llvm dev - Nov 2013 - [LLVMdev] Bug in Language Reference? %0 versus %1 as starting index.

Thanks for the lecture :)  But I was not planning on changing a single line
in LLVM/Clang.  I stick to the documentation until I've learned to swim,
perhaps even forever.  Ah, now I see.  You thought I meant "should I modify
the code to do this or that."  I only meant to change the documentation.
 Please refer to the patch I've sent on LLVM-commits.  That's about what
I
had in mind.  I am fully aware that you cannot simply dive in and hack away
on the handling of the %0 temporary.  I wouldn't ever dream of doing that!


-- Mikael




2013/11/27 Sean Silva <chisophugis at gmail.com>
> (gah, this turned into a huge digression, sorry)
>
> The implicit numbering of BB's seems to be a pretty frequent issue for
> people. Surprisingly, the issue boils down to simply changing the IR asm
> (.ll file) syntax so that it can have "unnamed BB's" in a
recognizable way
> that fits in with how unnamed values work (the asmprinter makes an effort
> to print a comment with the BB number, but the connection is hard to see
> and it's confusing).
>
> The thing that makes this not-as-easy-as-it-looks is doing it in a way
> that preserves compatibility with previous IR (and being able to convince
> yourself that this is the case), and the fact that the IR-parsing code is a
> bit twisty (it's not bad, but the way that some things work is subtly
> different from what you would expect) and you have to find something that
> "fits well" with what's there, doesn't require major
reworking of the
> existing code, etc.
>
> An alternative approach is to document very clearly this issue. That might
> be good in the short term, but IMO the time would be better spent
> ruminating over a way to fit this into the syntax, and thinking
> deeply/finding a way to convince yourself and others that this change
> doesn't break previous .ll files.
>
> It's just about thinking and coming up with a new syntax that fits well
> and that won't break existing .ll files. The key places for making this
> round-trip are AssemblyWriter::printBasicBlock in lib/IR/AsmWriter.cpp
> and LLParser::ParseBasicBlock in lib/AsmParser/LLParser.cpp. The parsing
> side is likely to be entirely in lib/AsmParser/LLLexer.cpp where you need
> to find a way to get a new token "LocalLabelID" returned for the
new syntax.
>
> To reiterate, the goal of such a change is solely to avoid people getting
> confused about the implicit numbering. It needs to be
> reminiscent/suggestive of the instruction numbering syntax to avoid this
> confusion.
>
> Heck, there may be something within the existing syntax that would work
> fine for this, but which we can recognize as being "unnamed",
rather than a
> unique name e.g. currently $1: will give the BB a name "$1" (in
the sense
> of getName()), and then "$2:" will give a name "$2",
etc., which will cause
> a lot of pointless string allocations; recognizing a decimal number here
> might be all that's needed (and updating the outputting code
accordingly),
> although I'm not sure a prefix $ is the best syntax.
>
> Maybe we could even get away with %42: as a BB label and that would be
> maximally reminiscent. The way that numbered local variables are handled is
> sort of ad-hoc (it is actually also handled in the Lexer; all the parser
> sees is lltok::LocalVarID). By just changing LLLexer::LexPercent in
> LLLexer.cpp to recognize a local label and emit a "LocalLabelID"
token,
> then adding an `else if` to the first `if` in LLParser::ParseBasicBlock,
> you could probably get a working solution too. However, this introduces an
> inconsistency in that now there's this pseudo-common syntax (%[0-9]+)
for
> unnamed things for both BB's and instructions, but in the case of
> instructions, the % sigil is always needed, while the label syntax
isn't
> sigilized by default, but permits this weird sigilized temporary numbered
> form. Maybe that slight inconsistency is worth it? If the inconsistency is
> really bothersome, we could also have BB's be able to start sigilized
with
