Walter's talk on D backend

[claptrap]

On Monday, 23 September 2024 at 11:03:45 UTC, user1234 wrote:
> On Monday, 23 September 2024 at 09:43:50 UTC, claptrap wrote:

> I cant reply for DMD but as I detect a confusion, let me add 
> something about that... SSA is not about variables it's about 
> nodes assigned uniquely once
>
> So if X is a variable then each use of it is either a load or a 
> store. (there can be bit cast and ptr arithmetic too, but let 
> keep thing simple.)
>
> 
> (pseudo LLVM IR, to illustrate, with X, a global uint)
>
> ```
> %1 = i32  load  ptr @X
> %3 = i32  add   i32 %1, i32 %1
>      i32  store ptr @X, i32 %3
>     (LLVM does not make store reuseable, that'd be be a 
> non-sense)
> 
>
> You see variables are not accessed as-is, either you have a 
> load or a store.
> An SSA node is what starts with "%"
> ```
> 

That's counter to everything I've read about it. It is defined as 
"variables are only assigned once", so each time you assign a new 
value to X, it gets a suffix, so each assignment to X becomes a 
new variable X0,X1... and so on.

It makes it explicit what definition reaches what uses of X. Even 
through all the control flow. That's actually what makes SSA 
useful.

A quick google says LLVM IR is SSA for registers but not memory 
locations. So I don't think your example shows anything.

What would show SSA is if you cant assign to a register multiple 
times. Eg..

```
%1 = i32  load  ptr @X
%2 = i32  load  ptr @Y
%2 = i32  add   i32 %1, i32 %2
```

That would be invalid in SSA