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Compute the proper struct alignment for TaggedUnion.

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Before this change, the alignment of TaggedAlgebraic was always 4, which in turn had a 50% chance of aligning 8-byte pointers on a non-8-byte boundary, causing the pointer to become invisible to the GC (and less bad, causing performance degradation when dereferencing the pointer). This would happen in particular when the total size of the TaggedUnion/TaggedAlgebraic was not divisible by 8 and an array of values was built.
This commit is contained in:
Sönke Ludwig 2019-04-16 09:41:16 +02:00
parent bb650bd5cf
commit fd85603576
1 changed files with 40 additions and 1 deletions
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41
source/taggedalgebraic/taggedunion.d
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@ -29,7 +29,8 @@ import std.traits : EnumMembers, FieldNameTuple, Unqual, isInstanceOf;
$(LI `getFoo` - equivalent to `get!(Kind.foo)`)
)
*/
struct TaggedUnion(U) if (is(U == union) || is(U == struct) || is(U == enum))
template TaggedUnion(U) if (is(U == union) || is(U == struct) || is(U == enum)) {
align(commonAlignment!(UnionKindTypes!(UnionFieldEnum!U))) struct TaggedUnion
{
import std.traits : FieldTypeTuple, FieldNameTuple, Largest,
hasElaborateCopyConstructor, hasElaborateDestructor, isCopyable;
@ -294,6 +295,7 @@ struct TaggedUnion(U) if (is(U == union) || is(U == struct) || is(U == enum))
package @trusted @property ref inout(T) trustedGet(T)() inout { return *cast(inout(T)*)m_data.ptr; }
}
}
///
@safe nothrow unittest {
@ -409,6 +411,33 @@ unittest { // non-copyable types
tu.setS(S.init);
}
unittest { // alignment
union S1 { int v; }
union S2 { ulong v; }
union S3 { void* v; }
// sanity check for the actual checks - this may differ on non-x86 architectures
static assert(S1.alignof == 4);
static assert(S2.alignof == 8);
version (D_LP64) static assert(S3.alignof == 8);
else static assert(S3.alignof == 4);
// test external struct alignment
static assert(TaggedUnion!S1.alignof == 4);
static assert(TaggedUnion!S2.alignof == 8);
version (D_LP64) static assert(TaggedUnion!S3.alignof == 8);
else static assert(TaggedUnion!S3.alignof == 4);
// test internal struct alignment
TaggedUnion!S1 s1;
assert((cast(ubyte*)&s1.vValue() - cast(ubyte*)&s1) % 4 == 0);
TaggedUnion!S1 s2;
assert((cast(ubyte*)&s2.vValue() - cast(ubyte*)&s2) % 8 == 0);
TaggedUnion!S1 s3;
version (D_LP64) assert((cast(ubyte*)&s3.vValue() - cast(ubyte*)&s3) % 8 == 0);
else assert((cast(ubyte*)&s3.vValue() - cast(ubyte*)&s3) % 4 == 0);
}
/** Dispatches the value contained on a `TaggedUnion` to a set of visitors.
@ -751,6 +780,16 @@ package template AmbiguousTypes(Types...) {
alias AmbiguousTypes = impl!0;
}
/// Computes the minimum alignment necessary to align all types correctly
private size_t commonAlignment(TYPES...)()
{
import std.numeric : gcd;
size_t ret = 1;
foreach (T; TYPES)
ret = (T.alignof * ret) / gcd(T.alignof, ret);
return ret;
}
package void rawEmplace(T)(void[] dst, ref T src)
{