> % in the other case like instructions are (there is no ambiguity because of
> the trailing `:`), but allow the unsigilized versions for compatibility;
> this may be more consistent from a semantic perspective too, since we refer
> to them sigilized when used as instruction operands.
>
> Or maybe you could have the BB be numbered just like `42:` without the
> sigil. We already lex a label like 42:, but we just have the issue that I
> mentioned with $1: that we set this string as the getName() value which
> creates a bunch of useless strings. If you just change the code to emit a
> "LocalLabelID" for this case and imitate how we handle locally
numbered
> instructions, that could be a pretty clean fix. However, that would change
> the behavior for how we handle a label like `0:`, for example, with this
> behavior, the following IR asm would work:
>
> define void @foo() {
> 0:
>   %1 = alloca i8*
>   ret void
> }
>
> but since with our current behavior we handle `0:` as a BB name and set
> it's getName() as "0", which causes it to not take up the
first unnamed
> value slot (the %0'th one), so then you get an error that %1 should be
%0.
> This may be an annoying forwards-compatibility issue for a while when we
> still have to work with not-trunk LLVM's, and this incompatibility may
not
> be worth it. Actually all the suggestions that I've made so far have
this
> same issue :/ Actually I think that it is unsolvable without a
> forwards-compatibility break due to this (any label that was previously
> accepted would not increment the unnamed local counter, which would cause
> all the existing unnamed locals to be off by one and cause an error). We do
> break forward-compatibility from time to time (e.g. the syntax for the new
> attributes system), so it might not be that big of an issue (although
> obviously the community will have to decide about the trade-off for a
> temporary nuisance vs. the issue this solves). If breaking
> forwards-compatibility is OK, then I would strongly suggest the `0:` syntax
> or `%0:`.
>
> Hopefully I've given you a bit of the flavor of the issues involved.
It's
> basically just a problem of sitting down and thinking hard, finding
> something cleanly-implementable that doesn't break backwards
compatibility,
> and checking with the community that the syntax is agreeable and that any
> forwards-compatibility break is ok.
>
> -- Sean Silva
>
>
> On Tue, Nov 26, 2013 at 8:02 PM, Mikael Lyngvig <mikael at
lyngvig.org>wrote:
>
>> The language reference states that local temporaries begin with index
0,
>> but if I try that on my not-entirely-up-to-date v3.4 llc (it is like a
week
>> old), I get an error "instruction expected to be numbered
'%1'".
>>
>> Also, quite a few examples in the LR uses %0 as a local identifier.
>>
>> Should I fix those or is it a problem in llc?
>>
>>
>> -- Mikael
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>>
>
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On Tue, Nov 26, 2013 at 9:58 PM, Mikael Lyngvig <mikael at lyngvig.org>
wrote:
> Thanks for the lecture :)  But I was not planning on changing a single
> line in LLVM/Clang.  I stick to the documentation until I've learned to
> swim, perhaps even forever.  Ah, now I see.  You thought I meant
"should I
> modify the code to do this or that."  I only meant to change the
> documentation.  Please refer to the patch I've sent on LLVM-commits.
>  That's about what I had in mind.  I am fully aware that you cannot
simply
> dive in and hack away on the handling of the %0 temporary.  I wouldn't
ever
> dream of doing that!
>
You should dream of doing that. Nobody else has stepped up to do it. Hack
on the code; ultimately that's where the action is and where you will gain
understanding.
(And I'm probably the worst person to give this advice since I do so little
code hacking during the school year. I swear, I really do prefer coding;
when I'm at work with a nice fast machine it's a lot nicer to hack, but
at
school with a crappy machine, the situation usually only permits reviewing
patches on the mailing lists or docs changes.)

