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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 - warnings for untaken branch of ?: expression, at file scope"
href="https://bugs.llvm.org/show_bug.cgi?id=38789">38789</a>
</td>
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<tr>
<th>Summary</th>
<td>warnings for untaken branch of ?: expression, at file scope
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>arnd@linaro.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>I have encountered a couple of warnings in the Linux kernel for range checking
of shift counts. The common macro
#define DMA_BIT_MASK(n) (((n) == 64) ? ~0ULL : ((1ULL<<(n))-1))
produces a warning at file scope for a '64' argument, but not inside of
a function. I tried using __builtin_choose_expr() to avoid that, which
made it worse: that version always warns, regardless of the scope (it
also does this with gcc).
The same thing happened with a less common macro, which I've simplifed
into 'COND_BIT()' in the test case below.
The behavior is the same for all clang versions I tried (3.8 through 8),
but my expectation was to see no warning at all.
$ clang-8 -Wall -O2 -c -xc - << EOF
#define DMA_BIT_MASK(n) (((n) == 64) ? ~0ULL : ((1ULL<<(n))-1))
#define DMA_BIT_MASK_2(n) __builtin_choose_expr((n) == 64, ~0ULL,
(1ULL<<(n))-1)
#define COND_BIT(x) (((x) >= 0) ? (1ul << x) : 0ul)
#define COND_BIT_2(x) __builtin_choose_expr(x >= 0, 1ul << x, 0ul)
// warning: shift count >= width of type [-Wshift-count-overflow]
long long a = DMA_BIT_MASK(64);
// warning: shift count >= width of type [-Wshift-count-overflow]
long long b = DMA_BIT_MASK_2(64);
// warning: shift count is negative [-Wshift-count-negative]
// [gcc: no warning]
unsigned long c = COND_BIT(-1);
// warning: shift count is negative [-Wshift-count-negative]
unsigned long d = COND_BIT_2(-1);
int f(void)
{
// no warning
long long a = DMA_BIT_MASK(64);
// warning: shift count >= width of type [-Wshift-count-overflow]
long long b = DMA_BIT_MASK_2(64);
// no warning
unsigned long c = COND_BIT(-1);
// warning: shift count is negative [-Wshift-count-negative]
unsigned long d = COND_BIT_2(-1);
// prevent unused variable wanings
return a & b & c & d;
}
EOF</pre>
</div>
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