[llvm-dev] Legalising seems to lose critical information needed for lowering return values properly?

[carl-llvm-dev@petosoft.com via llvm-dev]

I'm a new LLVM developer contributing patches for the AVR platform and I'm trying to understand which parts of the code base are malfunctioning in my case.
 
This LLVM IR...
 
define hidden i32 @setServoAngle3(i32) local_unnamed_addr {
entry:
 %1 = call i32 @myExternalFunction1(i32 %0, i32 119)
 ret i32 %1
}
declare i32 @myExternalFunction1(i32, i32)
 
Is being lowered to this assembly language...
 
setServoAngle3:                         ; @setServoAngle3
; %bb.0:                                ; %entry
 ldi r18, 119
 ldi r19, 0
 ldi r20, 0
 ldi r21, 0
 call myExternalFunction1
 mov r18, r22
 mov r19, r23
 mov r22, r24
 mov r23, r25
 mov r24, r18
 mov r25, r19
 ret
 
Which is clearly wrong.  It should just be...
setServoAngle3:                         ; @setServoAngle3
; %bb.0:                                ; %entry
ldi r18, 119
ldi r19, 0
ldi r20, 0
ldi r21, 0
call myExternalFunction1
ret
 
 
 
 
The AVR ABI is based on the AVR GCC ABI, which specifies for register passing and returning of arguments, a 32 bit value is passed in r22, r23, r24, r25 (the registers are 8 bit). And two 16 bit values would be passed, argument 1 in r24, r25 and argument 2 in r22, r23.
 
 
 
When I'm looking in the function lowering code in SelectionDAGISel::LowerArguments and 
AVRTargetLowering::LowerFormalArguments, we already have a problem because the 32 bit return value has been turned into two 16 bit values by the legaliser and the information has been lost that it was one 32 bit value. So the lowering code cannot correctly lower the return value and gets the two 16 bit words mixed up.
 
 
 
Which bit of the code do people think is at fault here? What hooks need changing on AVR?
 
Thanks for any advice or help you can give.
 
Carl
 
 
 
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