[LLVMdev] Meaning of byval?

[Alon Zakai]

Duncan wrote:
> Hi Alon,
> 
> > The docs say "[byval] indicates that the pointer parameter should
> > really be passed by value to the function. The attribute implies
> > that a hidden copy of the pointee is made between the caller and
> > the callee, so the callee is unable to modify the value in the
> > callee."
> >
> > I am not sure what this means though - when I generate code
> > from the LLVM assembly, do I need to do anything with byval?
> 
> yes, the pointee needs to be passed by-copy, which usually means on the
> stack but could mean in a bunch of registers.
> 
> > Either in the calling location or in the called function?
> 
> The caller does the copy IIRC.  If you look at the .s file you should see
> it happening.
> 
> >  I've
> > been trying to figure this out from generated LLVM assembly
> > and it puzzles me.
> 
> You can't really figure it out from LLVM IR.  Look at final target assembler
> (.s).
> 
> Ciao, Duncan.

Thanks Duncan, yes it is indeed much clearer from the x86
assembly. It's passed on the stack, so the copy is done in
the caller, in effect.

It's a little puzzling to me that the unoptimized LLVM
assembly also creates an explicit copy in the assembly
(allocating it & doing a memcpy), but I guess it gets
optimized out later so it doesn't matter.

Best,
  Alon Zakai