[LLVMdev] Inconsistencies or intended behaviour of LLVM IR?

Thu Jan 29 14:43:23 PST 2015

(forgot to cc the list)

Thank you for reviewing and commiting the patch Sean :) It was the first 
one I've ever submitted to LLVM and the whole process was really smooth! 
Using Phabricator with GitHub OAuth login was brilliant as it removed 
one more step for new contributors. I also feel very happy that the 
first patch ended up removing more code than it introduced :) Not likely 
to speed up the compilation process by a lot, but one can hope to keep 
the trend!

I read the blog post about the type system rewrite. Thank you for the 
link. It did clear up a lot of my uncertainties, but introduced a new 
one. Could you help me make sense of this part, which was presented 
under the "Identified structs have a 1-1 mapping with a name" section.

 > "... and the only types that can be named are identified structs"

Does this mean that other types cannot be named? What about type type 
"%x" in b.ll? It seems like I'm interpreting this in the wrong way. 
Could you help me make this clear? Is there a difference between a named 
type and an identified type (or are those two ways of saying the same 
thing)? If types other than structures can be given names, does this 
name impact type equality somehow?

To keep up with the spirit of the original topic here are a few more 
items :)

* Item 11 - hexadecimal integer constants

The lexer handles hexadecimal integer constants, e.g. from 
lib/AsmParser/LLLexer.cpp

 > ///    HexIntConstant  [us]0x[0-9A-Fa-f]+

This representation of integer constants is not mentioned in the 
language specification as far as I can tell.

* Item 12 - constant expressions

The documentation of sext states that the bit size of the constant must 
be smaller than the target type, but the implementation also accepts 
constants which have the same size as the target type. E.g. the 
documentation should be updated or the implementation made more strict.

 > sext (CST to TYPE)
 >    Sign extend a constant to another type. The bit size of CST must 
be smaller than the bit size of TYPE. Both types must be integers.

The same goes for the trunc, zext, sext, fptrunc and fpext operations. 
Some refer to larger instead of smaller but none states that types of 
equal size is allowed.

* Item 13 - LocalVar and LocalID for named types

This is more of a question. Why are types referred to using local names 
"%x" instead of global names "@x"? It seems inconsistent as local names 
are scoped to the function; a local variable name in one function refers 
to a different value from a local variable name in another. Since types 
are scoped to the module wouldn't a global name make more sense?

As always, I'm eager to hear more about the type system in particular. 
The compilation timed in at 120m36.240s while the test cases took 
32m10.111s. It will be interesting to see if this goes up or down as 
time passes :)

