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

Shiva Chen via llvm-dev llvm-dev at lists.llvm.org
Wed Apr 19 19:23:13 PDT 2017

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

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