a1f2: c0 f2 00 00 movt r0, #0x0
800a1f6: 49 46 mov r1, r9
800a1f8: 0b 58 ldr r3, [r1, r0]
800a1fa: 45 f6 c8 62 movw r2, #0x5ec8
800a1fe: c0 f2 00 02 movt r2, #0x0
800a202: 88 58 ldr r0, [r1, r2]
800a204: 18 44 add r0, r3
800a206: 88 50 str r0, [r1, r2]
800a208: 70 47 bx lr
Where #20 is the correct offset of uwTickFreq inside the .data segment :
Disassembly of section .data:
20000000 <_sdata>:
....
20000020 <uwTickFreq>:
and #5ec8 is the correct offset of uwTick inside the .bss segment :
Disassembly of section .bss:
20000458 <completed.1>:
....
20006320 <uwTick>:
But they both accessed with the same base which can be either 0x20000000 or
0x20000458.
The workaround I used was to add PHDRS command in my linker script with custom
rwdata segment and put there .data and .bss sections.
--------
This is the default behavior in gcc by the way. Same code compiled and linked
with gcc before I used PHDRS gave following placement :
Section to Segment mapping:
Segment Sections...
00 isr_vector .text .rodata .ARM .init_array .fini_array
01 .data .bss
Thanks.</pre>
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