bad closure if local variables have larger alignment requirements

[dlangBugzillaToGithub]

Rainer Schuetze (@rainers) reported this on 2019-08-23T09:35:35Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=20154

CC List

• Walter Bright (@WalterBright)

Description

If a local variable has larger alignment requirements, closures that capture function arguments can fail:

struct SpinLock
{
    align(64) int x;
}

void collectReferences(int x) //ref HashTab references)
{
	SpinLock lock; // dmd BUG: alignment causes bad capture!

	void mark() scope nothrow
	{
		assert (x == 7);
	}

	mark();
}

void main()
{
	collectReferences(7);
}


For Win64, RSP is aligned according to alignof(SpinLock) for alignments 32-128. This makes the closure offset for the function argument x non-constant.

Side note: no alignment happens for align(256) or higher.

For Win32, alignment is silently omitted.

[dlangBugzillaToGithub]

bugzilla (@WalterBright) commented on 2020-08-09T03:35:55Z

The alignment code is emitted for Win64, but not for Win32. But you listed the bug applying to x86_64.

So please clarify:

Is this working for 64 bit platforms?

[dlangBugzillaToGithub]

bugzilla (@WalterBright) commented on 2020-08-09T03:42:43Z

Currently, stack alignment is enforced only for 64 bit compilations and 32 bit OSX compilations. See the setting of `enforcealign` in codgen() in cgcod.d.

[dlangBugzillaToGithub]

bugzilla (@WalterBright) commented on 2020-08-09T04:17:41Z

A somewhat more illustrative test:


struct SpinLock
{
    align(64)
	int x;
}

void collectReferences(int x) //ref HashTab references)
{
    SpinLock lock = SpinLock(8); // dmd BUG: alignment causes bad capture!

    void mark() scope nothrow
    {
	assert(lock.x == 8);
	assert(x == 7);
    }

    mark();
}

void main()
{
    collectReferences(7);
}

[dlangBugzillaToGithub]

r.sagitario commented on 2020-08-09T05:53:25Z

(In reply to Walter Bright from comment #1)
> The alignment code is emitted for Win64, but not for Win32. But you listed
> the bug applying to x86_64.

Win32 passes the assert, but ignores alignments.

> 
> So please clarify:
> 
> Is this working for 64 bit platforms?

No.

[dlangBugzillaToGithub]

bugzilla (@WalterBright) commented on 2020-08-09T07:09:35Z

Thanks, I think I understand it now. But the solution isn't obvious :-/