Conditional compilation inside asm and enum declarations

[Walter Bright]

Julian Salazar wrote:
> Hi, I'm new here to the community but I've been using D for a while now, 
> and I have to say that it's a great programming language. I'd like to 
> get involved in this community and help shape this language.

Welcome!


> I'm just wondering about a minor issue: why are conditional blocks 
> invalid within expressions such as enum and asm? I mean, in trivial 
> cases it's fine, but in instances where code duplication is a big 
> maintainability nightmare, making conditional compilation more flexible 
> would have benefits for developers.

The request to do it for enums has been thrashed about before. 
Essentially, version works at the declaration and statement level, not 
at the expression level or in between tokens, etc. The reason for this 
is to encourage a more modular approach to versioning than the typical C 
method of doing it at the lowest level.


> Something like (I know it's a trivial example, but you get the point):
> 
> asm {
>    version(x86) mov EAX, 1;
>    else version(x86_64) mov EAX, 2;
> }
> 
> would trigger an error. Also, though I know enum qualifies as a 
> constant/datatype cross, structs and classes are perfectly fine with 
> conditional compilation. Couldn't the lexical stuff be changed to 
> support it for enum and asm as well?

Let me illustrate by a current example. The linker (optlink) is written 
100% in assembler. This makes it rather intractable. It's also loaded up 
with line-by-line nested conditional assembly (and a lot of macros). 
It's so hard to see what is *actually* being compiled that I'll assemble 
it, run OBJ2ASM on the output, and work off of the disassembled code.

So, in essence, the idea is to push conditional compilation to higher 
levels, not lower levels. Ideally, versioning would be done by 
abstracting all the version differences into an interface implemented by 
different modules.


> Also, I noticed that there is no formal specification page for x86-64 
> inline assembly. You define a predefined version identifier such as 
> D_InlineAsm_X86_64, but you don't define registers and instructions 
> pertaining to it. In GDC for example, using the RAX register in the D 
> inline ASM syntax is invalid. Not sure what the case is in LDC (they 
> probably do implement it for x86-64), and I know DMD does not have a 
> 64-bit version, but the spec should at least have a definition for 
> compilers that do implement 64-bit support.

The first approximation to the definition is to use the Intel asm syntax 
as outlined in their processor data sheets. I haven't written a spec 
more detailed than that because it's a lot of work and I'm lazy, and 
such work is not terribly exciting. But if you'd like to help with that, 
I'd welcome it.