[LLVMdev] MergeFunctions: reduce complexity to O(log(N))

[Stepan Dyatkovskiy]

Hi Nick,
Thank you for reviews! Here is my answers for first two patches.

Though as additional, I think its important answer to this question first:
 > Okay, I'm confused. We do all this work to produce a 3-way comparison,
 > then we wrap it up in an operator< for a std::set that will only look at
 > less-than...
Actually, on top level we don't need 3-way comparison. But we do need it 
to determine whether functions are equal on some stage just to proceed 
to the next "less meaningful" stage.
Consider two functions
"F1: {BB1, BB2}" and
"F2: {BB-equal-to-BB1, BB3}".
First we have to realize that BB1 and BB-equal-to-BB1 are equal. And 
then determine which one of BB2 and BB3 is less.

First patch.

 > +  /// 1. Bitcastability.
 > +  /// Check whether L's type could be losslessly bitcasted to R's type.
 > +  /// On this stage method, in case when lossless bitcast is not 
possible
 > +  /// method returns -1 or 1, thus also defining which type is 
greater in
 > +  /// context of bitcastability.
I have added detailed order description for this method. You also can 
find this description in cmpConstantsComment.txt.

 > Optional: "Return its pointers comparison result" -> "Return the result
 > of comparing the types"?
Oops! Fixed.

 > +    if (TyL->getTypeID() == Type::X86_MMXTyID)
 > +      TyLWidth = 64;
 > +    else if (const VectorType *VecTyL = dyn_cast<VectorType>(TyL))
 > +      TyLWidth = VecTyL->getBitWidth();
 >
 > Yikes! We never merge function A using a vector and function B using an
 > x86_mmx type together, right?
Fixed!

 > "Nor of TyL and TyR" -> "Neither Tyl nor TyR".
 >
 > Optional: "Return its pointers comparison result" -> "Return the result
 > of comparing the types"?
Fixed.

 > +  case Value::UndefValueVal: return cmpType(L->getType(), R->getType());
 >
 > You already have L->getType() and R->getType() in TyL and TyR.
Fixed. Sorry.

Second patch.

 > I'm curious whether we guarantee that
 > we get the same numbering when comparing two functions if I simply
 > reorder the BasicBlocks in one of the functions, will we still assign
 > the same numberings or not?
...
 > Why not just start at the beginning and run to
 > the end, relying on the SSA dominance property.
Good questions!

For first one I have added these notes:
[quote]
As follows from FunctionComparator::compare(), we do CFG walk, we start 
from entry, and then take each terminator. So it doesn't matter how in 
fact BBs are ordered in function. And since cmpValues are called during 
this walk, the numbering depends only on how BBs located inside the CFG.
[/quote]

As for second question, I have added next notes:

[quote]
We also can't use dominance properties. Consider situation:
If we compare two instruction operands: first is usage of local variable 
AL from function FL, and second is usage of local variable AR from FR. 
We are still not able to compare operands of PHI, since those could be 
operands from further BBs we didn't scan yet.
[/quote]

You can find comments also in sn_mapLRComment.txt
I have also described lexicographical ordering for cmpValues in method 
comments. I have also put them in separated txt file: cmpValuesComment.txt.

 > +    if (L == R) return 0;
 > +    // Compare C1 and the bitcast result.
 > +    if (int Res = cmpConstants(ConstL, ConstR))
 > +      return Res;
 > [...]
 > +    return 0;
 >
 > Isn't that just:
 >    return cmpConstants(ConstL, ConstR);
OOOPS! Fixed.

I'm working on rest of patches.
Thanks!

-Stepan.

