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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - __builtin_constant_p() appears to return true for non-constant argument"
href="https://bugs.llvm.org/show_bug.cgi?id=41459">41459</a>
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<th>Summary</th>
<td>__builtin_constant_p() appears to return true for non-constant argument
</td>
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<th>Product</th>
<td>clang
</td>
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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>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>-New Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>arnd@linaro.org
</td>
</tr>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>Building an s390 linux allmodconfig kernel produced an error message for a
misoptimized code path:
pblk-rb.i:...: error: invalid operand for inline asm constraint 'i'
I managed to reduce the test case and make it architecture independent, test
with "clang-9 pblk-rb.i -Wall -O2 -S":
typedef struct { long counter; } atomic64_t;
static inline void __atomic64_add_const(long val, long *ptr)
{
asm volatile (" agsi %[ptr],%[val]"
:[ptr] "+Q"(*ptr)
:[val] "i"(val)
:"cc", "memory");
}
void __atomic64_add_var(long val, long *ptr);
static inline void atomic64_add(long i, atomic64_t * v)
{
if (__builtin_constant_p(i) && (i > -129) && (i < 128))
__atomic64_add_const(i, &v->counter);
else
__atomic64_add_var(i, &v->counter);
}
void pblk_rb_read_to_bio(atomic64_t *pblk, int count)
{
if (count)
atomic64_add(count, pblk);
atomic64_add(count, pblk);
}
The __builtin_constant_p() should never hit here, since 'count' is clearly not
constant. If a special case is made for 'count=0', then I would expect 'val' to
be 0 in that branch and fit into an "i" constraint.</pre>
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