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Re: [Tinycc-devel] enforced immutability - proposed research project

From: Michael Matz
Subject: Re: [Tinycc-devel] enforced immutability - proposed research project
Date: Wed, 20 Jan 2021 15:29:22 +0100 (CET)
User-agent: Alpine 2.21 (LSU 202 2017-01-01)


On Tue, 19 Jan 2021, Steffen Nurpmeso wrote:

Bruno Haible wrote in
|Michael Matz wrote:
|> Okay, that's quite limited, and very easy to implement: disallow casting |> away immutability and you're done. But in this limited form you then |> can't infer anything useful from this, in particular you can't infer \ |> that |> the (immutably) pointed to object is unmodified when you allow mutable |> references to exist to it at the same time.
|Correct. The way I see it is:
|  - Code that got hold of a writable pointer (wp) may do modifications to
|    the object, as long as it wants.
|  - Code that got hold of a read-only pointer (p) should crash when it
|    attemps a modification through this pointer. Either the crash happens
|    when casting 'const struct data *' to 'struct data *', or it happens

memchr(3) would stop working.

memchr doesn't write, it would work just fine. (You can't write into the returned pointer, of course, at least not without mapping it back to a writable variant). But it's immaterial even if it wouldn't work: the proposal is contemplating a language extension, which might or might not be compatible with const qualifications. That might (or might not) require appropriate variants of standard functions.

|Very often the application lets all writable pointers to the object go |out-of-scope early, such that only read-only pointers to the object \ |survive. Then you have a true immutable object in the proper sense of |the word.

Shadow regions make this a 64-bit only thing then, i presume.

Nope, it works with 32bit just fine, it merely needs a MMU. Of course you reduce the available address space, which matters more on 32bit, but many programs don't need 1GB of address space in regular operation.


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