llvm dev - Aug 2013 - [LLVMdev] [cfe-dev] design for an accurate ODR-checker with clang

On 5 August 2013 15:33, John McCall <rjmccall at apple.com> wrote:
> On Aug 5, 2013, at 3:04 PM, Nick Lewycky <nlewycky at google.com>
wrote:
>
>  On 15 July 2013 15:12, John McCall <rjmccall at apple.com> wrote:
>
>> On Jul 11, 2013, at 6:13 PM, Nick Lewycky <nlewycky at
google.com> wrote:
>>
>> On 11 July 2013 18:02, John McCall <rjmccall at apple.com> wrote:
>>
>>> On Jul 11, 2013, at 5:45 PM, Nick Lewycky <nlewycky at
google.com> wrote:
>>> > Hi! A few of us over at Google think a nice feature in clang
would be
>>> ODR violation checking, and we thought for a while about how to do
this and
>>> wrote it down, but we aren't actively working on it at the
moment nor plan
>>> to in the near future. I'm posting this to share our design and
hopefully
>>> save anyone else the design work if they're interested in it.
>>> >
>>> > For some background, C++'s ODR rule roughly means that two
definitions
>>> of the same symbol must come from "the same tokens with the
same
>>> interpretation". Given the same token stream, the
interpretation can be
>>> different due to different name lookup results, or different types
through
>>> typedefs or using declarations, or due to a different point of
>>> instantiation in two translation units.
>>> >
>>> > Unlike existing approaches (the ODR checker in the gold linker
for
>>> example), clang lets us do this with no false positives and very
few false
>>> negatives. The basis of the idea is that we produce a hash of all
the
>>> ODR-relevant pieces, and to try to pick the largest possible
granularity.
>>> By granularity I mean that we would hash the entire definition of a
class
>>> including all methods defined lexically inline and emit a single
value for
>>> that class.
>>> >
>>> > The first step is to build a new visitor over the clang AST
that
>>> calculates a hash of the ODR-relevant pieces of the code.
(StmtProfiler
>>> doesn’t work here because it includes pointers addresses which will
be
>>> different across different translation units.) Hash the outermost
>>> declaration with external-linkage. For example, given a class with
a method
>>> defined inline, we start the visitor at the class, not at the
method. The
>>> entirety of the class must be ODR-equivalent across two translation
units,
>>> including any inline methods.
>>> >
>>> > Although the standard mentions that the tokens must be the
same, we do
>>> not actually include the tokens in the hash. The structure of the
AST
>>> includes everything about the code which is semantically relevant.
Any
>>> false positives that would be fixed by hashing the tokens either do
not
>>> impact the behaviour of the program or could be fixed by hashing
more of
>>> the AST. References to globals should be hashed by name, but
references to
>>> locals should be hashed by an ordinal number.
>>> >
>>> > Instantiated templates are also visited by the hashing
visitor. If we
>>> did not, we would have false negatives where the code is not
conforming due
>>> to different points of instantiation in two translation units. We
can skip
>>> uninstantiated templates since they don’t affect the behaviour of
the
>>> program, and we need to visit the instantiations regardless.
>>> >
>>> > In LLVM IR, create a new named metadata node
!llvm.odr_checking which
>>> contains a list of <mangled name, hash value> pairs. The
names do not
>>> necessarily correspond to symbols, for instance, a class will have
a hash
>>> value but does not have a corresponding symbol. For ease of
implementation,
>>> names should be mangled per the C++ Itanium ABI (demanglable with
c++filt
>>> -t). Merging modules that contain these will need to do ODR
checking as
>>> part of that link, and the resulting module will have the union of
these
>>> tables.
>>> >
>>> > In the .o file, emit a sorted table of <mangled name, hash
value> in a
>>> non-loadable section intended to be read by the linker. All entries
in the
>>> table must be checked if any symbol from this .o file is involved
in the
>>> link (note that there is no mapping from symbol to odr table name).
If two
>>> .o files contain different hash values for the same name, we have
detected
>>> an ODR violation and issue a diagnostic.
>>> >
>>> > Finally, teach the loader (RuntimeDyld) to do verification and
catch
>>> ODR violations when dlopen'ing a shared library.
>>>
>>> This is the right basic design, but I'm curious why you're
suggesting
>>> that the payload should just be a hash instead of an arbitrary
string.
>>
>>
>> What are you suggesting goes into this string?
>>
>>
>> The same sorts of things that you were planning on hashing, but maybe
not
>> hashed.  It's up to you; having a full string would let you
actually show a
>> useful error message, but it definitely inflates binary sizes.  If you
>> really think you can make this performant enough to do on every load, I
can
>> see how the latter would be important.
>>
>
> I was thinking we could add more things to help diagnostics, next to the
> hash. I *think* there are two cases that matter, but there may be more.
> Either we have an ODR violation where the file:line are different, or if
> file and line are the same then the preprocessor state was different.  We
> could emit file and line from the starting loc of each of the hashes, and
> we could emit a preprocessor table with the list of initial defines and
> changes to those defines as the TU went along -- at each hash we could
> point to an index into that table to indicate where we are. Both of those
> give us enough information for the linker to say why the ODRs failed to
> match.
>
>
> Do you have any idea how much data that is?  Your .o files are going to be
> *huge*.
>
The point of the design is to minimize the number of entries, so adding
more data per-entry doesn't seem like a big problem.

