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pointer to volatile, but no volatile object

From: Federico Kircheis <federico_at_[hidden]>
Date: Mon, 5 Jun 2023 19:43:03 +0200
Hello, I have a question about following snippet of code

void bar(volatile int& b){
void foo()
     int a = -1;
With GCC, clang and MSVC, and optimization enabled, a is set to 42, even 
if a is not a volatile object, and bar completely inlined inside foo:
bar(int volatile&):
         mov     DWORD PTR [rdi], 42
         mov     DWORD PTR [rsp-4], 42
This surprised me, because as far as I know, the standard mostly talks 
about glvalues
 > Accesses through volatile glvalues are evaluated strictly according 
to the rules of the abstract machine.
 > Reading an object designated by a volatile glvalue (7.2.1), modifying 
an object, calling a library I/O function, or calling a function that 
does any of those operations are all side effects, which are changes in 
the state of the execution environment.
and not about pointers marked as volatile to objects that are not.
Thus my understanding was that since a is not volatile, compilers are 
allowed to optimize the "mov     DWORD PTR [rsp-4], 42" away.
Is my interpretation incorrect and are those optimizations not allowed, 
or are compilers less aggressive when there are volatile pointers, as 
some code bases/users might "hide" non-volatile objects behind pointers 
to volatile objects to try to avoid some optimization?
(removing volatile from the function signature as bar optimizes foo as 

Received on 2023-06-05 17:43:08