[cfe-dev] [RFC] Clang SourceLocation overflow

[Richard Smith via cfe-dev]

On Wed, 2 Oct 2019 at 09:26, Mikhail Maltsev via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi all,
>
> We are experiencing a problem with Clang SourceLocation overflow.
> Currently source locations are 32-bit values, one bit is a flag, which
> gives
> a source location space of 2^31 characters.
>
> When the Clang lexer processes an #include directive it reserves the total
> size
> of the file being included in the source location space. An overflow can
> occur
> if a large file (which does not have include guards by design) is included
> many
> times into a single TU.
>
> The pattern of including a file multiple times is for example required by
> the AUTOSAR standard [1], which is widely used in the automotive industry.
> Specifically the pattern is described in the Specification of Memory
> Mapping [2]:
>
> Section 8.2.1, MEMMAP003:
> "The start and stop symbols for section control are configured with section
> identifiers defined in MemMap.h [...] For instance:
>
> #define EEP_START_SEC_VAR_16BIT
> #include "MemMap.h"
> static uint16 EepTimer;
> static uint16 EepRemainingBytes;
> #define EEP_STOP_SEC_VAR_16BIT
> #include "MemMap.h""
>
> Section 8.2.2, MEMMAP005:
> "The file MemMap.h shall provide a mechanism to select different code,
> variable
> or constant sections by checking the definition of the module specific
> memory
> allocation key words for starting a section [...]"
>
> In practice MemMap.h can reach several MBs and can be included several
> thousand
> times causing an overflow in the source location space.
>
> The problem does not occur with GCC because it tracks line numbers rather
> than
> file offsets. Column numbers are tracked separately and are optional.
> I.e., in
> GCC a source location can be either a (line+column) tuple packed into 32
> bits or
> (when the line number exceeds a certain threshold) a 32-bit line number.
>
> We are looking for an acceptable way of resolving the problem and propose
> the
> following approaches for discussion:
> 1. Use 64 bits for source location tracking.
> 2. Track until an overflow occurs after that make the lexer output
>    the <invalid location> special value for all subsequent tokens.
> 3. Implement an approach similar to the one used by GCC and start tracking
> line
>    numbers instead of file offsets after a certain threshold. Resort to (2)
>    when even line numbers overflow.
> 4. (?) Detect the multiple inclusion pattern and track it differently (for
> now
>    we don't have specific ideas on how to implement this)
>
> Is any of these approaches viable? What caveats should we expect? (we
> already
> know about static_asserts guarding the sizes of certain class fields which
> start
> failing in the first approach).
>
> Other suggestions are welcome.
>

I don't think any of the above approaches are reasonable; they would all
require fundamental restructuring of major parts of Clang, an efficiency or
memory size hit for all other users of Clang, or some combination of those.

Your code pattern seems unreasonable; including a multi-megabyte file
thousands of times is not a good idea. Can you split out parts of MemMap.h
into a separate header that is only included once, and keep only the parts
that actually change on repeated inclusion in MemMap.h itself?
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