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GCC has no options to handle shifts by negative amounts or by amounts outside the width of the type predictably or to trap on them; they are always treated as undefined. Processors may reduce the shift amount modulo the width of the type. For example, 32-bit right shifts are implemented using the following instructions on x86-32:
| Code Block |
|---|
sa[rl]lsarl %cl, %eax |
The sa[rl]l sarl instructions take a bit mask of the least significant 5 bits from %cl to produce a value in the range [0, 31] and then shift %eax that many bits:
| Code Block |
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// 64-bit right shifts on IA-32 platforms become sh[rl]dlshrdl %eax, %edx sa[rl]lsarl %cl, %eax |
where %eax stores the least significant bits in the doubleword to be shifted, and %edx stores the most significant bits.
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