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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Rewrite the 'CBR' instruction to an InstAlias in TableGen"
href="https://bugs.llvm.org/show_bug.cgi?id=38802">38802</a>
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<th>Summary</th>
<td>Rewrite the 'CBR' instruction to an InstAlias in TableGen
</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>All
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: AVR
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>me@dylanmckay.io
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>Rewrite the 'CBR' instruction, which is currently implemented as a hardware
instruction, to an instruction alias of 'ANDI Rd, COM(K)'.
This will fix this compilation warning:
===============
[423/492] Building AVRGenDisassemblerTables.inc...
Decoding Conflict:
0111............
01..............
................
ANDIRdK 0111____________
CBRRdK 0111____________
================
This is the only remaining instruction that needs to be converted to an alias.
This particular instruction is complicated because the alias is not so simple -
we need to complement an immediate operand before MCInst lowering.
This is currently done inside a PostEncoder method on a custom immediate
operand type in TableGen.
def imm_com8 : Operand<i8>
{
let EncoderMethod = "encodeComplement";
let MIOperandInfo = (ops i8imm);
}
Here is the naive InstAlias implementation
// CBR Rd, K
//
// Mnemonic alias to 'AND Rd, COM(K)'. Same bit pattern, same operands,
// same everything. K must be complemeted.
def : InstAlias<"cbr\t$rd, $k",
(ANDI LD8:$rd, imm_com8:$k), /* the immediate K must be
complemented. */
/* Disable display, so we don't override ORI */ 0>;
It appears that the PostEncoder method of 'imm_com8' is ignored. This is likely
because the InstAlias mapping is done at a high layer, directly substituting
the CBR instruction for the ANDI well before the PostEncoder method is called
or even queried.</pre>
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