https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95681
Bug ID: 95681 Summary: False positive uninitialized variable usage in decNumberCompareTotalMag Product: gcc Version: 11.0 Status: UNCONFIRMED Keywords: build, diagnostic Severity: normal Priority: P3 Component: middle-end Assignee: unassigned at gcc dot gnu.org Reporter: stefansf at linux dot ibm.com Target Milestone: --- Target: s390 The following error/warning shows up on S/390 while bootstrapping: libdecnumber/decNumber.c: In function 'decNumberCompareTotalMag': libdecnumber/decNumber.c:953:14: error: '*(allocbufa).bits' may be used uninitialized [-Werror=maybe-uninitialized] 953 | a->bits&=~DECNEG; /* .. and clear the sign */ | ^~ libdecnumber/decNumber.c:967:14: error: '*(allocbufb).bits' may be used uninitialized [-Werror=maybe-uninitialized] 967 | b->bits&=~DECNEG; /* .. and clear the sign */ | ^~ The part of interest is: if (decNumberIsNegative(lhs)) { /* lhs<0 */ a=bufa; needbytes=sizeof(decNumber)+(D2U(lhs->digits)-1)*sizeof(Unit); if (needbytes>sizeof(bufa)) { /* need malloc space */ allocbufa=(decNumber *)malloc(needbytes); if (allocbufa==NULL) { /* hopeless -- abandon */ status|=DEC_Insufficient_storage; break;} a=allocbufa; /* use the allocated space */ } decNumberCopy(a, lhs); /* copy content */ a->bits&=~DECNEG; /* .. and clear the sign */ lhs=a; /* use copy from here on */ } While calling `decNumberCopy` variable `a` either points to `bufa` or to `allocbufa`. Since `bufa` and `allocbufa` are both allocated inside function `decNumberCompareTotalMag` and argument `lhs` is not changed, it is guaranteed that `a != lhs` holds prior call `decNumberCopy(a, lhs)`. Function `decNumberCopy` initializes `a->bits`, if `a != lhs` holds. Since this is the case here, no warning should be printed. Note, no warning is printed if `dest->bits=src->bits;` is shifted above `if (dest==src) return dest;` of function `decNumberCopy` which endorses that this is a false positive resulting from a wrong assumption that `a == lhs` may hold. Analogous for the second warning/error which speaks about variable `b`.