That said, I haven't implemented even an experiment that counts how many
entries we would produce. I can do that and come back with numbers,
possibly with my tail between my legs.
>  There are other situations where the file, line and preprocessor
> settings were the same. Then I'd expect the md5 of the file was
different,
> and the file was changed between two builds. Anything beyond that is pretty
> exotic.
>
>
> If the diagnostics you want the linker to report are literally as coarse
> as “this file changed, maybe that has something to do with it?”, I think
> you might be able to find something that doesn’t require quite so much of a
> core dump.
>
>  This isn't going to be performant enough to do unconditionally at
every
>>> load no matter how much you shrink it.
>>>
>>
>> Every load of a shared object? That's not a fast operation even
without
>> odr checking, but the idea is to keep the total number of entries in
the
>> odr table small. It's less than the number of symbols, closer to
the number
>> of top-level decls.
>>
>>
>> Your ABI dependencies are every declaration *that you ever rely on*.
>>  You've got to figure that that's going to be very large.  For
a library of
>> any significance, I'd be expecting this check to touch about half a
>> megabyte of data, even with a 32-bit hash and some sort of clever
prefixing
>> scheme on the symbols.  That's a pretty major regression in library
loading.
>>
>
> Fair point. If we want to include less in the hash just to make it more
> palatable for dynamic library users, we can have a flag for that. Some sort
> of ODR-checking lite.
>
> I really don't care about .so files myself.
>
>
> Maybe you should just spec this out as a static-linker-only technology,
> then.
>
That's fair criticism. I did exactly that, then was asked to please
consider the dynamic linker case and lazily said "oh sure, I don't see
any
reason the same scheme can't verify at load time too". I'll retract
any
claims about handling the dynamic loading case.

  Also, you should have something analogous to symbol visibility as a
way
>>> to tell the static linker that something only needs to be
ODR-checked
>>> within a linkage unit.  It would be informed by actual symbol
visibility,
>>> of course.
>>>
>>
>> Great point, and that needs to flow into the .o files as well. If a
class
>> has one visibility and its method has another, we want to skip the
method
>> when hashing the class, and need to emit an additional entry for the
method
>> alone? Is that right?
>>
>>
>> Class hashes should probably only include virtual methods anyway, but
>> yes, I think this is a good starting point.
>>
>> What do you want in the hash for a function anyway?  Almost everything
is
>> already captured by (1) the separate hashes for the nominal types
mentioned
>> and (2) the symbol mangling.  You're pretty much only missing the
return
>> type.  Oh, I guess you need the body's dependencies for inline
functions.
>>
>
> On the contrary, this is the case I care about! Two different definitions
> of the same function.
>
>
> Allow me to suggest a structural hash of the statements, then, instead of
> dumping the entire preprocessor history.
>
But I am suggesting a structural hash of the statements!

Consider a system that produces a structural hash of the statements. We
should do this with something that behaves similarly to the StmtProfiler,
but with specialized handling for things like labels and global variables.
Suppose we get a hash for each function, and emit a table of hashed
function name to hashed AST. Great! The basis of an ODR checker.

But can we do better? Instead of storing a hash for each function, we can
group them (such that we produce a single hash for more than one function),
for instance by class they're in when the methods were written in the body?
What I proposed is to have a single has for each top-level-declaration and
all of its lexically-included contents, dramatically reducing the number of
hashes we need to store. The tradeoff is that we can't quickly map from
symbol-name to an entry in the ODR table. That in turn means need to check
all ODR entries in a .o if we bring in any symbol from that .o file (we
can't just check the individual function because it may not have been
hashed individually).

You were asking why we should hash things when we could just store the
source code. I don't see how storing the text of the .cc would be better
than storing a condensed version of the changes we care about (the
preprocessor history). It's a strict subset, and if we really care, we can
shrink it further by being clever (collapse multiple changes between the
start of two top-level-decls, don't include changes in macros that
weren't
expanded during the parse of the body of a top-level-decl). The sort of
output I'm expecting would be along the lines of "a.o was produced with
_DEBUG while b.o was produced with NDEBUG".

