llvm dev - Apr 2017 - Unsigned Bitwise Shift for Bit-field Structure

Hi,

I have a question about unsigned bitwise shift.

According the C99 6.5.7.4

The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated
bits are filled with zeros. If E1 has an unsigned type, the value of
the result is E1 × 2^E2, reduced modulo one more than the maximum
value representable in the result type.

So if

unsigned b = 0x80000000;
unsigned a = b << 1;
a will equal to 0 because a = (b << 1) mod (1<<32);
(1<< 32) is UINT_MAX+1

For the bit-field structure defined as
struct foo
{
  unsigned long long b:40;
} x;

According to C99 6.7.2.1
A bit-field is interpreted as a signed or unsigned integer type
consisting of the specified number of bits.
So the x.b will treat as 40 bit unsigned integer and it should follow 6.5.7.4.

if x.b = 0x80000000 00;
x.b << 1 = (x.b << 1) mod (1<<40)
So x.b << 1 should be 0, right ?

Please correct me if I miss understanding something.

Thanks,
Shiva

On 4/19/2017 7:23 PM, Shiva Chen via llvm-dev wrote:
> Hi,
>
> I have a question about unsigned bitwise shift.
>
> According the C99 6.5.7.4
>
> The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated
> bits are filled with zeros. If E1 has an unsigned type, the value of
> the result is E1 × 2^E2, reduced modulo one more than the maximum
> value representable in the result type.
>
> So if
>
> unsigned b = 0x80000000;
> unsigned a = b << 1;
> a will equal to 0 because a = (b << 1) mod (1<<32);
> (1<< 32) is UINT_MAX+1
>
> For the bit-field structure defined as
> struct foo
> {
>    unsigned long long b:40;
> } x;
>
> According to C99 6.7.2.1
> A bit-field is interpreted as a signed or unsigned integer type
> consisting of the specified number of bits.
> So the x.b will treat as 40 bit unsigned integer and it should follow
6.5.7.4.
The type of the field is long long; the bitfield size modifier is not 
part of the type.  x.b is a 64-bit integer, not a 40-bit integer.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
Foundation Collaborative Project

This discussion got moved to cfe-dev.

-Eli

On 4/20/2017 10:56 AM, Friedman, Eli via llvm-dev wrote:
> On 4/19/2017 7:23 PM, Shiva Chen via llvm-dev wrote:
>> Hi,
>>
>> I have a question about unsigned bitwise shift.
>>
>> According the C99 6.5.7.4
>>
>> The result of E1 << E2 is E1 left-shifted E2 bit positions;
vacated
>> bits are filled with zeros. If E1 has an unsigned type, the value of
>> the result is E1 × 2^E2, reduced modulo one more than the maximum
>> value representable in the result type.
>>
>> So if
>>
>> unsigned b = 0x80000000;
>> unsigned a = b << 1;
>> a will equal to 0 because a = (b << 1) mod (1<<32);
>> (1<< 32) is UINT_MAX+1
>>
>> For the bit-field structure defined as
>> struct foo
>> {
>>    unsigned long long b:40;
>> } x;
>>
>> According to C99 6.7.2.1
>> A bit-field is interpreted as a signed or unsigned integer type
>> consisting of the specified number of bits.
>> So the x.b will treat as 40 bit unsigned integer and it should follow 
>> 6.5.7.4.
>
> The type of the field is long long; the bitfield size modifier is not 
> part of the type.  x.b is a 64-bit integer, not a 40-bit integer.
>
> -Eli
>
-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
Foundation Collaborative Project