Do you think if statement as expression would be nice to have in D?

[Walter Bright]

On 6/7/2022 2:15 PM, max haughton wrote:
> They should already be filed I can find the  issues if required but they should 
> just a bugzilla search away.

Generally speaking, when one writes about bugs, having a bugzilla reference 
handy says a lot.


> Realistic for anyone other than you then ;)

I've never looked at the code generator for gdc or ldc. I doubt either is easy 
to contribute to. The x86 line is *extremely* complex and making a change that 
will work with all the various models is never ever going to be simple.


> With the register allocator as a specific example my attention span is rather 
> short but it was pretty impenetrable when I tried to explain to myself what 
> algorithm it actually uses.

It uses the usual graph coloring algorithm for register allocation. The 
kludginess in it comes from the first 8 registers all being special cases.


>> Triples *are* SSA, in that each node can be considered an assignment to a 
>> temporary.
> 
> Meanwhile I can assign to a variable in a loop? Or maybe you aren't supposed to 
> be able to and that was a bug in some IR I generated? The trees locally are SSA 
> but do you actually enforce SSA in overall dataflow?
> 
> Similarly SSA applies across control flow boundaries, phi nodes?
> 
> Anyway my point about SSA is more that one of the reasons it's prevalent is that 
> it makes writing passes quite a lot easier.

I don't know how you're trying to do things, but the optimizer generates 
variables all the time. Each node in the elem tree gets assigned exactly once 
and used exactly once.

> Implementing address sanitizer should be a doddle but it was not with the dmd 
> backend so I had to give up.
> 
> This is actually a really useful feature that we already link with at the moment 
> on Linux so I can tell you exactly the lowering needed if required. You'll be 
> able to do it within minutes I'm sure but I can't devote the time to working out 
> and testing all the bitvectors for the DFA etc.

I'm sure it's not trivial, but I'm also sure it isn't trivial in ldc, either.


> In the same way that structured programming has nothing to do with avoiding 
> bugs. I'm not saying it's impossible to do without SSA it's just indicative of 
> an IR that isn't very good compared to even simple modern ones.

I looked into doing SSA, and realized it was pointless because the binary tree 
does the same thing.

> They look like special cases, and maybe they are fundamentally, but other 
> compilers simply do not have this rate of mistakes when it comes to SIMD 
> instruction selection or code generation.

Other compilers have a massive test suite that detects problems up front. We 
don't. I also do not work full time on the back end. This is not a structural 
problem, it's a resource problem.

> I want to help with this and just today I have fixed a backend issue but I just 
> wanted to say some things about it.

That's fine, that's what the forums are for! And it gives me an opportunity to 
help out.

The only structural problem with the backend I've discovered is it cannot embed 
goto nodes in the tree structure, so it can't inline loops. I doubt it is that 
big a problem, because loops intrinsically are hardly worth inlining.