On Tue, 24 Jun 2014 00:24:50 -0400, deadalnix <[email protected]> wrote:
On Tuesday, 24 June 2014 at 01:41:13 UTC, Steven Schveighoffer wrote:
This is because most CPUs consider the instructions as immutable.
Even x86 do not provide any guarantee (which makes it very hard
to swap implementation outside of a VM).
Remember, these are not functions, but function pointers. You are not
modifying the function at all.
void* is not a function and can come from anything that is mutable.
Casting from void* to function is pretty guaranteed to be
undefined behavior on all plateforms, unless you emit the proper
barriers.
Can you demonstrate a C example that will fail on ARM?
Anything that JIT is affected. Consider the pseudocode:
void JIT() {
void* code = malloc(XXX);
// Write instructions into the allocated chunk of memory.
membar(); // Memory barrier to avoid reordering.
auto fun = cast(void function()) code;
fun(); // Even if your codegen is correct, anything can happen this
point.
}
This is not what we are talking about.
We are talking about whether a valid function pointer can be cast to void
* and back again. It is undefined for C/C++. Should it be defined for D?
-Steve
