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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Clang's _bittest* MSVC intrinsics are incorrect for values outside [0, 31]"
href="https://bugs.llvm.org/show_bug.cgi?id=33188">33188</a>
</td>
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<th>Summary</th>
<td>Clang's _bittest* MSVC intrinsics are incorrect for values outside [0, 31]
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<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>Headers
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>rnk@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>I discovered this while staring at Microsoft's platform adaption layer code in
ChakraCore:
<a href="https://github.com/Microsoft/ChakraCore/blob/master/lib/Common/CommonPal.h#L647">https://github.com/Microsoft/ChakraCore/blob/master/lib/Common/CommonPal.h#L647</a>
__forceinline unsigned char _BitTestAndSet(LONG *_BitBase, int _BitPos)
{
#if defined(__clang__) && !defined(_ARM_)
// Clang doesn't expand _bittestandset intrinic to bts, and it's
implemention also doesn't work for _BitPos >= 32
unsigned char retval = 0;
asm(
"bts %[_BitPos], %[_BitBase]\n\t"
"setc %b[retval]\n\t"
: [_BitBase] "+m" (*_BitBase), [retval] "+rm" (retval)
: [_BitPos] "ri" (_BitPos)
: "cc" // clobber condition code
);
return retval;
#else
return _bittestandset(_BitBase, _BitPos);
#endif
}
It's a shame that nobody filed this bug upstream. :(
The Intel manual supports confirms this view:
"""
BT—Bit Test
...
Selects the bit in a bit string (specified with the first operand, called the
bit base) at the bit-position designated by
the bit offset (specified by the second operand) and stores the value of the
bit in the CF flag. The bit base operand
can be a register or a memory location; the bit offset operand can be a
register or an immediate value:
• If the bit base operand specifies a register, the instruction takes the
modulo 16, 32, or 64 of the bit offset
operand (modulo size depends on the mode and register size; 64-bit operands are
available only in 64-bit
mode).
• If the bit base operand specifies a memory location, the operand represents
the address of the byte in memory
that contains the bit base (bit 0 of the specified byte) of the bit string. The
range of the bit position that can be
referenced by the offset operand depends on the operand size.
See also: Bit(BitBase, BitOffset) on page 3-11.
"""
We either need to codegen this with an intrinsic or inline asm that will
reliably select to bts, or we need to do an array indexing operation first.</pre>
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