POC: automatically move scope array literals to the stack

[dkorpel]

See #16888 (comment)

Just a proof of concept, I don't think this is worth merging at this stage. Maybe under -preview=dip1000 only.

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[Geod24]

I assume you will make scope foo = [...] @nogc ?

[dkorpel]

I assume you will make scope foo = [...] @nogc ?

If this is actually worth pursuing, yes. It's non-trivial because currently, @nogc is eagerly checked on each expression, while in this case, an array literal expression can only be checked at the end of the function.

[ryuukk]

This doesn't work for me, however this does:

dmd/compiler/src/dmd/dsymbolsem.d

Lines 1331 to 1338 in 6d2f652

	else if (auto ale = ex.isArrayLiteralExp())
	{
	// or an array literal assigned to a `scope` variable
	if (sc.useDIP1000 == FeatureState.enabled
	&& !dsym.type.nextOf().needsDestruction())
	ale.onstack = true;
	}
	}

                        else if (auto ale = ex.isArrayLiteralExp())
                        {
                            // or an array literal assigned to a `scope` variable
-                            if (sc.useDIP1000 == FeatureState.enabled
+                            if ((sc.useDIP1000 == FeatureState.enabled || global.params.betterC)
                                && !dsym.type.nextOf().needsDestruction())
                                ale.onstack = true;
                        }

[ryuukk]

My patch works with your tests too, please use it instead, more efficient, and let's merge

[dkorpel]

That patch could introduce dangling pointers in @safe code. -preview=dip1000 is required to verify the stack-allocated array literal doesn't escape.

-betterC simply disables language features that require druntime, it does not negate @safe or change language semantics in other ways, so checking for global.params.betterC likely isn't the right answer here.

[ryuukk]

From what i see, that DIP is 8 years old

I'm not gonna rely on a rotten DIP and i don't use attributes

new patch:

                        else if (auto ale = ex.isArrayLiteralExp())
                        {
                            // or an array literal assigned to a `scope` variable
-                            if (sc.useDIP1000 == FeatureState.enabled
+                            if ((sc.useDIP1000 == FeatureState.enabled || (global.params.betterC && !sc.func.isSafe()))
                                && !dsym.type.nextOf().needsDestruction())
                                ale.onstack = true;
                        }

Let's merge