• d-bugmail puremagic.com (39/39) Aug 23 2019 https://issues.dlang.org/show_bug.cgi?id=20154

d-bugmail puremagic.com writes:

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

          Issue ID: 20154
           Summary: bad closure if local variables have larger alignment
                    requirements
           Product: D
           Version: D2
          Hardware: x86_64
                OS: All
            Status: NEW
          Keywords: wrong-code
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: r.sagitario gmx.de

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.

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