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Re: Warnings in mingw64 build on emacs-28 branch

From: Óscar Fuentes
Subject: Re: Warnings in mingw64 build on emacs-28 branch
Date: Sun, 07 Nov 2021 16:51:56 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux)

Eli Zaretskii <eliz@gnu.org> writes:

>> 3) This warning is new with gcc 11.
>> C:/emacs/git/emacs/emacs-28/src/w32heap.c: In function 'getrlimit':
>> C:/emacs/git/emacs/emacs-28/src/w32heap.c:853:14: warning: 'm' may be used 
>> uninitialized [-Wmaybe-uninitialized]
>>   853 |         if (!VirtualQuery ((LPCVOID) &m, &m, sizeof m))
>>       |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> In file included from 
>> C:/msys64/mingw64/x86_64-w64-mingw32/include/winbase.h:25,
>>                  from 
>> C:/msys64/mingw64/x86_64-w64-mingw32/include/windows.h:70,
>>                  from C:/emacs/git/emacs/emacs-28/src/w32common.h:24,
>>                  from C:/emacs/git/emacs/emacs-28/src/w32heap.c:54:
>> C:/msys64/mingw64/x86_64-w64-mingw32/include/memoryapi.h:45:28: note: by 
>> argument 1 of type 'LPCVOID' {aka 'const void *'} to 'VirtualQuery' declared 
>> here
>>    45 |   WINBASEAPI SIZE_T WINAPI VirtualQuery (LPCVOID lpAddress, 
>>       |                            ^~~~~~~~~~~~
>> C:/emacs/git/emacs/emacs-28/src/w32heap.c:844:34: note: 'm' declared here
>>   844 |         MEMORY_BASIC_INFORMATION m;
>>       |                                  ^
> That's a compiler bug, I think.  I see nothing wrong in the call.

AFAIU that call to VirtualQuery does not do what we want. When lpAddress
is provided the function rounds down the address to a page boundary and
starts scanning pages *up*. Since we want information about the capacity
of the stack, the scan ignores all the space below the current page

There is GetCurrentThreadStackLimits, but requires _WIN32_WINNT >=
0x0602 and it works on allocated pages.

OTOH, we could call CreateThread with dwStackSize set to a fixed value
and return that value from getrlimit(RLIMIT_STACK...). However, we need
the main thread's stack size as well...

There is a hack that uses NtCurrentTeb to obtain the lower address of
the stack and pass it to VirtualQuery. I don't know how portable it is
across Windows versions.

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