[C++] Compiler warnings on mingw-w64

[asfimport]

In mingw64 we see the following warnings:

[ 54%] Building CXX object src/arrow/CMakeFiles/arrow_static.dir/util/io-util.cc.obj
C:/msys64/home/mingw-packages/mingw-w64-arrow/src/arrow/cpp/src/arrow/util/decimal.cc: In static member function 'static arrow::Status arrow::Decimal128::FromString(const string_view&, arrow::Decimal128*, int32_t*, int32_t*)':
C:/msys64/home/mingw-packages/mingw-w64-arrow/src/arrow/cpp/src/arrow/util/decimal.cc:313:35: warning: 'dec.arrow::{anonymous}::DecimalComponents::exponent' may be used uninitialized in this function [-Wmaybe-uninitialized]
       *scale = -adjusted_exponent + len - 1;
                ~~~~~~~~~~~~~~~~~~~^~~~

[ 56%] Building CXX object src/arrow/CMakeFiles/arrow_static.dir/util/string_builder.cc.obj
C:/msys64/home/mingw-packages/mingw-w64-arrow/src/arrow/cpp/src/arrow/util/io-util.cc: In static member function 'static arrow::Status arrow::internal::TemporaryDir::Make(const string&, std::unique_ptr<arrow::internal::TemporaryDir>*)':
C:/msys64/home/mingw-packages/mingw-w64-arrow/src/arrow/cpp/src/arrow/util/io-util.cc:897:3: warning: 'created' may be used uninitialized in this function [-Wmaybe-uninitialized]
   if (!created) {
   ^~

And on mingw32 we also see these:

In file included from C:/msys64/home/mingw-packages/mingw-w64-arrow/src/arrow/cpp/src/arrow/io/file.cc:25:
C:/msys64/home/mingw-packages/mingw-w64-arrow/src/arrow/cpp/src/arrow/io/mman.h: In function 'void* mmap(void*, size_t, int, int, int, off_t)':
C:/msys64/home/mingw-packages/mingw-w64-arrow/src/arrow/cpp/src/arrow/io/mman.h:94:62: warning: right shift count >= width of type [-Wshift-count-overflow]
   const DWORD dwMaxSizeHigh = static_cast<DWORD>((maxSize >> 32) & 0xFFFFFFFFL);
                                                              ^~

 54%] Building CXX object src/arrow/CMakeFiles/arrow_static.dir/util/logging.cc.obj
In file included from C:/msys64/home/mingw-packages/mingw-w64-arrow/src/arrow/cpp/src/arrow/util/io-util.cc:63:
C:/msys64/home/mingw-packages/mingw-w64-arrow/src/arrow/cpp/src/arrow/io/mman.h: In function 'void* mmap(void*, size_t, int, int, int, off_t)':
C:/msys64/home/mingw-packages/mingw-w64-arrow/src/arrow/cpp/src/arrow/io/mman.h:94:62: warning: right shift count >= width of type [-Wshift-count-overflow]
   const DWORD dwMaxSizeHigh = static_cast<DWORD>((maxSize >> 32) & 0xFFFFFFFFL);
                                                              ^~
C:/msys64/home/mingw-packages/mingw-w64-arrow/src/arrow/cpp/src/arrow/util/io-util.cc: In function 'arrow::Status arrow::internal::MemoryMapRemap(void*, size_t, size_t, int, void**)':
C:/msys64/home/mingw-packages/mingw-w64-arrow/src/arrow/cpp/src/arrow/util/io-util.cc:568:55: warning: right shift count >= width of type [-Wshift-count-overflow]
   LONG new_size_high = static_cast<LONG>((new_size >> 32) & 0xFFFFFFFFL);
                                 

Reporter: Jeroen / @jeroen
Assignee: Antoine Pitrou / @pitrou

PRs and other links:

• GitHub Pull Request #4804

_{Note: This issue was originally created as ARROW-5849. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
This is in 32-bit mode?
Can you propose a PR for this?

[asfimport]

Jeroen / @jeroen:
The first two -Wmaybe-uninitialized appear on both in 32 and 64 bit. The bottom two -Wshift-count-overflow are 32 bit only.

Can you propose a PR for this?

No sorry I'm not an arrow developer. I just need to build binaries.

[asfimport]

Antoine Pitrou / @pitrou:
No problem. Thanks for reporting anyway. At least the shift-count-overflow warnings look like they will need fixing (because they're UB AFAIK).

[asfimport]

Antoine Pitrou / @pitrou:
I'm already on this.

[asfimport]

Francois Saint-Jacques / @fsaintjacques:
Issue resolved by pull request 4804
#4804