[cfe-dev] BitstreamReader::GetCurrentBitNo() counterintuitive

[Ted Kremenek]

On Dec 23, 2008, at 5:01 PM, Zhongxing Xu wrote:

>
>
> On Wed, Dec 24, 2008 at 7:29 AM, Ted Kremenek <kremenek at apple.com>  
> wrote:
> Part of the problem is that "Deserialize.cpp" (in llvm) uses  
> GetCurrentBitNo() in a whole bunch of places.  AST serialization/ 
> deserialization is the only client of the serialization library, and  
> there aren't that many test cases in the clang test suite for AST  
> serialization because it just hasn't been completed yet.  I have a  
> feeling that changing the behavior of GetCurrentBitNo() will break  
> things.
>
> I'm actually a little surprised that GetCurrentBitNo() needs to be  
> changed.  While the AST serialization work is incomplete, it has  
> worked for non-trivial cases.  Since it uses GetCurrentBitNo() to  
> jump around the bitcode file, I'm suspicious that anything needs to  
> be changed.
>
> Also, what happens if NextChar == FirstChar?  Wouldn't (NextChar- 
> FirstChar-4)*8 be a negative number?
>
> It will be negative. But in that case,  BitsInCurWord will be 0, the  
> final result is 0.

Okay.  If GetCurrentBitNo() is broken then it should be fixed.  I  
personally haven't looked at it in detail recently, so I'm not certain  
what the ramifications would be if we changed it or why it seems to  
work correctly now (or maybe it doesn't?).
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