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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - aarch64: -march=armv8-a produces armv8.2 'bfc' instruction"
href="https://bugs.llvm.org/show_bug.cgi?id=42576">42576</a>
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<th>Summary</th>
<td>aarch64: -march=armv8-a produces armv8.2 'bfc' instruction
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<th>Product</th>
<td>clang
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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
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>C
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>arnd@linaro.org
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<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>Building arm64 linux randconfig kernels, I came across this assembler error:
/tmp/sdma_v4_0-f95fd3.s: Assembler messages:
/tmp/sdma_v4_0-f95fd3.s:44: Error: selected processor does not support `bfc
w0,#1,#5'
I reduced the test case to
int a, b, c;
int __order_base_2() { return a > 1 ? __builtin_constant_p(a) ?: b : 0; }
int sdma_v4_0_rb_cntl(int p1) {
int d = c ?: __order_base_2();
p1 = (p1 & ~0x0000003EL) | (62 & (d << 1));
return p1;
}
$ clang-8 --prefix=/home/arnd/cross/x86_64/gcc-8.1.0-nolibc/aarch64-linux/bin/
-no-integrated-as -O2 sdma_v4_0.i --target=aarch64-linux -S -o-
-march=armv8.1-a
<a href="https://godbolt.org/z/8Gcjl9">https://godbolt.org/z/8Gcjl9</a> shows the 'bfc' instruction being generated, but
not the error message as it is unable to call the external assembler correctly.
According to <a href="https://sourceware.org/ml/binutils/2015-11/msg00221.html">https://sourceware.org/ml/binutils/2015-11/msg00221.html</a>, the
'bfc' instruction is only valid in armv8.2 or higher but llvm uses it for any
armv8 version.</pre>
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