Re: [Boost-users] Memory mapped files

Subject: Re: [Boost-users] Memory mapped files
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2019-01-24 13:42:05

>> It's even stronger than that: the C++ standard makes no guarantee that
>> in-memory representation of storage does not change between program
>> executions, let alone between different compilers or architectures. To
>> use mapped memory at all outside a single program execution is pure
>> undefined behaviour. You are literally on your own wrt the standard.
> 
> While the standard doesn't explicitly define it, at least a POD which
> doesn't contain pointers generally should maintain representation within
> the same executable, even if run multiple times.

It depends on your execution environment. An implementation based on an
interpreter or JIT may readily perform WPO which changes compile time
constants across program runs.

You may not think such execution environments common, but I'd remind you
of C++ on GPUs, or C++ cross compiled into Javascript.

Further on down the line we may have Modules from a Modules store be
late stage optimised into locally optimised binaries. Padding, in
particular, might be optimisable, especially as we start to support ever
larger integer types. Padding with 512-bit integers, for example, is
wasteful.

What I will agree with is that for a given version of a given toolchain
on a given architecture e.g. GCC 7.3 on x64, you currently get stronger
guarantees. But you shouldn't rely on anything not explicitly guaranteed
by the standard or your toolchain vendor.

Niall