Nick Lewycky wrote:
> Stepan Dyatkovskiy wrote:
>> Hello Nick!
>>
>> Can we proceed to work on patch series?
>
>> Just in case, I have reattached them to this post.
>> Would you prefer to see them in http://reviews.llvm.org ?
>
> Oddly, I prefer attachments, but we can use reviews.llvm.org if you prefer?
>
> 0001-constants-comparison.patch
>
> +  /// 1. Bitcastability.
> +  /// Check whether L's type could be losslessly bitcasted to R's type.
> +  /// On this stage method, in case when lossless bitcast is not possible
> +  /// method returns -1 or 1, thus also defining which type is greater in
> +  /// context of bitcastability.
>
> Looking at the code, it looks like the order is:
>    1. first class types, by typeid
>    2. vectors (by element size? by number of elements? the two
> multiplied together?)
>
> +      // Nor of TyL and TyR are values of first class type. Return
> +      // its pointers comparison result.
>
> "Nor of TyL and TyR" -> "Neither Tyl nor TyR".
>
> Optional: "Return its pointers comparison result" -> "Return the result
> of comparing the types"?
>
> +    if (TyL->getTypeID() == Type::X86_MMXTyID)
> +      TyLWidth = 64;
> +    else if (const VectorType *VecTyL = dyn_cast<VectorType>(TyL))
> +      TyLWidth = VecTyL->getBitWidth();
>
> Yikes! We never merge function A using a vector and function B using an
> x86_mmx type together, right? Note what the LangRef says about x86_mmx:
> "The operations allowed on it are quite limited: parameters and return
> values, load and store, and bitcast."
>
> +  case Value::UndefValueVal: return cmpType(L->getType(), R->getType());
>
> You already have L->getType() and R->getType() in TyL and TyR. You also
> already have cmpType(TyL, TyR) in TypesRes. Please just reuse them in
> the rest of this function.
>
> 0002-values-comparison-replacement-for-enumerate.patch
>
> +  // Assign serial numbers to values from left function, and values from
> +  // right function.
> +  // Explanation:
> +  // Being comparing functions we need to compare values we meet at
> left and
> +  // right sides.
> +  // Its easy to sort things out for external values. It just should be
> +  // the same value at left and right.
> +  // But for local values (those were introduced inside function body)
> +  // we have to ensure they were introduced at exactly the same place,
> +  // and plays the same role. How to do it?
> +  // Just assign serial number to each value when we meet it first time.
> +  // Values that were met at same place will be with same serial numbers.
>
> This explanation is correct, but I'm curious whether we guarantee that
> we get the same numbering when comparing two functions if I simply
> reorder the BasicBlocks in one of the functions, will we still assign
> the same numberings or not?
>
> Also, it's not clear from the comment why we need the complexity of
> numbering them anyways. Why not just start at the beginning and run to
> the end, relying on the SSA dominance property to guarantee that we
> visit definitions before uses, and assume we'll visit each value in the
> same order (look at each instruction and operand in order and just
> complain about differences)? The answer is that PHI nodes mean we will
> visit some uses before some definitions.
>
> Anyways, changing this text is optional in my mind. I think it could be
> better, but it's not worth spending much time on it.
>
> +int FunctionComparator::cmpValues(const Value *L, const Value *R) {
>
> I wouldn't mind an explanation of the lexographic ordering on this one
> either. Constants before non-constants, then InlineAsm (ordered by
> address), then visit order.
>
> +    if (L == R) return 0;
> +    // Compare C1 and the bitcast result.
> +    if (int Res = cmpConstants(ConstL, ConstR))
> +      return Res;
> [...]
> +    return 0;
>
> Isn't that just:
>    return cmpConstants(ConstL, ConstR);
> ?
>
> 0003-attributes-comparison.patch
>
> +int FunctionComparator::cmpStrings(StringRef L, StringRef R) const {
> +  // Prevent heavy comparison, compare sizes first.
> +  if (int Res = cmpNumbers(L.size(), R.size()))
> +    return Res;
> +
> +  return L.compare(R);
> +}
>
> Optional: I'm curious whether that's actually worth it? I checked
> StringRef and it does the length comparison last. It could be that you
> know that your strings tend to be longer than StringRef which tries to
> be efficient in general.
>
> 0004-operations-comparison.patch
>
> +int FunctionComparator::cmpOperation(const Instruction *L,
>
> Optional: Again, a description of the ordering would be interesting.
> Opcode number, then number of operands, then type, then
> 'nsw/nuw/exact/volatile'.
>
> 0005-GEP-comparison.patch
>
> +int FunctionComparator::cmpGEP(const GEPOperator *GEPL,
> +                               const GEPOperator *GEPR) {
>
> +  unsigned int ASL = GEPL->getPointerAddressSpace();
>
> Extra line after {
>
>     if (DL) {
> [...]
> +    unsigned BitWidth = DL ? DL->getPointerSizeInBits(ASL) : 1;
>
> BitWidth doesn't need to test DL for null-ness.
>
> 0006-top-level-compare-method.patch
>
> Perfect!
>
> 0007-sanity-check.patch
>
> +          // Check symmetricity.
>
> "symmetricity" -> "symmetry".
