https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64003
--- Comment #12 from dmalcolm at gcc dot gnu.org --- (In reply to dmalcolm from comment #11) > Running valgrind with vgdb to get the precise location of its > warnings indicates they are here within get_attr_length_nobnd in > insn-attrtab.c:19152: > > 19147 if ((((INSN_ADDRESSES_SET_P ()? > 19148 INSN_ADDRESSES (INSN_UID > 19149 (GET_CODE (operands[0]) == > 19150 LABEL_REF ? XEXP (operands[0], > 19151 0) : operands[0])) : 0) > - > 19152 (insn_current_reference_address (insn))) >= (-126)) > 19153 && > 19154 (((INSN_ADDRESSES_SET_P ()? > 19155 INSN_ADDRESSES (INSN_UID > 19156 (GET_CODE (operands[0]) == > > i.e. at the logical-AND at line 19153. > > Valgrind presumably is noticing the "uninitialized" trait of this > read, then propagating it through to the result of align_fuzz, > and thence to insn_current_reference_address, and hence to the whole > of the first argument of the logical-AND. > > Hence the decision about whether to process the second argument of > the logical-AND is a jump that relies on an uninitialized value, and > hence valgrind complains. i.e. the issue is that evaluating both sides of the (and) expression at line 10931 in: 10920 (define_insn "*jcc_1" 10921 [(set (pc) 10922 (if_then_else (match_operator 1 "ix86_comparison_operator" 10923 [(reg FLAGS_REG) (const_int 0)]) 10924 (label_ref (match_operand 0)) 10925 (pc)))] 10926 "" 10927 "%!%+j%C1\t%l0" 10928 [(set_attr "type" "ibr") 10929 (set_attr "modrm" "0") 10930 (set (attr "length_nobnd") >10931 (if_then_else (and (ge (minus (match_dup 0) (pc)) 10932 (const_int -126)) 10933 (lt (minus (match_dup 0) (pc)) 10934 (const_int 128))) 10935 (const_int 2) 10936 (const_int 6)))]) 10937 for a forward jump, leads to reads of uninitialized items from insn_lengths in align_fuzz for the uid for the jump target. Hence we have an (and (UNINITIALIZED_1) (WILL_BE_UNINITIALIZED_2)) and hence the decision about whether to short-circuit the read of WILL_BE_UNINITIALIZED_2 is a conditional jump that depends on UNINITIALIZED_1.