Cheers /Robin Eklind

On 01/28/2015 08:31 PM, Sean Silva wrote:
> On Wed, Jan 28, 2015 at 6:28 PM, Robin Eklind <carl.eklind at myport.ac.uk>
> wrote:
>
>> Hello Sean,
>>
>> Thank you for your reply. I'll give your suggestion to item 6 and 7 a try
>> tonight. I'll start a compilation and let it run throughout the night. My
>> laptop (x61s) is 8 years old by know, so compiling LLVM takes a little time
>> :)
>>
>
> This is why I did so much documentation work when in college. The docs
> build much faster.
>
>
>>
>> Regarding item 8. I don't know if anyone is using "": in the wild so
>> fixing the implementation might make sense. If not the documentation (e.g.
>> the QuoteLabel comment) should be updated to be in line with the
>> implementation.
>>
>
> FYI the textual IR doesn't have a compatibility guarantee (we try not to
> egregiously change it, but users don't expect .ll to work across versions).
>
>
>>
>> I only included item 9 since I stumbled upon it once cross-referencing the
>> source code with the language specification. Bitrot for a project of this
>> size is to be expected.
>>
>> I'm still very interested to hear about the items related to types, e.g.
>> item 1 and 2. Is there a good reference which describes how type equality
>> works in LLVM IR? If the source code is the reference, could someone with
>> the high level knowledge get me up to speed?
>>
>
> Off the top of my head maybe
> http://blog.llvm.org/2011/11/llvm-30-type-system-rewrite.html
>
>
>>
>> Item 1 still confuses me, so I'd be very happy if someone with more
>> insight could clarify if this is the intended behaviour and if so the
>> motivation behind it.
>>
>> As it so happens, I forgot to include item 10 :)
>>
>> * Item 10 - lli vs. clang output
>>
>> Using the same source files as before, it seems like lli and clang treats
>> common linkage and constant variables differently. The following execution
>> demonstrates the return value after executing i.ll, j.ll, k.ll and l.ll
>> with lli and clang respectively:
>>
>>> $ clang i.ll && ./a.out ; echo $?
>>> 37
>>>
>>> $ lli i.ll ; echo $?
>>> 37
>>>
>>>
>>> $ clang j.ll && ./a.out ; echo $?
>>> 0
>>>
>>> $ lli j.ll ; echo $?
>>> 42
>>>
>>>
>>> $ clang k.ll && ./a.out ; echo $?
>>> 37
>>>
>>> $ lli k.ll ; echo $?
>>> 37
>>>
>>>
>>> $ clang l.ll && ./a.out ; echo $?
>>> Segmentation fault
>>> 139
>>>
>>> $ lli l.ll ; echo $?
>>> 37
>>
>
> Some of these linkage combinations and operations have dubious semantics.
> Talking briefly with Rafael Espindola over a build, sounds like we should
> mostly tighten up the verifier to remove some of these weird cases. For
> example, storing to a constant is sort of .... I'm sort of surprised it
> works at all.
>
> -- Sean Silva
>
>
>>
>> Looking forward to hear more about type equality, or get a pointer as to
>> where I can read up about it.
>>
>> Cheers /Robin Eklind
>>
>>
>>
>> On 01/28/2015 03:45 PM, Sean Silva wrote:
>>
>>> A couple quick comments inline (didn't touch on all points):
>>>
>>> On Wed, Jan 28, 2015 at 1:49 AM, Robin Eklind <carl.eklind at myport.ac.uk>
>>> wrote:
>>>
>>>   Hello everyone!
>>>>
>>>> I've recently had a chance to familiarize myself with the nitty-gritty
>>>> details of LLVM IR. It has been a great learning experience, sometimes
>>>> frustrating or confusing but mostly rewarding.
>>>>
>>>> There are a few cases I've come across which seems odd to me. I've tried
>>>> to cross reference with the language specification and the source code to
>>>> the best of my abilities, but would like to reach out to an experienced
>>>> crowd with a few questions.
>>>>
>>>> Could you help me out by taking a look at these examples? To my novice
>>>> eyes they seem to highlight inconsistencies in LLVM IR (or the reference
>>>> implementation), but it is quite likely that I've overlooked something.
>>>> Please help me out.
>>>>
>>>> Note: the example source files have been attached and a copy is made
>>>> available at https://github.com/mewplay/ll
>>>>
>>>> * Item 1 - named pointer types
>>>>
>>>> It is possible to create a named array pointer type (and many others),
>>>> but
>>>> not a named structure pointer type. E.g.
>>>>
>>>> %x = type [1 x i32]* ; valid.
>>>> %x = type {i32}*     ; invalid.
>>>>
>>>> Is this the intended behaviour? Attaching a.ll, b.ll, c.ll and d.ll for
>>>> reference. All files except d.ll compiles without error using clang
>>>> version
>>>> 3.5.1 (tags/RELEASE_351/final).
>>>>
>>>>   $ clang d.ll
>>>>> d.ll:3:16: error: expected top-level entity
>>>>> %x = type {i32}*
>>>>>                  ^
>>>>> 1 error generated.
>>>>>
>>>>
>>>> Does it have anything to do with type equality? (just a hunch)
>>>>
>>>> * Item 2 - equality of named types
>>>>
>>>> A named integer type is equivalent to its literal type counterpart, but
