[Issue 16087] New: Alignment (.alignof) and stack space incorrect for SIMD types.

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16087

          Issue ID: 16087
           Summary: Alignment (.alignof) and stack space incorrect for
                    SIMD types.
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Keywords: SIMD, wrong-code
          Severity: blocker
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: Marco.Leise at gmx.de

The following code compiled and run with dmd-2.071 prints:

SIMD vector types inherit their base type's alignment:
float8.alignof yields 4 but should yield 32.
void16.alignof yields 1 but should yield 16.
When embedded, a float4 exposes some hidden alignment, as it changes to 16.
The float8 needs a 32 byte alignment though or we SEGFAULT as soon as we load
c[0] or c[1].
Even worse, the stack space occupied by these 32 byte vectors is only 16!

-------------------------

void main()
{
    import core.simd;
    import std.stdio;
    import std.math;

    union Matrix4x4 { float[16] a; float4[4] b; float8[2] c; alias a this; }
    float8 onStackA;
    float8 onStackB;

    writeln("SIMD vector types inherit their base type's alignment:");
    writefln("float8.alignof yields %s but should yield 32.", float8.alignof);
    writefln("void16.alignof yields %s but should yield 16.", void16.alignof);
    writefln("When embedded, a float4 exposes some hidden alignment, as it
changes to %s.", Matrix4x4.alignof);
    writeln("The float8 needs a 32 byte alignment though or we SEGFAULT as soon
as we load c[0] or c[1].");
    writefln("Even worse, the stack space occupied by these %s byte vectors is
only %s!",
        float8.sizeof, abs(cast(ptrdiff_t)&onStackB -
cast(ptrdiff_t)&onStackA));
}

-------------------------

Compiled programs with too little stack space or incorrect alignment of SIMD
types crash.

--