[cfe-dev] Question about valid FileID values and assert

[Richard Smith]

On Tue, Jul 29, 2014 at 8:07 AM, Jason Dagit <dagit at galois.com> wrote:

> Hello,
>
> I’m very new to the clang source and I’m trying to understand how AST
> serialization works. As far as I can tell valid FileIDs can be positive or
> negative but not 0. Yet I see the following in ASTReader:
> https://github.com/llvm-mirror/clang/blob/master/lib/Serialization/ASTReader.cpp#L1036
>
> I’m wondering, shouldn’t that line be:
> assert(FID > 0 && "Serialized line entries for non-local file.”);
>

Yes, I think it should.


> It looks like SLocEntryBaseID is always initialized to 0, and I can’t find
> a place that changes it.
>

It's changed here:

https://github.com/llvm-mirror/clang/blob/master/lib/Serialization/ASTReader.cpp#L2890


> if FID was 0, then `FID += F.SLocEntryBaseID - 1;` would be -1? I see
> that FileIDs can be negative, but in this case it looks like an error to me.
>
> Perhaps I missed something? Does the serialization sometimes write out
> invalid FileIDs (meaning, 0 values) for some reason?
>

The table is written out here:

https://github.com/llvm-mirror/clang/blob/master/lib/Serialization/ASTWriter.cpp#L1887

I would not expect this table to contain any entries for the invalid FileID
0. You get some basic confidence in this (or a counterexample) by changing
the assert and seeing if any tests fail.
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