>
> 0008-removed-unused-methods.patch
>
> Perfect
>
> 0009-FnSet-replaced-with-FnTree.patch
>
> Okay, I'm confused. We do all this work to produce a 3-way comparison,
> then we wrap it up in an operator< for a std::set that will only look at
> less-than, but never uses the equals result. We want to do one
> comparison between two functions, not two (ie., std::set does
> func1<func2 and func2<func1 to see whether they're equal or less than,
> and we don't need that since cmp(func1, func2) tells us directly). Maybe
> this comes in a future patch?
>
> One other question, I should probably know the answer to this but it's
> been a while. :) When we start detecting equal functions, do we merge
> them immediately? If so, can't that cause functions already in the set
> to have their equality comparisons change (ie., A calls X, B calls Y, A
> and B are in the set, then we merge X and Y for being equal, now we've
> changed A and B to both called the same merged function, so they start
> to compare the same). Sets tend not to like having their comparison
> functions change on elements that they contain.
>
> 0010-updated-comments.patch
>
> Okay.
>
> 0011-removed-DenseMap-helpers.patch
>
> Yay deleting dead code!
>
> Nick
>
>>
>> Thanks!
>> -Stepan.
>>
>> Stepan Dyatkovskiy wrote:
>>> ping
>>> Stepan Dyatkovskiy wrote:
>>>> Hi Nick,
>>>>
>>>> Please find the fixed patches in attachment.
>>>> Series starts from "constants comparison".
>>>>
>>>> Below small report of what has been fixed, and answers on your
>>>> questions.
>>>>
>>>> cmpTypes:
>>>> > Please add a comment for this method. Include the meaning of the
>>>> returned value. ("man strcmp" for inspiration.)
>>>> Fixed and committed. So you can look in trunk, may be I forgot to do
>>>> something (hope not :-) )
>>>>
>>>> checkForLosslessBitcast, cmpConstants:
>>>> > Why isn't this using cmpType?
>>>> Fixed.
>>>>
>>>> > Please put the else on the same line as the closing brace.
>>>> Fixed.
>>>>
>>>> >> + else if (const VectorType *thisPTy = dyn_cast<VectorType>(L))
>>>> > Missing initial capital
>>>> Sorry. Fixed. Actually these typos has been cloned from
>>>> Type::canLosslesslyBitCastTo.
>>>>
>>>> >> + return cmpNumbers((uint64_t)L, (uint64_t)R);
>>>> >>
>>>> > Please assert on unknown constant.
>>>> That's possible. But what if we really got unknown constant? New
>>>> comming
>>>> constant types merely depends on MergeFunctions implementation. So we
>>>> get crash if it will happen. And we loose nothing comparing them
>>>> just as
>>>> pointers. So, do we really need an assertion? Currently I kept it as it
>>>> was. If your answer is "yes, we need it", it would easy to add it:
>>>>
>>>> case Value::FunctionVal:
>>>> case Value::GlobalVariableVal:
>>>> - case Value::GlobalAliasVal:
>>>> - default: // Unknown constant
>>>> - return cmpNumbers((uint64_t)L, (uint64_t)R);
>>>> + case Value::GlobalAliasVal:
>>>> + return cmpNumbers((uint64_t)L, (uint64_t)R);
>>>> + default:
>>>> + llvm_unreachable("Unknown constant.");
>>>>
>>>> About variable names: Var1, Var2 vs VarL,VarR.
>>>> I think its better to use L/R concept. Easer to understand what to
>>>> return (-1, or 1) when you see L/R rather than Var1/Var2.
>>>> Var1/Var2 has been kept for functions that are to be removed till the
>>>> end of patch series.
>>>> F1 and F2 names were also kept till the "top level compare method"
>>>> patch.
>>>>
>>>> > Alternatively, any reason not to have cmpPointers(const void *L,
>>>> const void *R)?
>>>> I could do it. I just wanted to show that pointers are compared like a
>>>> numbers in some situations. Actually we do it in cases when we have
>>>> absolutely no idea of smarter comparison.
>>>> And back to cmpPonters. Its rather about what intuition tells. If
>>>> pointers are equal as numbers, could they be different somehow? Could
>>>> they be non-castable to numbers on some platforms? The last one is big
>>>> trouble for DenseMapInfo implementation..
>>>> If there is any (even very small) possibility of such cases - then yes,
>>>> I vote for cmpPointers. The last word is up to you anyway.
>>>>
>>>> Attributes (0005):
>>>> > Attributes already have operator< and operator==. Please reuse them.
>>>> Fixed. I used simple "if":
>>>>
>>>> if (LA < RA)
>>>> return -1;
>>>> if (RA < LA)
>>>> return 1;
>>>>
>>>> Though its possible to use:
>>>>
>>>> if (int Res = (LA < RA) ? -1 : (RA < LA) ? 1 : 0)
>>>> return Res;
>>>>
>>>> Which one is more preferable?
>>>>
>>>> cmpGEP (0007):
>>>> > + int cmpGEP(const GEPOperator *GEP1, const GEPOperator *GEP2);
>>>> > + int cmpGEP(const GetElementPtrInst *GEP1,
>>>> > + const GetElementPtrInst *GEP2) {
>>>> >
>>>> > Const members?
>>>> We can't make it constant, since it compares values (cmpValues, aka
>>>> enumerate). So, if we see Value first time, we have to add it into
>>>> sn_mapL/R.
>>>>
>>>> > I think you should put this under if (DL) { /* declare Offset1,
>>>> Offset2, etc. */ }
>>>> Fixed.
>>>>
>>>> About 0008:
>>>> > Did you mean "cmpOperation"?
>>>> Yes. We could keep it in one place. I have fixed this case and removed
>>>> TODO.
>>>>
>>>> > "compare" still returns "bool". I'm going to assume this was meant to
