[llvm-dev] Unsigned Bitwise Shift for Bit-field Structure (original) (raw)

Friedman, Eli via llvm-dev llvm-dev at lists.llvm.org
Thu Apr 20 10:56:38 PDT 2017

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 UINTMAX+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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