How to trigger warning: 'di' is used uninitialized here

Xi Ruoyao xry111@xry111.site
Wed Aug 30 12:53:15 GMT 2023


On Wed, 2023-08-30 at 14:49 +0200, Mathieu Malaterre wrote:
> On Wed, Aug 30, 2023 at 2:32 PM Xi Ruoyao <xry111@xry111.site> wrote:
> > 
> > On Wed, 2023-08-30 at 14:25 +0200, Mathieu Malaterre via Gcc-help wrote:
> > > Hi all,
> > > 
> > > Consider the following silly c++ code (*), it compiles nicely using
> > > g++ 13.2 no warnings using either:
> > > 
> > > % g++  -c -O2 -Wall -Wextra alt.cxx
> > > 
> > > or
> > > 
> > > % g++  -c -O0 -Wall -Wextra alt.cxx
> > > 
> > > Is it possible to trigger a warning for this code (maybe I missed some
> > > gcc options) ?
> > 
> > Analyzer does a better job for such a "complex" pattern:
> > 
> > $ gcc -Wall -Wextra t.cc -fanalyzer -S
> 
> Cool ! Documentation is somewhat unclear to me as `-fanalyzer` seems
> to be required (I cannot simply use
> -Wanalyzer-use-of-uninitialized-value) (*)
> 
> Could someone confirm that my code cannot trigger one of the
> `fsanitize=undefined` logic ? valgrind sees the uninitialised read:

-fsanitize=undefined cannot detect any uninitialised use.  OTOH -
fsanitize=memory is capable:

$ clang++ t.cc -fsanitize=memory
$ ./a.out
==969846==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x5603db7b65f7 in cl(d) (/home/xry111/a.out+0xb55f7)
    #1 0x5603db7b6709 in k() (/home/xry111/a.out+0xb5709)
    #2 0x5603db7b67d8 in main (/home/xry111/a.out+0xb57d8)
    #3 0x7ff2d9c27f59 in __libc_start_call_main /home/xry111/sources/lfs/glibc-2.38/csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #4 0x7ff2d9c28024 in __libc_start_main@GLIBC_2.2.5 /home/xry111/sources/lfs/glibc-2.38/csu/../csu/libc-start.c:360:3
    #5 0x5603db71f2d0 in _start /home/xry111/sources/lfs/glibc-2.38/csu/../sysdeps/x86_64/start.S:115

SUMMARY: MemorySanitizer: use-of-uninitialized-value (/home/xry111/a.out+0xb55f7) in cl(d)
Exiting

But unfortunately the memory sanitizer is only supported by Clang, not
GCC :(.
-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University


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