llvm dev - Feb 2007 - [LLVMdev] problem with function arguments in ARM EABI

I'm facing a problem with functions arguments. ARM EABI defines that
8-bytes arguments must be 8-bytes aligned. For example:

void @f(i32 %a, i64 %b)

ARM EABI:
r0 <- %a
r2,r3 <- %b

Darwin:
r0 <- %a
r1,r2 <- %b

void @g(i32 %a, i32 %b, i32 %c)

ARM EABI or Darwin:
r0 <- %a
r1 <- %b
r2 <- %c

The problem is: I can't differ a i64 argument of two i32 arguments in
ISD::CALL (ARM doesn't have i64 registers). The function
TargetLowering::LowerCallTo transforms all i64 arguments in two i32
arguments.

A possible solution is override TargetLowering::LowerCallTo and do not
expand i64 arguments. I don't know if it's a good solution. None
backend overrides this function.

Any ideas?

Lauro

On Feb 8, 2007, at 10:46 AM, Lauro Ramos Venancio wrote:
> I'm facing a problem with functions arguments. ARM EABI defines that
> 8-bytes arguments must be 8-bytes aligned. For example:
>
> void @f(i32 %a, i64 %b)
>
> ARM EABI:
> r0 <- %a
> r2,r3 <- %b
Ok. I suppose this makes it easier to use 64 bit store instructions.
>
> Darwin:
> r0 <- %a
> r1,r2 <- %b
>
> void @g(i32 %a, i32 %b, i32 %c)
>
> ARM EABI or Darwin:
> r0 <- %a
> r1 <- %b
> r2 <- %c
>
> The problem is: I can't differ a i64 argument of two i32 arguments in
> ISD::CALL (ARM doesn't have i64 registers). The function
> TargetLowering::LowerCallTo transforms all i64 arguments in two i32
> arguments.
>
> A possible solution is override TargetLowering::LowerCallTo and do not
> expand i64 arguments. I don't know if it's a good solution. None
> backend overrides this function.
Yes, you need to override both LowerArguments and LowerCallTo. All of  
the current targets / abi's use the default implementation in  
SelectionDAGISel.cpp But I guess ARM EABI will be the first. :-) As  
far as I can see, this is the only clean way to do it.  I expect the  
ARM EABI specific implementation will be quite a bit simpler than the  
default implementation though.

Evan

>
> Any ideas?
>
> Lauro
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>
> Yes, you need to override both LowerArguments and LowerCallTo. All of
> the current targets / abi's use the default implementation in
> SelectionDAGISel.cpp But I guess ARM EABI will be the first. :-) As
> far as I can see, this is the only clean way to do it.  I expect the
> ARM EABI specific implementation will be quite a bit simpler than the
> default implementation though.
>
If I override these functions, I would copy a lot of code and change
only few lines. I found another solution: adding two items to
arguments Flags. Flags[27:24] = number of pieces that an argument was
expanded. Flags[31:28] = sequential piece ID.

What do you think? May I add these items?

Lauro