On Thu, 22 Mar 2012, Eric Botcazou wrote: > > This is what I have applied after bootstrapping and testing on > > x86_64-unknown-linux-gnu. > > Thanks. Last identified problem: the miscompilation with variant record. > > gnat.dg/pack17.adb > > raised PROGRAM_ERROR : pack17.adb:36 explicit raise > FAIL: gnat.dg/pack17.adb execution test > > > Things start to go wrong at line 28: > > 24 procedure Fill (R : out Record_With_QImode_Variants) is > 25 begin > 26 R.C_Filler := (True, False, True, False, True, False, True); > 27 R.C_Map := (True, False, True); > 28 R.T_Int := 17; > 29 end; > > R is a packed record with variant part and -gnatR1 gives its layout: > > for Record_With_Qimode_Variants'Object_Size use 24; > for Record_With_Qimode_Variants'Value_Size use 19; > for Record_With_Qimode_Variants'Alignment use 1; > for Record_With_Qimode_Variants use record > D at 0 range 0 .. 0; > C_Filler at 0 range 1 .. 7; > C_Map at 1 range 0 .. 2; > F_Bit at 1 range 3 .. 3; > F_Filler at 1 range 4 .. 10; > T_Int at 1 range 3 .. 10; > > Now, since T_Int is within a variant, the assignment is translated into: > > r_1(D)->d___XVN.O.t_int = 17; > > d___XVN is QUAL_UNION_TYPE and O a RECORD_TYPE (fields of QUAL_UNION_TYPE are > always of RECORD_TYPE, that's why the overloading of qualifier is indeed OK). > > The compiler now generates: > > movl 8(%ebp), %eax > movb $17, 1(%eax) > > which is of course wrong given the layout above. > > It seems to me that the new code implicitly assumes that the first field in a > bitfield group starts on a byte boundary, which doesn't hold in Ada.
Yes, I have noticed that from what you can see in the code. So the issue is that get_bit_range tells us that it's ok to touch bits outside of the field - but that's obviously required. We may not change bits that are not covered by the FIELD_DECL which now somehow happens? That sounds like a latent bug in the bitfield expander to me - it should never _change_ bits outside of bitpos/bitsize as returned by get_inner_reference. I guess I'll have to debug this if you don't beat me on it. Richard.
