[llvm-dev] Unsigned Bitwise Shift for Bit-field Structure

[Friedman, Eli via llvm-dev]

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

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