AFAIK nobody is an "expert" in that code (its probably long out of
core for
even the people that wrote it); if you dive into it, you can become a local
expert in it.

-- Sean Silva

>
>
> -- Mikael
>
>
>
>
> 2013/11/27 Sean Silva <chisophugis at gmail.com>
>
>> (gah, this turned into a huge digression, sorry)
>>
>> The implicit numbering of BB's seems to be a pretty frequent issue
for
>> people. Surprisingly, the issue boils down to simply changing the IR
asm
>> (.ll file) syntax so that it can have "unnamed BB's" in a
recognizable way
>> that fits in with how unnamed values work (the asmprinter makes an
effort
>> to print a comment with the BB number, but the connection is hard to
see
>> and it's confusing).
>>
>> The thing that makes this not-as-easy-as-it-looks is doing it in a way
>> that preserves compatibility with previous IR (and being able to
convince
>> yourself that this is the case), and the fact that the IR-parsing code
is a
>> bit twisty (it's not bad, but the way that some things work is
subtly
>> different from what you would expect) and you have to find something
that
>> "fits well" with what's there, doesn't require major
reworking of the
>> existing code, etc.
>>
>> An alternative approach is to document very clearly this issue. That
>> might be good in the short term, but IMO the time would be better spent
>> ruminating over a way to fit this into the syntax, and thinking
>> deeply/finding a way to convince yourself and others that this change
>> doesn't break previous .ll files.
>>
>> It's just about thinking and coming up with a new syntax that fits
well
>> and that won't break existing .ll files. The key places for making
this
>> round-trip are AssemblyWriter::printBasicBlock in lib/IR/AsmWriter.cpp
>> and LLParser::ParseBasicBlock in lib/AsmParser/LLParser.cpp. The
parsing
>> side is likely to be entirely in lib/AsmParser/LLLexer.cpp where you
need
>> to find a way to get a new token "LocalLabelID" returned for
the new syntax.
>>
>> To reiterate, the goal of such a change is solely to avoid people
getting
>> confused about the implicit numbering. It needs to be
>> reminiscent/suggestive of the instruction numbering syntax to avoid
this
>> confusion.
>>
>> Heck, there may be something within the existing syntax that would work
>> fine for this, but which we can recognize as being "unnamed",
rather than a
>> unique name e.g. currently $1: will give the BB a name "$1"
(in the sense
>> of getName()), and then "$2:" will give a name
"$2", etc., which will cause
>> a lot of pointless string allocations; recognizing a decimal number
here
>> might be all that's needed (and updating the outputting code
accordingly),
>> although I'm not sure a prefix $ is the best syntax.
>>
>> Maybe we could even get away with %42: as a BB label and that would be
>> maximally reminiscent. The way that numbered local variables are
handled is
>> sort of ad-hoc (it is actually also handled in the Lexer; all the
parser
>> sees is lltok::LocalVarID). By just changing LLLexer::LexPercent in
>> LLLexer.cpp to recognize a local label and emit a
"LocalLabelID" token,
>> then adding an `else if` to the first `if` in
LLParser::ParseBasicBlock,
>> you could probably get a working solution too. However, this introduces
an
>> inconsistency in that now there's this pseudo-common syntax
(%[0-9]+) for
>> unnamed things for both BB's and instructions, but in the case of
>> instructions, the % sigil is always needed, while the label syntax
isn't
>> sigilized by default, but permits this weird sigilized temporary
numbered
>> form. Maybe that slight inconsistency is worth it? If the inconsistency
is
>> really bothersome, we could also have BB's be able to start
sigilized with
>> % in the other case like instructions are (there is no ambiguity
because of
>> the trailing `:`), but allow the unsigilized versions for
compatibility;
>> this may be more consistent from a semantic perspective too, since we
refer
>> to them sigilized when used as instruction operands.
>>
>> Or maybe you could have the BB be numbered just like `42:` without the
>> sigil. We already lex a label like 42:, but we just have the issue that
I
>> mentioned with $1: that we set this string as the getName() value which
>> creates a bunch of useless strings. If you just change the code to emit
a
>> "LocalLabelID" for this case and imitate how we handle
locally numbered
>> instructions, that could be a pretty clean fix. However, that would
change
>> the behavior for how we handle a label like `0:`, for example, with
this
>> behavior, the following IR asm would work:
>>
>> define void @foo() {
>> 0:
>>   %1 = alloca i8*
>>   ret void
>> }
>>
>> but since with our current behavior we handle `0:` as a BB name and set
>> it's getName() as "0", which causes it to not take up the
first unnamed
>> value slot (the %0'th one), so then you get an error that %1 should
be %0.
>> This may be an annoying forwards-compatibility issue for a while when
we
>> still have to work with not-trunk LLVM's, and this incompatibility
may not
>> be worth it. Actually all the suggestions that I've made so far
have this
>> same issue :/ Actually I think that it is unsolvable without a
>> forwards-compatibility break due to this (any label that was previously
>> accepted would not increment the unnamed local counter, which would
cause
>> all the existing unnamed locals to be off by one and cause an error).
We do
>> break forward-compatibility from time to time (e.g. the syntax for the
new
>> attributes system), so it might not be that big of an issue (although
>> obviously the community will have to decide about the trade-off for a
>> temporary nuisance vs. the issue this solves). If breaking
>> forwards-compatibility is OK, then I would strongly suggest the `0:`
syntax
>> or `%0:`.
>>
>> Hopefully I've given you a bit of the flavor of the issues
involved. It's
>> basically just a problem of sitting down and thinking hard, finding
>> something cleanly-implementable that doesn't break backwards
compatibility,
>> and checking with the community that the syntax is agreeable and that
any
>> forwards-compatibility break is ok.
>>
>> -- Sean Silva
>>
>>
>> On Tue, Nov 26, 2013 at 8:02 PM, Mikael Lyngvig <mikael at
lyngvig.org>wrote:
>>
>>> The language reference states that local temporaries begin with
index 0,
>>> but if I try that on my not-entirely-up-to-date v3.4 llc (it is
like a week
>>> old), I get an error "instruction expected to be numbered
'%1'".
>>>
>>> Also, quite a few examples in the LR uses %0 as a local identifier.
>>>
>>> Should I fix those or is it a problem in llc?
>>>
>>>
>>> -- Mikael
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>
>>
>
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Without ANY intent of offending anybody, I simply don't like C++.  I did
code in it for some 12 years back from 1990 to 2002, but then I left it
behind with a feeling of happiness.  The main reason I am _trying_ to make
a new language is that I hope to one day come up with something that can
help retiring C++.  I love C# but that language is yet too slow for many
demanding problem domains.

