Pure dynamic casts?

[bearophile]

I don't know much about this topic, but this post is mostly in question form.

Are dynamic casts pure? I have compiled the following small program with LDC:


version (Tango)
    import tango.stdc.stdio: printf;
else version (D1)
    import std.c.stdio: printf;
else
    import core.stdc.stdio: printf;

class A { int a; }

void main() {
    Object o = new A();
    A a = cast(A)o;
    a.a = 10;
    printf("%d %d\n", a.a, cast(A)o);
}


LDC based on DMD v1.045 and llvm 2.6 (Thu Sep 10 23:50:27 2009)
ldc -O5 -release -inline -output-s dyncast_test.d

The asm it produces shows two calls to _d_dynamic_cast:

_Dmain:
    pushl   %esi
    subl    $16, %esp
    movl    $_D5cast51A7__ClassZ, (%esp)
    call    _d_allocclass
    movl    %eax, %esi
    movl    $_D5cast51A6__vtblZ, (%esi)
    movl    $0, 4(%esi)
    movl    $0, 8(%esi)
    movl    %esi, (%esp)
    movl    $_D5cast51A7__ClassZ, 4(%esp)
    call    _d_dynamic_cast
    movl    $10, 8(%eax)
    movl    %esi, (%esp)
    movl    $_D5cast51A7__ClassZ, 4(%esp)
    call    _d_dynamic_cast
    movl    %eax, 8(%esp)
    movl    $10, 4(%esp)
    movl    $.str1, (%esp)
    call    printf
    xorl    %eax, %eax
    addl    $16, %esp
    popl    %esi
    ret $8


If the dynamic cast are pure the compiler can call it only once here (LLVM already has two functions attributes for pure functions).

(Related: can the code that performs the dynamic casts be in some situations inlined by LDC?)

Bye,
bearophile