Nick
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On Aug 5, 2013, at 4:43 PM, Nick Lewycky <nlewycky at google.com>
wrote:
> On 5 August 2013 15:33, John McCall <rjmccall at apple.com> wrote:
> On Aug 5, 2013, at 3:04 PM, Nick Lewycky <nlewycky at google.com>
wrote:
>> On 15 July 2013 15:12, John McCall <rjmccall at apple.com> wrote:
>> The same sorts of things that you were planning on hashing, but maybe
not hashed.  It's up to you; having a full string would let you actually
show a useful error message, but it definitely inflates binary sizes.  If you
really think you can make this performant enough to do on every load, I can see
how the latter would be important.
>> 
>> I was thinking we could add more things to help diagnostics, next to
the hash. I *think* there are two cases that matter, but there may be more.
Either we have an ODR violation where the file:line are different, or if file
and line are the same then the preprocessor state was different.  We could emit
file and line from the starting loc of each of the hashes, and we could emit a
preprocessor table with the list of initial defines and changes to those defines
as the TU went along -- at each hash we could point to an index into that table
to indicate where we are. Both of those give us enough information for the
linker to say why the ODRs failed to match.
> 
> Do you have any idea how much data that is?  Your .o files are going to be
*huge*.
> 
> The point of the design is to minimize the number of entries, so adding
more data per-entry doesn't seem like a big problem.
No, I mean the “preprocessor table with the list of initial defines and changes
to those defines as the TU went along”.  That’s global, sure, but it’s enormous.
>>> This isn't going to be performant enough to do unconditionally
at every load no matter how much you shrink it.
>>> 
>>> Every load of a shared object? That's not a fast operation even
without odr checking, but the idea is to keep the total number of entries in the
odr table small. It's less than the number of symbols, closer to the number
of top-level decls.
>> 
>> Your ABI dependencies are every declaration *that you ever rely on*. 
You've got to figure that that's going to be very large.  For a library
of any significance, I'd be expecting this check to touch about half a
megabyte of data, even with a 32-bit hash and some sort of clever prefixing
scheme on the symbols.  That's a pretty major regression in library loading.
>> 
>> Fair point. If we want to include less in the hash just to make it more
palatable for dynamic library users, we can have a flag for that. Some sort of
ODR-checking lite.
>> 
>> I really don't care about .so files myself.
> 
> Maybe you should just spec this out as a static-linker-only technology,
then.
> 
> That's fair criticism. I did exactly that, then was asked to please
consider the dynamic linker case and lazily said "oh sure, I don't see
any reason the same scheme can't verify at load time too". I'll
retract any claims about handling the dynamic loading case.
The same scheme could work, but doing it at load time forces completely
different performance considerations, so it really demands a different design.
>>> Also, you should have something analogous to symbol visibility as a
way to tell the static linker that something only needs to be ODR-checked within
a linkage unit.  It would be informed by actual symbol visibility, of course.
>>> 
>>> Great point, and that needs to flow into the .o files as well. If a
class has one visibility and its method has another, we want to skip the method
when hashing the class, and need to emit an additional entry for the method
alone? Is that right?
>> 
>> Class hashes should probably only include virtual methods anyway, but
yes, I think this is a good starting point.
>> 
>> What do you want in the hash for a function anyway?  Almost everything
is already captured by (1) the separate hashes for the nominal types mentioned
and (2) the symbol mangling.  You're pretty much only missing the return
type.  Oh, I guess you need the body's dependencies for inline functions.
>> 
>> On the contrary, this is the case I care about! Two different
definitions of the same function.
> 
> Allow me to suggest a structural hash of the statements, then, instead of
dumping the entire preprocessor history.
> 
> But I am suggesting a structural hash of the statements!
With pointers into a complete preprocessor history for some reason!

By “structural”, I mean “considering only the result of preprocessing and
semantic analysis”.