That being said, I don't seriously believe I'll ever finish up my own
language, but as long as I am having a good time along the way, I don't
mind.  Now I spend the majority of my spare time on LLVM documentation
(most of it still pending submission because of various factors).  Once the
dust settles from all the documentation projects I've started on (Arch
Linux build doc, Debian build doc, Windows build doc, Mapping High-Level
Constructs to LLVM IR), I plan to resume work on my own language, which
will be something like Python-syntax C# without .NET and perhaps with
optional garbage collection.

Perhaps I'll some day gather up the courage to pick an easy bug report and
fix that, but it is not very likely that I ever become a core coder on LLVM.


-- Mikael


2013/11/27 Sean Silva <chisophugis at gmail.com>
>
>
>
> On Tue, Nov 26, 2013 at 9:58 PM, Mikael Lyngvig <mikael at
lyngvig.org>wrote:
>
>> Thanks for the lecture :)  But I was not planning on changing a single
>> line in LLVM/Clang.  I stick to the documentation until I've
learned to
>> swim, perhaps even forever.  Ah, now I see.  You thought I meant
"should I
>> modify the code to do this or that."  I only meant to change the
>> documentation.  Please refer to the patch I've sent on
LLVM-commits.
>>  That's about what I had in mind.  I am fully aware that you cannot
simply
>> dive in and hack away on the handling of the %0 temporary.  I
wouldn't ever
>> dream of doing that!
>>
>
> You should dream of doing that. Nobody else has stepped up to do it. Hack
> on the code; ultimately that's where the action is and where you will
gain
> understanding.
> (And I'm probably the worst person to give this advice since I do so
> little code hacking during the school year. I swear, I really do prefer
> coding; when I'm at work with a nice fast machine it's a lot nicer
to hack,
> but at school with a crappy machine, the situation usually only permits
> reviewing patches on the mailing lists or docs changes.)
>
> AFAIK nobody is an "expert" in that code (its probably long out
of core
> for even the people that wrote it); if you dive into it, you can become a
> local expert in it.
>
> -- Sean Silva
>
>
>>
>>
>> -- Mikael
>>
>>
>>
>>
>> 2013/11/27 Sean Silva <chisophugis at gmail.com>
>>
>>> (gah, this turned into a huge digression, sorry)
>>>
>>> The implicit numbering of BB's seems to be a pretty frequent
issue for
>>> people. Surprisingly, the issue boils down to simply changing the
IR asm
>>> (.ll file) syntax so that it can have "unnamed BB's"
in a recognizable way
>>> that fits in with how unnamed values work (the asmprinter makes an
effort
>>> to print a comment with the BB number, but the connection is hard
to see
>>> and it's confusing).
>>>
>>> The thing that makes this not-as-easy-as-it-looks is doing it in a
way
>>> that preserves compatibility with previous IR (and being able to
convince
>>> yourself that this is the case), and the fact that the IR-parsing
code is a
>>> bit twisty (it's not bad, but the way that some things work is
subtly
>>> different from what you would expect) and you have to find
something that
>>> "fits well" with what's there, doesn't require
major reworking of the
>>> existing code, etc.
>>>
>>> An alternative approach is to document very clearly this issue.
That
>>> might be good in the short term, but IMO the time would be better
spent
>>> ruminating over a way to fit this into the syntax, and thinking
>>> deeply/finding a way to convince yourself and others that this
change
>>> doesn't break previous .ll files.
>>>
>>> It's just about thinking and coming up with a new syntax that
fits well
>>> and that won't break existing .ll files. The key places for
making this
>>> round-trip are AssemblyWriter::printBasicBlock in
lib/IR/AsmWriter.cpp
>>> and LLParser::ParseBasicBlock in lib/AsmParser/LLParser.cpp. The
parsing
>>> side is likely to be entirely in lib/AsmParser/LLLexer.cpp where
you need
>>> to find a way to get a new token "LocalLabelID" returned
for the new syntax.
>>>
>>> To reiterate, the goal of such a change is solely to avoid people
>>> getting confused about the implicit numbering. It needs to be
>>> reminiscent/suggestive of the instruction numbering syntax to avoid
this
>>> confusion.
>>>
>>> Heck, there may be something within the existing syntax that would
work
>>> fine for this, but which we can recognize as being
"unnamed", rather than a
