https://bugs.llvm.org/show_bug.cgi?id=41358
Bug ID: 41358
Summary: Bad codegen for rotate ops
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: AVR
Assignee: unassignedb...@nondot.org
Reporter: he...@dsprenkels.com
CC: llvm-bugs@lists.llvm.org
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:
https://godbolt.org/z/Cs-dif
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
```
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