| Message ID | 20200109182705.5309-1-msc@linux.ibm.com |
|---|---|
| State | New |
| Headers | show |
| Series | Fix maybe-uninitialized error on powerpc | expand |
On Thu, 9 Jan 2020, Matheus Castanho wrote: > The build has been failing on powerpc64le-linux-gnu with GCC 10 > due to a maybe-uninitialized error: > > ../sysdeps/ieee754/dbl-64/mpa.c:875:6: error: ‘w.e’ may be used > uninitialized in this function [-Werror=maybe-uninitialized] > 875 | EY -= EX; > | ^~ > > This commits adds proper initialization to avoid it. glibc practice is not to add such initializations just to avoid warnings, if the warnings are false positives (rather, DIAG_* macros with appropriate comments, or __builtin_unreachable, are used instead). I think what's going on here is that __dbl_mp may not set the exponent of a returned 0 value, but in fact a returned 0 value is impossible in the case where GCC is reporting uninitialized data. If you agree with that analysis, does putting if (t == 0) __builtin_unreachable (); (with a comment justifying *why* t cannot be 0), just before the __dbl_mp call in __inv, help with the warning?
On 1/9/20 3:48 PM, Joseph Myers wrote: > On Thu, 9 Jan 2020, Matheus Castanho wrote: > >> The build has been failing on powerpc64le-linux-gnu with GCC 10 >> due to a maybe-uninitialized error: >> >> ../sysdeps/ieee754/dbl-64/mpa.c:875:6: error: ‘w.e’ may be used >> uninitialized in this function [-Werror=maybe-uninitialized] >> 875 | EY -= EX; >> | ^~ >> >> This commits adds proper initialization to avoid it. > > glibc practice is not to add such initializations just to avoid warnings, > if the warnings are false positives (rather, DIAG_* macros with > appropriate comments, or __builtin_unreachable, are used instead). > > I think what's going on here is that __dbl_mp may not set the exponent of > a returned 0 value, but in fact a returned 0 value is impossible in the > case where GCC is reporting uninitialized data. If you agree with that > analysis, does putting > > if (t == 0) > __builtin_unreachable (); > > (with a comment justifying *why* t cannot be 0), just before the __dbl_mp > call in __inv, help with the warning? Thanks for the help, that seems the problem indeed, and this check does solve it. I'll do this instead (with the justification comment) and send a new patch later.
diff --git a/sysdeps/ieee754/dbl-64/mpa.c b/sysdeps/ieee754/dbl-64/mpa.c index 3bb8bff90d..8ad0fc7535 100644 --- a/sysdeps/ieee754/dbl-64/mpa.c +++ b/sysdeps/ieee754/dbl-64/mpa.c @@ -895,6 +895,7 @@ SECTION __dvd (const mp_no *x, const mp_no *y, mp_no *z, int p) { mp_no w; + w.e = 0; if (X[0] == 0) Z[0] = 0;
The build has been failing on powerpc64le-linux-gnu with GCC 10 due to a maybe-uninitialized error: ../sysdeps/ieee754/dbl-64/mpa.c:875:6: error: ‘w.e’ may be used uninitialized in this function [-Werror=maybe-uninitialized] 875 | EY -= EX; | ^~ This commits adds proper initialization to avoid it. Tested on powerpc64le. --- sysdeps/ieee754/dbl-64/mpa.c | 1 + 1 file changed, 1 insertion(+)