>>> unique name e.g. currently $1: will give the BB a name
"$1" (in the sense
>>> of getName()), and then "$2:" will give a name
"$2", etc., which will cause
>>> a lot of pointless string allocations; recognizing a decimal number
here
>>> might be all that's needed (and updating the outputting code
accordingly),
>>> although I'm not sure a prefix $ is the best syntax.
>>>
>>> Maybe we could even get away with %42: as a BB label and that would
be
>>> maximally reminiscent. The way that numbered local variables are
handled is
>>> sort of ad-hoc (it is actually also handled in the Lexer; all the
parser
>>> sees is lltok::LocalVarID). By just changing LLLexer::LexPercent in
>>> LLLexer.cpp to recognize a local label and emit a
"LocalLabelID" token,
>>> then adding an `else if` to the first `if` in
LLParser::ParseBasicBlock,
>>> you could probably get a working solution too. However, this
introduces an
>>> inconsistency in that now there's this pseudo-common syntax
(%[0-9]+) for
>>> unnamed things for both BB's and instructions, but in the case
of
>>> instructions, the % sigil is always needed, while the label syntax
isn't
>>> sigilized by default, but permits this weird sigilized temporary
numbered
>>> form. Maybe that slight inconsistency is worth it? If the
inconsistency is
>>> really bothersome, we could also have BB's be able to start
sigilized with
>>> % in the other case like instructions are (there is no ambiguity
because of
>>> the trailing `:`), but allow the unsigilized versions for
compatibility;
>>> this may be more consistent from a semantic perspective too, since
we refer
>>> to them sigilized when used as instruction operands.
>>>
>>> Or maybe you could have the BB be numbered just like `42:` without
the
>>> sigil. We already lex a label like 42:, but we just have the issue
that I
>>> mentioned with $1: that we set this string as the getName() value
which
>>> creates a bunch of useless strings. If you just change the code to
emit a
>>> "LocalLabelID" for this case and imitate how we handle
locally numbered
>>> instructions, that could be a pretty clean fix. However, that would
change
>>> the behavior for how we handle a label like `0:`, for example, with
this
>>> behavior, the following IR asm would work:
>>>
>>> define void @foo() {
>>> 0:
>>>   %1 = alloca i8*
>>>   ret void
>>> }
>>>
>>> but since with our current behavior we handle `0:` as a BB name and
set
>>> it's getName() as "0", which causes it to not take up
the first unnamed
>>> value slot (the %0'th one), so then you get an error that %1
should be %0.
>>> This may be an annoying forwards-compatibility issue for a while
when we
>>> still have to work with not-trunk LLVM's, and this
incompatibility may not
>>> be worth it. Actually all the suggestions that I've made so far
have this
>>> same issue :/ Actually I think that it is unsolvable without a
>>> forwards-compatibility break due to this (any label that was
previously
>>> accepted would not increment the unnamed local counter, which would
cause
>>> all the existing unnamed locals to be off by one and cause an
error). We do
>>> break forward-compatibility from time to time (e.g. the syntax for
the new
>>> attributes system), so it might not be that big of an issue
(although
>>> obviously the community will have to decide about the trade-off for
a
>>> temporary nuisance vs. the issue this solves). If breaking
>>> forwards-compatibility is OK, then I would strongly suggest the
`0:` syntax
>>> or `%0:`.
>>>
>>> Hopefully I've given you a bit of the flavor of the issues
involved.
>>> It's basically just a problem of sitting down and thinking
hard, finding
>>> something cleanly-implementable that doesn't break backwards
compatibility,
>>> and checking with the community that the syntax is agreeable and
that any
>>> forwards-compatibility break is ok.
>>>
>>> -- Sean Silva
>>>
>>>
>>> On Tue, Nov 26, 2013 at 8:02 PM, Mikael Lyngvig <mikael at
lyngvig.org>wrote:
>>>
>>>> The language reference states that local temporaries begin with
index
>>>> 0, but if I try that on my not-entirely-up-to-date v3.4 llc (it
is like a
>>>> week old), I get an error "instruction expected to be
numbered '%1'".
>>>>
>>>> Also, quite a few examples in the LR uses %0 as a local
identifier.
>>>>
>>>> Should I fix those or is it a problem in llc?
>>>>
>>>>
>>>> -- Mikael
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>>
>>>>
>>>
>>
>
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