>>>> go in 0009.
>>>> Yes.
>>>>
>>>> About 0011:
>>>> > std::set is frowned upon, see
>>>> http://llvm.org/docs/ProgrammersManual.html#set . Do you actually rely
>>>> on the stable iterator guarantee? Or would another set-like container
>>>> be a
>>>> better fit?
>>>> Huh.. I have looked for alternatives. Unfortunately SmallSet is not
>>>> suitable, since we need to lookup existing items, and SmallSet::find
>>>> method is not present. May be I have missed something.
>>>> Actually we need binary tree implementation. Do we have better
>>>> analogue?
>>>> I could implement new one. Think it should be just one more patch
>>>> afterwards.
>>>>
>>>> >No. An identifier with underscore then capital letter is reserved for
>>>> the implementation. You can just call them "F" and "DL", then the ctor
>>>> initializer list can be "F(F), DL(DL)" and that will work fine.
>>>> Fixed.
>>>>
>>>> 0013:
>>>> > Sorry, I'm not sure what this sentence means? The thing your example
>>>> is talking about is showing is that two functions in an SCC may be
>>>> equivalent, and detecting them requires "blinding" (ignoring) certain
>>>> details of the function when you do the comparison. We blind ourselves
>>>> to the pointee types, but not to callees of functions in the same SCC.
>>>>
>>>> I meant generalising of cross-reference case:
>>>> in old implementation, while comparing F and G functions, we treat as
>>>> equal case when F uses G, with case when G uses F (at the same place).
>>>> It could happen that F uses G1, while G1 uses G2, while G2 uses F.
>>>> So it
>>>> still the same. But currently we have to invent way how to detect such
>>>> cases.
>>>>
>>>> Thanks for reviews!
>>>> -Stepan
>>>>
>>>> Stepan Dyatkovskiy wrote:
>>>>> Hi Nick,
>>>>> I have committed 0001 as r203788.
>>>>> I'm working on fixes for 0002 - 0014.
>>>>>
>>>>> > After reading through this patch series, I feel like I'm missing
>>>>> > something important. Where's the sort function? It looks like we're
>>>>> > still comparing all functions to all other functions.
>>>>> When you insert functions into std::set or its analogs it does all
>>>>> the
>>>>> job for you. Since internally it builds a binary tree using "less"
>>>>> comparison, and each insert/look-up operation takes O(log(N)) time.
>>>>>
>>>>> -Stepan.
>>>>>
>>>>> Nick Lewycky wrote:
>>>>>> On 27 February 2014 08:23, Stepan Dyatkovskiy <stpworld at narod.ru
>>>>>> <mailto:stpworld at narod.ru>> wrote:
>>>>>>
>>>>>> Hi Nick,
>>>>>>
>>>>>> I tried to rework changes as you requested. One of patches (0004
>>>>>> with extra assertions) has been removed.
>>>>>>
>>>>>>
>>>>>> > + bool isEquivalentType(Type *Ty1, Type *Ty2) const {
>>>>>> > + return cmpType(Ty1, Ty2) == 0;
>>>>>> > + }
>>>>>> >
>>>>>> > Why do we still need isEquivalentType? Can we nuke this?
>>>>>> Yup. After applying all the patches isEquivalentType will be
>>>>>> totally
>>>>>> replaced with cmpType. All isEqXXXX and friends will be removed in
>>>>>> 0011 (old 0012). No traces left.
>>>>>> Old function wasn't removed in 0001 just for keeping patches
>>>>>> without
>>>>>> extra noise like:
>>>>>>
>>>>>> - something that uses isEquivalentType
>>>>>> + something that uses cmpType
>>>>>>
>>>>>> The point is, that "something" that uses isEquivalentType, will be
>>>>>> also replaced with one of next patches in this series.
>>>>>>
>>>>>>
>>>>>> >
>>>>>> > +static int cmpNumbers(uint64_t L, uint64_t R) {
>>>>>> > + if (L < R) return -1;
>>>>>> > + if (L > R) return 1;
>>>>>> > + return 0;
>>>>>> > +}
>>>>>> >
>>>>>> > At a high level, you don't actually need a <=> operator to use a
>>>>>> sort. A
>>>>>> > strict ordering ( < operator) is sufficient. (Note that for
>>>>>> mergefunc, a
>>>>>> > strict weak ordering is not sufficient, it must be a total
>>>>>> ordering.)
>>>>>> That could be done with int FunctionComparator::compare(). We can
>>>>>> replace it with bool FunctionComparator::less(). Though for all
>>>>>> other cmp methods need at least 2 bits of information as result:
>>>>>> 1. Whether things are equal.
>>>>>> 2. Whether left less than right.
>>>>>>
>>>>>> As for FunctionComparator::compare(), conversion into less() will
>>>>>> increase time of sanity check (patch #0010).
>>>>>> Sanity check is just a sweet bonus. It checks that ordering
>>>>>> implemented properly (checks order relation properties).
>>>>>> Turning compare() into less() mean, that we'll have to run
>>>>>> comparison two times: L.less(R) and R.less(L). But may be sanity
>>>>>> check is not a thing to be published at all.
>>>>>>
>>>>>>
>>>>>> >
>>>>>> > Consider hoisting this inside the FunctionComparator class? That
>>>>>> class
>>>>>> > should have a bunch of implementations of comparisons between
>>>>>> various
>>>>>> > different things, which can pass down to other methods in the
>>>>>> same class.
>>>>>> In new patch series attached to this post, I have moved all static
>>>>>> methods into FunctionComparator.
>>>>>>
>>>>>>
>>>>>> > + // Replacement for type::canLosslesslyBitCastTo, that
>>>>>> > + // establish order relation on this kind of properties.
>>>>>> > + int checkForLosslessBitcast(const Type *L, const Type *R);
