[Bug c/85233] New: Incorrect -Wmaybe-uninitialized with -fpartial-inlining -finline-small-functions

[erik.carstensen at intel dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85233

            Bug ID: 85233
           Summary: Incorrect -Wmaybe-uninitialized with
                    -fpartial-inlining -finline-small-functions
           Product: gcc
           Version: 7.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: erik.carstensen at intel dot com
  Target Milestone: ---

Created attachment 43858
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43858&action=edit
Reproducer

When compiling the attachment with
gcc -Wall -c foo.c -o foo.o -O1 -fpartial-inlining -finline-small-functions
I get the warning:
foo.c:26:11: warning: ‘x’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
         c = x;
though it can be easily proven that x is not used uninitialized.

I tried this with 7.3.1 from fedora 27,
$ gcc -v
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap
--enable-languages=c,c++,objc,obj-c++,fortran,ada,go,lto --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared
--enable-threads=posix --enable-checking=release --enable-multilib
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
--enable-gnu-unique-object --enable-linker-build-id
--with-gcc-major-version-only --with-linker-hash-style=gnu --enable-plugin
--enable-initfini-array --with-isl --enable-libmpx
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-gnu-indirect-function --with-tune=generic --with-arch_32=i686
--build=x86_64-redhat-linux
Thread model: posix
gcc version 7.3.1 20180303 (Red Hat 7.3.1-5) (GCC) 

I also tried some other gcc versions I had lying around. I can reproduce it in
all versions I have that support -fpartial-inlining (down to gcc 5.2.0)

While minimizing the test case I could see that the warning only seems to
happen when f is partially inlined, so a workaround is to pass
-fno-partial-inlining for affected files.