mojmir svoboda wrote:
> the time has come and i am resolving these strict aliasing issues
> in our project i mentioned about a year ago.
>
> few questions popped up:
>
> Q1: what is the difference between
> W1: dereferencing type-punned pointer will break...
> and
> W2: dereferencing type-punned pointer might break...?
>
> i'm getting the W1 version in constructs like:
> C_VertexPosCompression
> {
> Vector3 m_Offset;
> float m_Scale;
> const Vector4 & GetOffsetAndScale() const
> { return *(const Vector4*) &m_Offset; }
> };
>
> i'm getting the W2 version in constructs like:
> class Vector3
> {
> float x, y, z;
> const Vector2 & GetVector2 () const { return *(const Vector2*)&x; }
> };
>
> but i do not see any difference between them. enlighten me, please :)
I'm not sure. It doesn't much matter, as the code is wrong in both cases.
I don't know what type a Vector2 is.
> Q2: this is excerpt from noname commercial software. i'd like to know
> whether the cast through pointer-to-union is valid solution and whether
> the note in the comment is really true.
No, it isn't really true. Casting through a pointer to union doesn't work.
All it does is shut up the compiler, and I'm glad to see that -Wstrict-aliasing=2
still works despite this.
> Q3: i know already that following solution is common
>
> template<class Tgt, class Src> TargetT type_pun (Src & s) {
> union convert { Src src; Tgt tgt; } conv;
> conv.src = s; return conv.tgt;
> }
>
> so i would resolve Q2 this way:
> float separation = type_pun<float>(binary);
> is it better than the original u32f32 * cast?
Yes. Much.
> Q4: will this version do the same as in Q3?
>
> template<class Tgt, class Src> Tgt & type_pun (Src & s)
> {
> union convert { Src * src; Tgt * tgt; } conv;
> conv.src = &s; return *conv.tgt;
> }
No. What matters is the type of the object, not the type of the pointer.
> Q5: when i fix for example the
> C_VertexPosCompression
> {
> Vector3 m_Offset; float m_Scale;
> Vector4 GetOffsetAndScale() const
> { return C_Vector4(m_Offset, m_Scale); }
> };
> is there a chance that the temporary will be eventually optimized out?
I think so, but this isn't a full example. You can easily check.
Andrew.