>>>>>> >
>>>>>> > Type:: not type:: . Please make this comment more descriptive.
>>>>>> Done.
>>>>>> [new comment]
>>>>>> Replacement for Type::canLosslesslyBitCastTo, that
>>>>>>
>>>>>> establish order relation on this kind of properties
>>>>>> Returns 0, if L and R types could be converted to each other
>>>>>> without
>>>>>> reinterpretation of bits.
>>>>>> Otherwise method returns -1 or 1, defining total ordering between
>>>>>> types in context of lossless bitcastability trait.
>>>>>> E.g.: if L is less than R (result is -1), than every type that
>>>>>> could be
>>>>>> losslessly bitcasted to L is less than R.
>>>>>> [/new comment]
>>>>>>
>>>>>>
>>>>>> >
>>>>>> > + /// Replacement for C1 == getBitCast(C2, C1Ty)
>>>>>> > + /// Its more controllable, and supposed to be simpler and
>>>>>> more
>>>>>> > predictionable.
>>>>>> > + /// As very important advantage: it allows to introduce order
>>>>>> relation on
>>>>>> > + /// constants set, and thus use it as trait in refinement
>>>>>> routines.
>>>>>> >
>>>>>> > "Its" --> "It's". "predictionable" --> "predictable". And how is
>>>>>> it more
>>>>>> > predictable? I think this comment would be better if it
>>>>>> described the
>>>>>> > function instead of making comparisons between it and other
>>>>>> functions.
>>>>>> > Something like, "Compare constants under a system where pointer
>>>>>> to X and
>>>>>> > pointer to Y are considered equal" or whatever is actually true
>>>>>> here.
>>>>>> Done.
>>>>>> [new comment]
>>>>>>
>>>>>> Replacement for C1 == getBitCast(C2, C1Ty)
>>>>>> Parses constants contents, assuming that types are losslessly
>>>>>> bitcasted between each other. So actually it ignores types and only
>>>>>> compares bits from L and R.
>>>>>> Returns 0, if L and R has equivalent content.
>>>>>> -1 or 1 if values are different. Maintaining total ordering
>>>>>> requires
>>>>>> two values that indicates non-equivalence (L less R, L greater R).
>>>>>> [/new comment]
>>>>>>
>>>>>>
>>>>>> >
>>>>>> > +enum ConstantType {
>>>>>> > I'm not sure that this buys you much. All the "isa" tests can be
>>>>>> broken
>>>>>> > down into a switch on getValueID() with the one exception of
>>>>>> isNullValue().
>>>>>> Done.
>>>>>>
>>>>>>
>>>>>> > + assert(
>>>>>> > +
>>>>>> C1->getType()->__canLosslesslyBitCastTo(C2->__getType()) &&
>>>>>> > + "Pass is healthless. checkForLosslessBitcast should be
>>>>>> twin of "
>>>>>> > + "canLosslesslyBitCastTo method, except the case when the
>>>>>> last one "
>>>>>> > + "returns false, the first one should return -1 or 1");
>>>>>> ...
>>>>>>
>>>>>> > I think we can skip the asserts here. They aren't detecting a
>>>>>> specific
>>>>>> > bug, they're checking whether the new code does a certain task
>>>>>> relative
>>>>>> > to the old code. Drop the old code, your new code is the new
>>>>>> sheriff in
>>>>>> > town.
>>>>>> Asserts has been removed.
>>>>>>
>>>>>>
>>>>>> >
>>>>>> > + DenseMap<const Value*, int> sn_map1, sn_map2;
>>>>>> >
>>>>>> > What is sn short for ("seen")? Why are there two of these?
>>>>>> Serial number :-)
>>>>>>
>>>>>> >
>>>>>> > + std::pair<DenseMap<const Value *, int>::iterator, bool>
>>>>>> > + LeftSN = sn_map1.insert(std::make_pair(__V1,
>>>>>> sn_map1.size())),
>>>>>> > + RightSN = sn_map2.insert(std::make_pair(__V2,
>>>>>> sn_map2.size()));
>>>>>> >
>>>>>> > So I think I get it, this is easy to reason about. You number
>>>>>> each value
>>>>>> > going down both functions. But I think you can eliminate one of
>>>>>> these
>>>>>> > maps because you know that if the left and right were ever
>>>>>> different
>>>>>> > then we terminate the comparison immediately. You can at least
>>>>>> share one
>>>>>> > map with both V1 and V2, but I think you can reduce the number
>>>>>> of map
>>>>>> > insertions.
>>>>>> Not sure. Consider that in left you have:
>>>>>> %A = alloca i32
>>>>>> %B = alloca i32
>>>>>> store i32 1, i32* %A
>>>>>>
>>>>>> And in right:
>>>>>> %A = alloca i32
>>>>>> %B = alloca i32
>>>>>> store i32 1, i32* %B
>>>>>>
>>>>>> When we meet 'store' instruction we need to check in which order %A
>>>>>> was allocated at left and in which order %B was allocated at right.
>>>>>> So for each value in function we have to assign serial number. Then
>>>>>> comparing to local values from different functions we can just
>>>>>> check
>>>>>> their serial numbers.
>>>>>> Though, may be I missed something? May be there is another way to
>>>>>> determine "who was first".
>>>>>>
>>>>>>
>>>>>> > High-level question, are attributes ordered? Your comparison is
>>>>>> ordered.
>>>>>> > So if I have function F with string attributes "sse2",
>>>>>> "gc=hemisphere"
>>>>>> > and another function G with string attributes "gc=hemisphere",
>>>>>> "sse2"
>>>>>> > then they would be considered different. I don't think that's
>>>>>> right.
>>>>>> I didn't check it. But if it is not ordered, we could order it
>>>>>> lexicographically.
>>>>>>
>>>>>>
>>>>>> >
>>>>>> > + int cmpOperation(const Instruction *I1, const Instruction
>>>>>> *I2)
>>>>>> const;
>>>>>> > +
>>>>>> > bool isEquivalentOperation(const Instruction *I1,
>>>>>> > - const Instruction *I2) const;
>>>>>> > + const Instruction *I2) const {
