[Bug target/106755] Incorrect code gen for altivec intrinsics with constant inputs

[bergner at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106755

--- Comment #4 from Peter Bergner <bergner at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #3)
> So yes there is aliasing violation as you do the store as vui128_t aka
> "__vector unsigned __int128" (which has the same aliasing set) as "__int128"
> but then do the load from it as vui32_t aka "__vector unsigned int vui32_t"
> (which is the same aliasing set as int). 

Yes, I came to the same conclusion and sent Steve a patch offline that fixes
the issue.

> -Wstrict-aliasing=3 might warn about this case I can't remember exactly of
> the levels of the options but the -Wstrict-aliasing warnings are always not
> going to happen, sometimes it is easier to read the code.

Ah, I didn't realize we could increase the warning level.  Cool!  It seems
-Wstrict-aliasing=2 is enough to catch the problem:

gcc -O3 -Wall -c -fmessage-length=0 -mcpu=power8 -mtune=power8
-Wstrict-aliasing=2  -m64 ../src/vec_bug6.c
../src/vec_bug6.c: In function ‘test_muludq’:
../src/vec_bug6.c:969:45: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
  969 |   k = (vui32_t) test_vec_muludq((vui128_t* )&l, (vui128_t)i,
(vui128_t)j);
...