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Consider the following code:
int i = 3 << 65;
I would expect that the result is i==0, however the actual result is i==6. With some testing I found that with the following code:
int i, s;
int a = i << s;
int b = i << (s & 31);
the values of a and b are always the same.
Does the C standard say anything about shifting more than 32 bits (the width of type int) or is this unspecified behavior?
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On simple low-cost processors, typically, bitwise operations are substantially faster than division, several times faster than multiplication, and sometimes significantly faster than addition.
The right-shift operator causes the bit pattern in shift-expression to be shifted to the right by the number of positions specified by additive-expression . For unsigned numbers, the bit positions that have been vacated by the shift operation are zero-filled.
The bitwise shift operators move the bit values of a binary object. The left operand specifies the value to be shifted. The right operand specifies the number of positions that the bits in the value are to be shifted.
From my WG12/N1124 draft (not the standard, but Good Enough For Me), there's the following block in 6.5.7 Bitwise shift operators:
If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined.
So, undefined. Be careful.
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