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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 code generated for bit field access"
href="https://bugs.llvm.org/show_bug.cgi?id=45265">45265</a>
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<th>Summary</th>
<td>wrong code generated for bit field access
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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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: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>carrot@google.com
</td>
</tr>
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
<div>
<pre>The source code is:
struct S {
long long f1 : 40;
long long f2 : 48;
char f3;
} __attribute__((__packed__));
extern void foo();
void bar(int i, struct S *p) {
if (p[i].f2 == i)
foo();
}
Command line:
clang -c bitfield.c -O3 -save-temps
generates:
# %bb.0: # %entry
movslq %edi, %rax
leaq (%rax,%rax,2), %rcx
cmpq %rax, 5(%rsi,%rcx,4)
jne .LBB0_1
# %bb.2: # %if.then
xorl %eax, %eax
jmp foo # TAILCALL
.LBB0_1: # %if.end
retq
The access of p[i].f2 should load 48bit value from memory, but instruction
cmpq %rax, 5(%rsi,%rcx,4)
loads 64 bit value from memory and uses it directly. The high 16bit contains
unrelated value, and causes run time error.</pre>
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