John.
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On 5 August 2013 16:55, John McCall <rjmccall at apple.com> wrote:
> On Aug 5, 2013, at 4:43 PM, Nick Lewycky <nlewycky at google.com>
wrote:
>
>  On 5 August 2013 15:33, John McCall <rjmccall at apple.com> wrote:
>
>> On Aug 5, 2013, at 3:04 PM, Nick Lewycky <nlewycky at google.com>
wrote:
>>
>> On 15 July 2013 15:12, John McCall <rjmccall at apple.com> wrote:
>>
>>> The same sorts of things that you were planning on hashing, but
maybe
>>> not hashed.  It's up to you; having a full string would let you
actually
>>> show a useful error message, but it definitely inflates binary
sizes.  If
>>> you really think you can make this performant enough to do on every
load, I
>>> can see how the latter would be important.
>>>
>>
>> I was thinking we could add more things to help diagnostics, next to
the
>> hash. I *think* there are two cases that matter, but there may be more.
>> Either we have an ODR violation where the file:line are different, or
if
>> file and line are the same then the preprocessor state was different. 
We
>> could emit file and line from the starting loc of each of the hashes,
and
>> we could emit a preprocessor table with the list of initial defines and
>> changes to those defines as the TU went along -- at each hash we could
>> point to an index into that table to indicate where we are. Both of
those
>> give us enough information for the linker to say why the ODRs failed to
>> match.
>>
>>
>> Do you have any idea how much data that is?  Your .o files are going to
>> be *huge*.
>>
>
> The point of the design is to minimize the number of entries, so adding
> more data per-entry doesn't seem like a big problem.
>
>
> No, I mean the “preprocessor table with the list of initial defines and
> changes to those defines as the TU went along”.  That’s global, sure, but
> it’s enormous.
>
Ah. Even more convincing to me is that this data is not normally included
in .o files at all. You're right. This could get big.
>    This isn't going to be performant enough to do unconditionally at
>>>> every load no matter how much you shrink it.
>>>>
>>>
>>> Every load of a shared object? That's not a fast operation even
without
>>> odr checking, but the idea is to keep the total number of entries
in the
>>> odr table small. It's less than the number of symbols, closer
to the number
>>> of top-level decls.
>>>
>>>
>>> Your ABI dependencies are every declaration *that you ever rely
on*.
>>>  You've got to figure that that's going to be very large. 
For a library of
>>> any significance, I'd be expecting this check to touch about
half a
>>> megabyte of data, even with a 32-bit hash and some sort of clever
prefixing
>>> scheme on the symbols.  That's a pretty major regression in
library loading.
>>>
>>
>> Fair point. If we want to include less in the hash just to make it more
>> palatable for dynamic library users, we can have a flag for that. Some
sort
>> of ODR-checking lite.
>>
>> I really don't care about .so files myself.
>>
>>
>> Maybe you should just spec this out as a static-linker-only technology,
>> then.
>>
>
> That's fair criticism. I did exactly that, then was asked to please
> consider the dynamic linker case and lazily said "oh sure, I don't
see any
> reason the same scheme can't verify at load time too". I'll
retract any
> claims about handling the dynamic loading case.
>
>
> The same scheme could work, but doing it at load time forces completely
> different performance considerations, so it really demands a different
> design.
>
I wonder whether the people who ask for ODR-violation detection at dynamic
link time are looking for something different? A way to verify that a new
release of their C++ library is free from a certain class of ABI mismatch
against the previous version. This is something entirely different from
what my goal was, even though it's also under the banner of enforcing
C++'s
ODR rule.
>   Also, you should have something analogous to symbol visibility as a way
>>>> to tell the static linker that something only needs to be
ODR-checked
>>>> within a linkage unit.  It would be informed by actual symbol
visibility,
>>>> of course.
>>>>
>>>
>>> Great point, and that needs to flow into the .o files as well. If a
>>> class has one visibility and its method has another, we want to
skip the
>>> method when hashing the class, and need to emit an additional entry
for the
>>> method alone? Is that right?
>>>
>>>
>>> Class hashes should probably only include virtual methods anyway,
but
>>> yes, I think this is a good starting point.
>>>
>>> What do you want in the hash for a function anyway?  Almost
everything
>>> is already captured by (1) the separate hashes for the nominal
types
>>> mentioned and (2) the symbol mangling.  You're pretty much only
missing the
>>> return type.  Oh, I guess you need the body's dependencies for
inline
>>> functions.
>>>
>>
>> On the contrary, this is the case I care about! Two different
definitions
>> of the same function.
>>
>>
>> Allow me to suggest a structural hash of the statements, then, instead
of
>> dumping the entire preprocessor history.
>>
>
> But I am suggesting a structural hash of the statements!
>
>
> With pointers into a complete preprocessor history for some reason!
>
> By “structural”, I mean “considering only the result of preprocessing and
> semantic analysis”.
>
Yes. There's two things here being conflated. The ODR detection itself --
determining whether there is an ODR violation -- should be done considering
only the result of preprocessing and semantic analysis. Absolutely. Violent
agreement.

Once an ODR violation is found, how can the linker produce a better error
message than "nope!"? That's where my suggestion of tagging each
entry with
the macro changes since previous top-level-decl came from. That should be
small in most cases (who defines things in the middle of their TU?) but in
reality we can't discard macros that come in from the system headers (and
are consequently used to declare functions that are used by definitions
later on). Ouch.

If I were to suppose a flag we could provide that means "help, I
encountered an ODR violation and need the compiler's help to solve it"
then
there's no reason we couldn't just have that flag include snippets of
source code in the .o instead of a condensed list of preprocessor changes.
Though upon further consideration, maybe that should just be a clang-tool.

Nick
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