[cfe-dev] [RFC] Clang SourceLocation overflow

Thu Oct 10 02:13:46 PDT 2019

Hi Richard.

Thanks for the clarification, I certainly had not realized that location tracking was needed for correctness of clang. Decoupling this sounds like a painful processes, and the only thing we get is errors without any location information. Sounds like not a great trade-off. We’ll go experiment a bit with the size impact of using 64-bits and will attempt to take the route of a cmake configuration option.

If there are people that have already done some experiments on the size impact of 64-bits location pointers, we’re welcome your insights. @Matt Asplund<mailto:mwasplund at gmail.com> I believe you said that you’ve done some prototyping, is there something you can share?

Thanks,
Christof

From: Richard Smith <richard at metafoo.co.uk>
Sent: 08 October 2019 19:53
To: Christof Douma <Christof.Douma at arm.com>
Cc: Mikhail Maltsev <Mikhail.Maltsev at arm.com>; Clang Dev <cfe-dev at lists.llvm.org>; nd <nd at arm.com>
Subject: Re: [cfe-dev] [RFC] Clang SourceLocation overflow

On Tue, 8 Oct 2019, 10:42 Christof Douma via cfe-dev, <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
Hi Richard, Paul and other.

Thanks for the input so far. I wanted to point out that it’s not our code-base. Rather, we’re seeing more use of the LLVM technology in the automotive market and as usual we’re faced with existing code bases that are tried and tested with other toolchains (gcc or others) and when LLVM comes along things don’t always work directly.

We’ve suggested better ways of structuring their code and your suggestions are certainly good input. However, legacy code is especially sticky in any market that has to handle ‘safety’ concerns, like automotive, aerospace and medical markets. Code changes are pretty expensive in those fields. So while I hope that over time we see more sensible coding structures, I don’t expect that to happen any time soon. In the mean time, we’re searching for a solution for this coding pattern that doesn’t play well with clang.

Hope that gave some more background of where this question comes from.

Do all options that were suggested by Mikhail really require fundamental restructuring of major parts of clang? This surprised me, I had expected that the option 2 to be possible without a complete overhaul. (2 is “Track until an overflow occurs after that make the lexer output the <invalid location> special value for all subsequent tokens.”)
Clang uses source locations as part of the semantic representation of the AST in some cases (simple example: some forms of initialization might use parens or not, with different semantics, and we distinguish between them based on whether we have paren locations; there are also some checks that look at which of two source locations came first when determining which warnings or errors to produce, and so on). Maybe we could go through and fix all of those, but that's still a very large task and it'd be hard to check we got them all.
Not nice user experience but maybe doable? I was hoping there was something slightly better that still works without a major restructuring (maybe something that at least gives a rough location or something that only gives the location of the error and not the include stack under an option or using some kind of heuristic to detect that things go haywire).

As an alternative, I was curious if it would be possible and acceptable to make the switch between 32-bit and 64-bit location tracking a build-time/cmake decision? I’ve not done any estimation on the memory size growth, so maybe this is a dead end.
If someone is prepared to do the work to add and maintain this build mode, I think this might be a feasible option.
Thanks,
Christof

From: cfe-dev <cfe-dev-bounces at lists.llvm.org<mailto:cfe-dev-bounces at lists.llvm.org>> On Behalf Of Richard Smith via cfe-dev
Sent: 07 October 2019 20:36
To: Mikhail Maltsev <Mikhail.Maltsev at arm.com<mailto:Mikhail.Maltsev at arm.com>>
Cc: nd <nd at arm.com<mailto:nd at arm.com>>; cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] [RFC] Clang SourceLocation overflow

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