5 Dec
2014
5 Dec
'14
3:12 a.m.
On 12/04/2014 03:50 PM, Peter Corlett wrote:
On Tue, Dec 02, 2014 at 03:32:38PM -0800, Fred Cisin
wrote:
Well, that's the problem! The destination
was 64 bits, the
operand was 32 bits.
if the 32-bit operand got promoted to 64-bits, it would have
worked as the
original programmer intended. But, the whole operation was
done at 32 bits,
and the desired bits fell off the end. I thought that C did
NOT promote smaller
operands, so the behavior was actually expected, once I saw
what was really
going on there.
Jon
. . .
>>> long long int A;
>>> int B;
>>> A = B << 32;
[...]
So, should the compiler optimize that to
MOV X,0
or
NOP
?
That basically depends on what "long long" and "int" means
:)
The C99 (draft) standard says of << and >> that "[t]he behavior is
undefined if
the right operand is negative, or greater than or equal to the length in bits
of the promoted left operand"