>>>> the same is not true for named and literal structures. I am certain that
>>>> I've read about this before, but can't seem to locate the right section
>>>> of
>>>> the language specification; could anyone point me in the right direction?
>>>> Also, what is the motivation behind this decision? I've skimmed over the
>>>> code which handles named structure types (in lib/IR/core.cpp), but would
>>>> love to hear the high level idea.
>>>>
>>>> Attaching e.ll, f.ll, g.ll and h.ll for reference. All compile just file
>>>> except h.ll, which produces the following error message (using the same
>>>> version of clang as above):
>>>>
>>>>   $ clang h.ll
>>>>> h.ll:10:23: error: argument is not of expected type '%x = type { i32 }'
>>>>>           call void (%x)* @foo({i32} {i32 0})
>>>>>                                ^
>>>>> 1 error generated.
>>>>>
>>>>
>>>> * Item 3 - zero initialized common linkage variables
>>>>
>>>> According to the language specification common linkage variables are
>>>> required to have a zero initializer [1]. If so, why are they also
>>>> required
>>>> to provide an initial value?
>>>>
>>>> Attaching i.ll and j.ll for reference. Both compiles just fine and once
>>>> executed i.ll returns 37 and j.ll return 0. If the common linkage
>>>> variable
>>>> @x was not initialized to 0, j.ll would have returned 42.
>>>>
>>>> * Item 4 - constant common linkage variables
>>>>
>>>> The language specification states that common linkage variables may not
>>>> be
>>>> marked as constant [1]. The parser doesn't seem to enforce this
>>>> restriction. Would doing so cause any problems?
>>>>
>>>> Attaching k.ll and l.ll for reference. Both compiles just fine, but once
>>>> executed k.ll returns 37 (e.g. the constant variable was overwritten)
>>>> while
>>>> l.ll segfaults as expected when it tries to overwrite a read-only memory
>>>> location.
>>>>
>>>> * Item 5 - appending linkage restrictions
>>>>
>>>> An extract from the language specification [1]:
>>>>
>>>>   "appending" linkage may only be applied to global variables of pointer
>>>>>
>>>> to array type.
>>>>
>>>> Similarly to item 4 this restriction isn't enforced by the parser. Would
>>>> it make sense doing so, or is there any problem with such an approach?
>>>>
>>>> * Item 6 - hash token
>>>>
>>>> The hash token (#) is defined in lib/AsmParser/LLToken.h (release version
>>>> 3.5.0 of the LLVM source code) but doesn't seem to be used anywhere else
>>>> in
>>>> the source tree. Is this token a historical artefact or does it serve a
>>>> purpose?
>>>>
>>>>
>>> Try deleting it. If the tests pass send a patch. Same for item 7.
>>>
>>>
>>>
>>>> * Item 7 - backslash token
>>>>
>>>> Similarly to item 7 the backslash token doesn't seem to serve a purpose
>>>> (with regards to release version 3.5.0 of the LLVM source code). Is it
>>>> used
>>>> somewhere?
>>>>
>>>> * Item 8 - quoted labels
>>>>
>>>> A comment in lib/AsmParser/LLLexer.cpp (once again, release version 3.5.0
>>>> of the LLVM source code) describes quoted labels using the following
>>>> regexp
>>>> (e.g. at least one character between the double quotes):
>>>>
>>>>   ///   QuoteLabel        "[^"]+":
>>>>>
>>>>
>>>> In contrast the reference implementation accepts quoted labels with zero
>>>> or more characters between the double quotes. Which is to be trusted? The
>>>> comment makes more sense as the variable name would effectively be blank
>>>> otherwise.
>>>>
>>>>
>>> Looks an empty name just results in the thing becoming unnamed. That's
>>> sort
>>> of confusing, but probably not harmful. Maybe we use an empty name as a
>>> sentinel for "unnamed", so it sort of just was an accident of the
>>> implementation.
>>>
>>>
>>>
>>>> * Item 9 - undocumented calling conventions
>>>>
>>>> The following calling conventions are valid tokens but not described in
>>>> the language references as of revision 223189:
>>>>
>>>> intel_ocl_bicc, x86_stdcallcc, x86_fastcallcc, x86_thiscallcc,
>>>> kw_x86_vectorcallcc, arm_apcscc, arm_aapcscc, arm_aapcs_vfpcc,
>>>> msp430_intrcc, ptx_kernel, ptx_device, spir_kernel, spir_func,
>>>> x86_64_sysvcc, x86_64_win64cc, kw_ghccc
>>>>
>>>>
>>>>   This is just bitrot.
>>>
>>> -- Sean Silva
>>>
>>>
>>>
>>>>
>>>> Lastly I'd just like to thank the LLVM developers for all the time and
>>>> hard work they've put into this project. I'd especially like to thank you
>>>> for providing a language specification along side of the reference
>>>> implementation! Keeping it up to date is a huge task, but also hugely
>>>> important. Thank you!
>>>>
>>>> Kind regards
>>>> /Robin Eklind
>>>>
>>>> [1]: http://llvm.org/docs/LangRef.html#linkage-types
>>>>
>>>> _______________________________________________
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>>>>
>>>>
>>>>
>>>
>