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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Bad codegen for rotate ops"
href="https://bugs.llvm.org/show_bug.cgi?id=41358">41358</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Bad codegen for rotate ops
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<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>Backend: AVR
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>hello@dsprenkels.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>I am aware the AVR target is still experimental, so I am a bit unsure wether
this report is actually useful. Anyway, here goes:
I think the codegen for rotate instructions on AVR is incorrect. For example,
the IR
```
define i8 @example(i8, i8) {
%a = tail call i8 @llvm.fshl.i8(i8 %0, i8 %0, i8 %1)
ret i8 %a
}
declare i8 @llvm.fshl.i8(i8, i8, i8)
```
compiles to:
```
.text
.file "<stdin>"
.globl example ; -- Begin function example
.p2align 1
.type example,@function
example: ; @example
; %bb.0:
cpi r22, 0
breq LBB0_2
LBB0_1: ; =>This Inner Loop Header: Depth=1
rol r24
subi r22, 1
brne LBB0_1
LBB0_2:
ret
.Lfunc_end0:
.size example, .Lfunc_end0-example
; -- End function
; Declaring this symbol tells the CRT that it should
;copy all variables from program memory to RAM on startup
.globl __do_copy_data
; Declaring this symbol tells the CRT that it should
;clear the zeroed data section on startup
.globl __do_clear_bss
```
However, the Microchip documentation mentions that on AVR, rotates pass through
the carry bit in the status register. Should the codegen be updated?
For context, avr-gcc also produces a different result:
<a href="https://godbolt.org/z/Cs-dif">https://godbolt.org/z/Cs-dif</a>
As a fix, I would propose something like this:
```
.text
.file "<stdin>"
.globl example ; -- Begin function example
.p2align 1
.type example,@function
example: ; @example
; %bb.0:
cpi r22, 0
breq LBB0_2
eor r20 ; get zero in a register
clc ; clear the carry bit
LBB0_1: ; =>This Inner Loop Header: Depth=1
rol r24 ; do a carried rotate to the left
adc r24, r20 ; add the carry bit and set the carry
bit to 0
dec r22 ; dec loop counter (without tainting
carry bit)
brne LBB0_1
LBB0_2:
ret
.Lfunc_end0:
.size example, .Lfunc_end0-example
; -- End function
; Declaring this symbol tells the CRT that it should
;copy all variables from program memory to RAM on startup
.globl __do_copy_data
; Declaring this symbol tells the CRT that it should
;clear the zeroed data section on startup
.globl __do_clear_bss
```</pre>
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