>>>>>> > + return cmpOperation(I1, I2) == 0;
>>>>>> > + }
>>>>>> >
>>>>>> > Hopefully this patch series ultimately eliminates calls of
>>>>>> > isEquivalentOperation?
>>>>>> Of course. Just like isEquivalentType.
>>>>>>
>>>>>>
>>>>>> > By the way, this is an interesting one. Should "add x, y" and
>>>>>> "add nsw
>>>>>> > x, y" be equal or not?
>>>>>> Yeah. Those are interesting questions. We could even treat 'mul a,
>>>>>> 2' and 'shl a,1' as equal in some cases. I think it's matter of
>>>>>> time
>>>>>> and further optimization.
>>>>>>
>>>>>> Please find first reworked patches in attachment.
>>>>>>
>>>>>>
>>>>>> I am extremely concerned that you may end up with A < B and B < C
>>>>>> but A
>>>>>> > C. There are cases where you use cmpTypes to compare two types,
>>>>>> then
>>>>>> others where you compare them via cmpNumbers. The only way to ensure
>>>>>> that the sort function is self-consistent is to be entirely
>>>>>> consistent
>>>>>> with how we traverse the function.
>>>>>>
>>>>>> 0001:
>>>>>> + int cmpType(Type *Ty1, Type *Ty2) const;
>>>>>> +
>>>>>>
>>>>>> Please add a comment for this method. Include the meaning of the
>>>>>> returned value. ("man strcmp" for inspiration.)
>>>>>>
>>>>>> + static int cmpNumbers(uint64_t L, uint64_t R);
>>>>>>
>>>>>> Optional: is there any benefit to making this static instead of a
>>>>>> const
>>>>>> method? It doesn't need access to the 'this' pointer, but it seems
>>>>>> like
>>>>>> that's an incidental artifact of the implementation. The other cmp*
>>>>>> members are const methods.
>>>>>>
>>>>>> + return cmpNumbers(FTy1->isVarArg(), FTy2->isVarArg());;
>>>>>>
>>>>>> Extra semi-colon.
>>>>>>
>>>>>> I trust you to apply the fixes above, you can choose to commit the
>>>>>> patch
>>>>>> without going through another round of review with me.
>>>>>>
>>>>>> 0002:
>>>>>>
>>>>>> +int FunctionComparator::checkForLosslessBitcast(const Type *L, const
>>>>>> Type *R) {
>>>>>> ...
>>>>>> + int TypesRes = cmpNumbers((uint64_t) L, (uint64_t) R);
>>>>>> ...
>>>>>> + return TypesRes;
>>>>>>
>>>>>> Why isn't this using cmpType? Alternatively, any reason not to have
>>>>>> cmpPointers(const void *L, const void *R)?
>>>>>>
>>>>>> + }
>>>>>> + else {
>>>>>>
>>>>>> Please put the else on the same line as the closing brace.
>>>>>>
>>>>>> + else if (const VectorType *thisPTy = dyn_cast<VectorType>(L))
>>>>>>
>>>>>> Missing initial capital, see
>>>>>> http://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> . Also, I find it odd that you switch from "L" and "R" to "this" and
>>>>>> "that". Please make those consistent, probably on left and right.
>>>>>>
>>>>>> 0003:
>>>>>>
>>>>>> + int cmpConstants(const Constant *L, const Constant *R);
>>>>>>
>>>>>> Any reason this method isn't const?
>>>>>>
>>>>>> +int FunctionComparator::cmpConstants(const Constant *L, const
>>>>>> Constant
>>>>>> *R) {
>>>>>> +
>>>>>> + // Pack null value as one more Value ID
>>>>>> + unsigned LType = L->isNullValue() ? 0 : L->getValueID() + 1;
>>>>>> + unsigned RType = R->isNullValue() ? 0 : R->getValueID() + 1;
>>>>>> +
>>>>>> + if (int Res = cmpNumbers(LType, RType))
>>>>>> + return Res;
>>>>>> +
>>>>>> + if (LType == 0)
>>>>>> + return 0;
>>>>>> +
>>>>>> + switch (LType-1) {
>>>>>>
>>>>>> Optional: how about:
>>>>>>
>>>>>> if (L->isNullValue() && R->isNullValue())
>>>>>> return cmpType(L->getType(), R->getType());
>>>>>> if (L->isNullValue() && !R->isNullValue())
>>>>>> return 1;
>>>>>> if (!L->isNullValue() && R->isNullVaue())
>>>>>> return -1;
>>>>>>
>>>>>> if (int Res = cmpNumbers(L->getValueID(), R->getValueID()))
>>>>>> return Res;
>>>>>>
>>>>>> switch (L->getValueID()) {
>>>>>>
>>>>>> ? In particular I'm not a fan of how LType/RType are equal to
>>>>>> valueID+1,
>>>>>> even with the comment.
>>>>>>
>>>>>> + case Value::FunctionVal:
>>>>>> + case Value::GlobalVariableVal:
>>>>>> + case Value::GlobalAliasVal:
>>>>>> + default: // Unknown constant
>>>>>> + return cmpNumbers((uint64_t)L, (uint64_t)R);
>>>>>>
>>>>>> Please assert on unknown constant.
>>>>>>
>>>>>> How are function, global variable and alias reachable here?
>>>>>>
>>>>>> 0004:
>>>>>>
>>>>>> Thanks for the comment on sn_mapL/R!
>>>>>>
>>>>>> +int FunctionComparator::cmpEnumerate(const Value *V1, const Value
>>>>>> *V2) {
>>>>>>
>>>>>> "Compare enumerate"? This name doesn't make sense to me. "cmpValue"
>>>>>> perhaps?
>>>>>>
>>>>>> + const Constant *C1 = dyn_cast<Constant>(V1);
>>>>>> + const Constant *C2 = dyn_cast<Constant>(V2);
>>>>>> + if (C1 && C2) {
>>>>>> + if (V1 == V2) return 0;
>>>>>> // TODO: constant expressions with GEP or references to F1 or F2.
>>>>>> - if (C1->isNullValue() && C2->isNullValue() &&
>>>>>> - isEquivalentType(C1->getType(), C2->getType()))
>>>>>> - return true;
>>>>>> - // Try bitcasting C2 to C1's type. If the bitcast is legal and
>>>>>> returns C1
>>>>>> - // then they must have equal bit patterns.
>>>>>> - return checkForLosslessBitcast(C1->getType(), C2->getType()) ==
>>>>>> 0 &&
>>>>>> - cmpConstants(C1, C2) == 0;
>>>>>> + if (C1->isNullValue() && C2->isNullValue()) {
>>>>>> + if (int Res = cmpType(C1->getType(), C2->getType()))
>>>>>> + return Res;
>>>>>> + }
>>>>>> +
