* long long bit-fields
@ 2003-03-30 19:14 Falk Hueffner
2003-03-31 8:46 ` Segher Boessenkool
0 siblings, 1 reply; 4+ messages in thread
From: Falk Hueffner @ 2003-03-30 19:14 UTC (permalink / raw)
To: gcc
Hi,
PR c/6346, middle-end/7018 and target/9620 deal with long long
bit-fields. I guess part of the confusion comes from the fact that the
C standard doesn't specify how they should behave (at least I couldn't
find it). So what is the following program supposed to print?
#include <stdio.h>
int
main(int argc, char *argv[]){
struct {
unsigned long long ull31a: 31;
unsigned long long ull31b: 31;
} x;
x.ull31a = -1;
x.ull31b = 1;
printf("%016llx %016llx %016llx\n",
(x.ull31a << 48) >> 48,
(x.ull31a << 16),
(x.ull31b << 48));
}
Candidates are:
ffffffffffffffff ffffffffffff0000 0000000000010000 (gcc)
ffffffffffffffff 00007fffffff0000 0001000000000000 (Compaq C)
000000000000ffff 00007fffffff0000 0001000000000000 (g++)
0000000000000000 ffffffffffff0000 0000000000000000 (Compaq C++)
--
Falk
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: long long bit-fields
2003-03-30 19:14 long long bit-fields Falk Hueffner
@ 2003-03-31 8:46 ` Segher Boessenkool
2003-03-31 9:22 ` Falk Hueffner
0 siblings, 1 reply; 4+ messages in thread
From: Segher Boessenkool @ 2003-03-31 8:46 UTC (permalink / raw)
To: Falk Hueffner; +Cc: gcc
Falk Hueffner wrote:
> Hi,
>
> PR c/6346, middle-end/7018 and target/9620 deal with long long
> bit-fields. I guess part of the confusion comes from the fact that the
> C standard doesn't specify how they should behave (at least I couldn't
> find it). So what is the following program supposed to print?
>
> #include <stdio.h>
>
> int
> main(int argc, char *argv[]){
> struct {
> unsigned long long ull31a: 31;
> unsigned long long ull31b: 31;
> } x;
> x.ull31a = -1;
> x.ull31b = 1;
>
> printf("%016llx %016llx %016llx\n",
> (x.ull31a << 48) >> 48,
> (x.ull31a << 16),
> (x.ull31b << 48));
> }
>
> Candidates are:
>
> ffffffffffffffff ffffffffffff0000 0000000000010000 (gcc)
Correct (the integer promotions promote an unsigned integer
bit-field to a signed integer, if it fits).
> ffffffffffffffff 00007fffffff0000 0001000000000000 (Compaq C)
Not correct.
> 000000000000ffff 00007fffffff0000 0001000000000000 (g++)
> 0000000000000000 ffffffffffff0000 0000000000000000 (Compaq C++)
Don't know about C++, sorry.
Segher
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: long long bit-fields
2003-03-31 8:46 ` Segher Boessenkool
@ 2003-03-31 9:22 ` Falk Hueffner
2003-04-01 0:11 ` Segher Boessenkool
0 siblings, 1 reply; 4+ messages in thread
From: Falk Hueffner @ 2003-03-31 9:22 UTC (permalink / raw)
To: Segher Boessenkool; +Cc: gcc
Segher Boessenkool <segher@koffie.nl> writes:
> Falk Hueffner wrote:
> > PR c/6346, middle-end/7018 and target/9620 deal with long long
> > bit-fields. [...]
>
> Correct (the integer promotions promote an unsigned integer
> bit-field to a signed integer, if it fits).
Okay. Then we can close middle-end/7018. Most other bit-field bugs
seem to be caused by casts to the bit-field's type getting discarded
erraneously, by the folder presumably. I'll try to have a look at that
some time in the future.
How about:
struct { unsigned long long ull31a: 33; } x = { -1 };
printf("%016llx\n", (x.ull31a << 48) >> 48);
We don't currently promote to signed long long if it fits. Should we?
--
Falk
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: long long bit-fields
2003-03-31 9:22 ` Falk Hueffner
@ 2003-04-01 0:11 ` Segher Boessenkool
0 siblings, 0 replies; 4+ messages in thread
From: Segher Boessenkool @ 2003-04-01 0:11 UTC (permalink / raw)
To: Falk Hueffner; +Cc: gcc
Falk Hueffner wrote:
> How about:
>
> struct { unsigned long long ull31a: 33; } x = { -1 };
> printf("%016llx\n", (x.ull31a << 48) >> 48);
>
> We don't currently promote to signed long long if it fits. Should we?
Yes. But IMHO, of course.
Segher
^ permalink raw reply [flat|nested] 4+ messages in thread
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2003-03-30 19:14 long long bit-fields Falk Hueffner
2003-03-31 8:46 ` Segher Boessenkool
2003-03-31 9:22 ` Falk Hueffner
2003-04-01 0:11 ` Segher Boessenkool
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