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signed bit fields

Kirill Frolov

Does anybody knows, Keil C compiler V7.xx supports bit fields with signed types or not?
The code always generated same as for unsigned types. This is compiler bug or feature? My code compiled with Keil C fails on MCU, but
works excellent on PC when compiled with gcc.

Example:

struct xxx {
signed y: 7;
};


struct xxx *s;
volatile int i;

i = s->y;
/* This operator does not extends y to i
with sign, instead all higher bits masked
with zeros */
; i = s->y;
MOV DPTR,#s?25158
MOVX A,@DPTR
MOV R3,A
INC DPTR
MOVX A,@DPTR
MOV R2,A
INC DPTR
MOVX A,@DPTR
MOV R1,A
LCALL ?C?ILDPTR
MOV R5,A
MOV R4,B
ANL A,#07FH
MOV R7,A
MOV DPTR,#i?25159
CLR A
MOVX @DPTR,A
INC DPTR
MOV A,R7
MOVX @DPTR,A


i=-2, s->y=i;
/* This operator again only masking
higher bits of i without carying of
sign bit */
; i=-2;
MOV DPTR,#i?25159
MOV A,#0FFH
MOVX @DPTR,A
INC DPTR
DEC A
MOVX @DPTR,A
; s->y=i;
MOV DPTR,#i?25159+01H
MOVX A,@DPTR
ANL A,#07FH
MOV R7,A
MOV A,R5
ANL A,#080H
MOV R5,A
MOV A,R4
MOV R6,A
MOV A,R5
ORL A,R7
MOV R7,A
MOV A,R6
MOV B,R7
LCALL ?C?ISTPTR

Parents
  • 0 in reply to Andy Neil

    Per the ANSI standard, bit fields are always of type "int". Every compiler I've ever used gives you the (extended) ability to declare bitfields in chunks of different widths (char, short, long, etc), but that is a non-standard capability. Also per standard, and despite the fact that bitfields are "ints", the signed-ness of a bitfield and whether or not to do sign extension is up to the implementation. There is no requirement by the language that you can store a signed quantity in a bitfield.

    Note that order of locating fields within that int is also up to the implementation. Some compilers will start with the LSB and work up, others with the MSB and work down.

    I developed the same philosophy as Andrew - bitfield portability and efficiency issues make them not worth using. I just define masks and use bitwise operators instead.

Reply
  • 0 in reply to Andy Neil

    Per the ANSI standard, bit fields are always of type "int". Every compiler I've ever used gives you the (extended) ability to declare bitfields in chunks of different widths (char, short, long, etc), but that is a non-standard capability. Also per standard, and despite the fact that bitfields are "ints", the signed-ness of a bitfield and whether or not to do sign extension is up to the implementation. There is no requirement by the language that you can store a signed quantity in a bitfield.

    Note that order of locating fields within that int is also up to the implementation. Some compilers will start with the LSB and work up, others with the MSB and work down.

    I developed the same philosophy as Andrew - bitfield portability and efficiency issues make them not worth using. I just define masks and use bitwise operators instead.

Children