If the code does jit_getarg* after a temporary was used, and the argument
was in a register, it can be clobbered in a temporary.
I did some experiments, and the code generated by GNU lightning actually spills and reloads register arguments when it thinks they are live. (I tried with %rdx on x64, which is used by qdiv and is a register argument.)
Which is great because with it, one can generate more performant code with lightning.
It is a good practice to read arguments at the function prolog.
This would make the (ABI) optimization of passing arguments in registers a bit moot, wouldn't it?
> On the other hand, this example from the documentation
> fact_entry: ! This is the tail call entry point
> ac = arg ! The accumulator is the first argument
> in = arg ! The factorial argument
> getarg R0, ac ! Move the accumulator to R0
> getarg R1, in ! Move the argument to R1
> blei fact_out, R1, 1 ! Done if argument is one or less
> mulr R0, R0, R1 ! accumulator *= argument
> putargr R0, ac ! Update the accumulator
> subi R1, R1, 1 ! argument -= 1
> putargr R1, in ! Update the argument
> jmpi fact_entry ! Tail Call Optimize it!
> retr R0 ! Return the accumulator
> makes it clear, that simple operations like subtraction actually do preserve
> Could the set of safe instructions (that is those instructions not modifying
> arguments) be documented? Is it safe to assume that all ALU operations
> (except for division and multiplication (?)), the compare and transfer
> operations, the network, load and store operations, and the branch
> instructions do not modify (register) arguments?
jit_getarg* should be called at the function prolog, otherwise the argument
will be considered dead from the prolog to the jit_getarg* call, what may be
too late if it was used as a temporary. But as the example you quote tells,
one can restore (or change) an argument, with jit_putarg*.
My point was that between the two putargr instructions in the above code, there is a subi instruction. The subi instruction obviously has to respect the value in the "ac" argument and not clobber it.
And thank you very much!