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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 - Linker defined variable used in asm() call results in incorrect value for 32-bit code"
href="https://bugs.llvm.org/show_bug.cgi?id=48461">48461</a>
</td>
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<th>Summary</th>
<td>Linker defined variable used in asm() call results in incorrect value for 32-bit code
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>10.0
</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>LLVM Codegen
</td>
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<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>Jared.candelaria@intel.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=24256" name="attach_24256" title="Repro consisting of C source and linker script with build script">attachment 24256</a> <a href="attachment.cgi?id=24256&action=edit" title="Repro consisting of C source and linker script with build script">[details]</a></span>
Repro consisting of C source and linker script with build script
I'm compiling 32-bit x86 code that uses inline assembly, linker defined
variables, and some arithmetic. Clang generates unexpected code around this
compared to gcc.
For example, given the following C:
extern uint8_t BASE[]; // This is the linker defined variable (0x2000).
#define BASE_ADDRESS ((uint32_t) BASE)
#define TOP (0xFF100000UL)
#define OFFSET (0x000000a5UL)
#define REGISTERS_BASE (TOP - BASE_ADDRESS)
#define WRITE_CTL_REG(Register, Value) WRITE_REG(REGISTERS_BASE + (Register),
Value)
#define WRITE_REG(Register, Value) \
__asm__( \
"movl %1, %%fs:(%0) \n\t" \
: \
: "ir"(Register), "ir"(Value) \
: "memory" \
) \
WRITE_CTL_REG(OFFSET, 0xdead);
Clang generates the corresponding:
64 c7 05 a5 20 f0 00 movl $0xdead,%fs:0xf020a5
While gcc generates:
b8 00 20 00 00 mov $0x2000,%eax
ba a5 00 10 ff mov $0xff1000a5,%edx
29 c2 sub %eax,%edx
89 d0 mov %edx,%eax
64 c7 00 ad de 00 00 movl $0xdead,%fs:(%eax)
Clang generates the expected code when compiling as 64-bit.
I tested this on Ubuntu clang version
10.0.1-++20201013091236+ef32c611aa2-1~exp1~20201013191834.199. Attached is a
small repro that compiles, links, and disassembles the problem spot. You can
uncomment a gcc invocation in build.sh to see what gcc will do in the same
situation. Also attached is example.txt that does this for you.</pre>
</div>
</p>
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