Re: [sdcc-devel] No longer omitting the integer promotion on explicitly typecast arguments

[Maarten B. <sou...@ds...>]

Philipp Klaus Krause schreef op 2025-11-23 16:02:
> Currently, SDCC omits integer promotion of explictly typecast
> arguments passed as variable arguments. This is done only when the
> selected standard dialect is an SDCC variant, and the target is not a
> pdk port (see section 3.5.15 of the manual).
> 
> The feature allows to save a byte of stack space (and some code size /
> speed if pushing an 8-bit value is simpler than pushing a 16-bit one)
> when passing a char, bool or _BitInt as variable argument.
> 
> I'd like to remove support for this langauge extension from the stm8
> port, f8 port, the z80-related ports, hc08 and s08.
> 
> Rationale:
> 
> * The main benefit of the feature is saving a little bit of stack
> space. But stm8, f8, the z80-related ports, hc08, s08, and mcs51 (for
> the large and huge memory model) have a large stack (with the
> exception of some stm8 devices, the whole RAM can be used as stack).

I would not qualify the stack as large for the mcs51 large & huge
memory models. Medium, large and huge do not place every variable in
the <= 248 bytes data memory, but still use it for itemps. So stack
space (= unallocated idata) remains very limited.

And when using --xstack the stack still is limited to 256 bytes pdata.
There is no stack implementation in xdata.

> * Omitting the integer promotion violates the C standard, and could
> thus result in surprising behavior for the user. After all that cast
> is not necessarily explicitly written with intent to omit the
> promotion, the cast might come e.g. from a macro expansion.
> * The omission makes code compiled with the SDCC dialect incompatible
> with code compiled without the dialect. This results in silent
> breakage when there are calls between such code that pass explicitly
> cast variable arguments (AFAIK even if that happens within the same
> file, and there is a #pragma std_sdccXX / #pragma std_cXX in between).
> * stm8, f8, the z80-related ports, hc08, s08 do have instructions to
> efficiently push a 16-bit value onto the stack, so there is not much
> benefit in code size and speed.
> * For the z80-related ports, we do not have instructions to
> efficiently put a single 8-bit value on the stack, we instead need to
> push a 16-bit register or value, then adjust the stack pointer, so
> there typically is no or even a negative benenefit in code size and
> speed.
> 
> Philipp

So my recommendation is to keep this behaviour for all targets with a
stack space of 256 bytes or less.

Maarten