>>>>>> + // Check whether C2 and C1 has equal bit patterns.
>>>>>> + if (int Res = checkForLosslessBitcast(C1->getType(),
>>>>>> C2->getType()))
>>>>>> + return Res;
>>>>>> +
>>>>>> + // Compare C1 and the bitcast result.
>>>>>> + if (int Res = cmpConstants(C1, C2))
>>>>>> + return Res;
>>>>>> +
>>>>>> + return 0;
>>>>>> }
>>>>>> + if (C1)
>>>>>> + return -1;
>>>>>> + if (C2)
>>>>>> + return 1;
>>>>>>
>>>>>> I'm confused why this isn't just using cmpConstants? I think the
>>>>>> isNullValue to cmpType check is already in cmpConstants, why not move
>>>>>> the checkForLosslessBitcast there? Is cmpConstants used from
>>>>>> elsewhere
>>>>>> and needs to not have that check?
>>>>>>
>>>>>> + std::pair<DenseMap<const Value *, int>::iterator, bool>
>>>>>> + LeftSN = sn_mapL.insert(std::make_pair(V1, sn_mapL.size())),
>>>>>> + RightSN = sn_mapR.insert(std::make_pair(V2, sn_mapR.size()));
>>>>>>
>>>>>> "auto"?
>>>>>>
>>>>>> 0005:
>>>>>>
>>>>>> Okay, this makes attributes ordered.
>>>>>>
>>>>>> + enum AttrType {
>>>>>> + Enum,
>>>>>> + Align,
>>>>>> + Other
>>>>>> + } LeftAttrType = Other, RightAttrType = Other;
>>>>>> +
>>>>>> + if (LA.isAlignAttribute()) LeftAttrType = Align;
>>>>>> + else if (LA.isEnumAttribute()) LeftAttrType = Enum;
>>>>>> + if (RA.isAlignAttribute()) RightAttrType = Align;
>>>>>> + else if (RA.isEnumAttribute()) RightAttrType = Enum;
>>>>>> +
>>>>>> + if (int Res = cmpNumbers(LeftAttrType, RightAttrType))
>>>>>> + return Res;
>>>>>> +
>>>>>> + switch (LeftAttrType) {
>>>>>> + case Enum:
>>>>>> + if (int Res = cmpNumbers(LA.getKindAsEnum(),
>>>>>> RA.getKindAsEnum()))
>>>>>> + return Res;
>>>>>> + break;
>>>>>> + case Align:
>>>>>> + if (int Res = cmpNumbers(LA.getValueAsInt(),
>>>>>> RA.getValueAsInt()))
>>>>>> + return Res;
>>>>>> + break;
>>>>>> + case Other:
>>>>>> + if (int Res = cmpStrings(LA.getKindAsString(),
>>>>>> RA.getKindAsString()))
>>>>>> + return Res;
>>>>>> + if (int Res = cmpStrings(LA.getValueAsString(),
>>>>>> RA.getValueAsString()))
>>>>>> + return Res;
>>>>>> + break;
>>>>>> + }
>>>>>>
>>>>>> Attributes already have operator< and operator==. Please reuse them.
>>>>>>
>>>>>> 0006:
>>>>>>
>>>>>> This looks fine.
>>>>>>
>>>>>> 0007:
>>>>>>
>>>>>> + int cmpGEP(const GEPOperator *GEP1, const GEPOperator *GEP2);
>>>>>> + int cmpGEP(const GetElementPtrInst *GEP1,
>>>>>> + const GetElementPtrInst *GEP2) {
>>>>>>
>>>>>> Const members?
>>>>>>
>>>>>> + unsigned BitWidth = DL ? DL->getPointerSizeInBits(AS1) : 1;
>>>>>> + APInt Offset1(BitWidth, 0), Offset2(BitWidth, 0);
>>>>>> + if (DL &&
>>>>>> + (GEP1->accumulateConstantOffset(*DL, Offset1) &&
>>>>>> + GEP2->accumulateConstantOffset(*DL, Offset2)))
>>>>>> + return cmpAPInt(Offset1, Offset2);
>>>>>>
>>>>>> I think you should put this under if (DL) { /* declare Offset1,
>>>>>> Offset2,
>>>>>> etc. */ }
>>>>>>
>>>>>> 0008:
>>>>>>
>>>>>> + // TODO: Already checked in cmpOps
>>>>>>
>>>>>> Did you mean "cmpOperation"?
>>>>>>
>>>>>> - if (!enumerate(F1BB, F2BB) || !compare(F1BB, F2BB))
>>>>>> + if (!enumerate(F1BB, F2BB) || compare(F1BB, F2BB) != 0)
>>>>>>
>>>>>> "compare" still returns "bool". I'm going to assume this was meant
>>>>>> to go
>>>>>> in 0009.
>>>>>>
>>>>>> 0009:
>>>>>>
>>>>>> Looks fine.
>>>>>>
>>>>>> 0010: not reviewing this now.
>>>>>>
>>>>>> 0011:
>>>>>>
>>>>>> Yay!
>>>>>>
>>>>>> 0012:
>>>>>>
>>>>>> + typedef std::set<FunctionPtr> FnTreeType;
>>>>>>
>>>>>> std::set is frowned upon, see
>>>>>> http://llvm.org/docs/ProgrammersManual.html#set . Do you actually
>>>>>> rely
>>>>>> on the stable iterator guarantee? Or would another set-like
>>>>>> container be
>>>>>> a better fit?
>>>>>>
>>>>>> + FunctionPtr(Function *_F, const DataLayout *_DL) : F(_F),
>>>>>> DL(_DL) {}
>>>>>>
>>>>>> No. An identifier with underscore then capital letter is reserved for
>>>>>> the implementation. You can just call them "F" and "DL", then the
>>>>>> ctor
>>>>>> initializer list can be "F(F), DL(DL)" and that will work fine.
>>>>>>
>>>>>> 0013:
>>>>>>
>>>>>> +// a ² a (reflexivity)
>>>>>>
>>>>>> I'm seeing a "squared" between the two 'a's. Could you represent
>>>>>> this in
>>>>>> plain ASCII?
>>>>>>
>>>>>> +// Comparison iterates through each instruction in each basic block.
>>>>>>
>>>>>> Two spaces before "iterates" should be one.
>>>>>>
>>>>>> +// While ability to deal with complex references could be really
>>>>>> perspective.
>>>>>>
>>>>>> Sorry, I'm not sure what this sentence means? The thing your
>>>>>> example is
>>>>>> talking about is showing is that two functions in an SCC may be
>>>>>> equivalent, and detecting them requires "blinding" (ignoring) certain
>>>>>> details of the function when you do the comparison. We blind
>>>>>> ourselves
>>>>>> to the pointee types, but not to callees of functions in the same
>>>>>> SCC.
>>>>>>
>>>>>> 0014:
>>>>>>
>>>>>> Yay again!
>>>>>>
>>>>>>
>>>>>> After reading through this patch series, I feel like I'm missing
>>>>>> something important. Where's the sort function? It looks like we're
>>>>>> still comparing all functions to all other functions.
>>>>>>
>>>>>> Nick
>>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>>
>>
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>

