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Question on Undefined Right Shifts
- From: Jeffrey Walton <noloader at gmail dot com>
- To: gcc at gcc dot gnu dot org
- Date: Tue, 19 Feb 2013 19:19:08 -0500
- Subject: Question on Undefined Right Shifts
- Reply-to: noloader at gmail dot com
Hi All,
I've been using John Regehr's Integer Overflow Checker (IOC) on a few
libraries. It is a Clang plug-in and can be found at
http://embed.cs.utah.edu/ioc/.
The checker has flagged two libraries I use for performing undefined
right shifts:
CLANG ARITHMETIC UNDEFINED at <./xxx, (595:22)> : Op: >>, Reason :
Unsigned Right Shift: Right operand is negative or is greater than or
equal to the width of the promoted left operand, BINARY OPERATION:
left (uint32): 2196358166 right (uint32): 32
The fix seems easy enough on the surface: simply validate the
parameters before performing the shift. However, that could introduce
a timing difference in the function.
How could one fix the issue without introducing timing differences?
Jeff