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-------------- next part --------------
Full comment for sn_mapL and sn_mapR fields.

Assign serial numbers to values from left function, and values from
right function.
Explanation:
Being comparing functions we need to compare values we meet at left and
right sides.
Its easy to sort things out for external values. It just should be
the same value at left and right.
But for local values (those were introduced inside function body)
we have to ensure they were introduced at exactly the same place,
and plays the same role. How to do it?
Just assign serial number to each value when we meet it first time.
Values that were met at same place will be with same serial numbers.

Several notes about values assigned to BBs and other
ways of implementation.

What will happen if we reorder the BasicBlock in one of functions.
Will we still get the same numbering?

As follows from FunctionComparator::compare(), we do CFG walk, we start
from entry, and then take each terminator. So it doesn't matter how in
fact BBs are ordered in function. And since cmpValues are called during
this walk, the numbering depends only on how BBs located inside the CFG.
So the answer is - yes. We will get the same numbering.

Why not to use dominance properties of values? We could just go from
operands to their origins and check whether they are defined at the same
place.

If we compare two instruction operands: first is usage of local
variable AL from function FL, and second is usage of local variable AR
from FR, we could compare their origins.
But, we are still not able to compare operands of PHI nodes, since those
could be operands from further BBs we didn't scan yet.
-------------- next part --------------
Full comment for cmpValues method declaration.

Assign or look up previously assigned numbers for the two values, and
return whether the numbers are equal. Numbers are assigned in the order
visited.
Comparison order:
Stage 0: Value that is function itself is always greater then others.
         If left and right values are references to their functions, then
         they are equal.
Stage 1: Constants are greater than non-constants.
         If both left and right are constants, then the result of
         cmpConstants is used as cmpValues result.
Stage 2: InlineAsm instances are greater than others. If both left and
         right are InlineAsm instances, InlineAsm* pointers casted to
         integers and compared as numbers.
Stage 3: For all other cases we compare order we meet these values in
         their functions. If right value was meet first during scanning,
         then left value is greater.
         In another words, we compare serial numbers, see comments for
         sn_mapL and sn_mapR.
-------------- next part --------------
Full comment for cmpConstants method declaration.

Constants comparison.
Its analog to lexicographical comparison between hypothetical numbers
of next format:
<bitcastability-trait><raw-bit-contents>

1. Bitcastability.
Check whether L's type could be losslessly bitcasted to R's type.
On this stage method, in case when lossless bitcast is not possible
method returns -1 or 1, thus also defining which type is greater in
context of bitcastability.
Stage 0: If types are equal in terms of cmpTypes, then we can go straight
         to the contents comparison.
         If types differ, remember types comparison result and check
         whether we still can bitcast types.
Stage 1: Type that satisfies isFirstClassType conditions are always
         greater then others.
Stage 2: Vector is greater then non-vector.
         If both types are vectors, then vector with greater bitwidth is
         greater.
         If both types are vectors with the same bitwidth, then types
         are bitcastable, and we can skip other stages, and go to contents
         comparison.
Stage 3: Pointer types are greater than non-pointers. If both types are
         pointers of same address space - go to contents comparison.
         Different address spaces: pointer with greater address space is
         greater.
Stage 4: Types are neither vectors, nor pointers. And they differ.
         We don't know how to bitcast them. So, we better don't do it,
         and return types comparison result (so it determines the
         relationship among constants we don't know how to bitcast).

Just for clearance, let's see how the set of constants could look
on single dimension axis:

[NFCT], [FCT, "others"], [FCT, pointers], [FCT, vectors]
Where: NFCT - Not a FirstClassType
       FCT - FirstClassTyp:

2. Compare raw contents.
It ignores types on this stage and only compares bits from L and R.
Returns 0, if L and R has equivalent contents.
-1 or 1 if values are different.
Pretty trivial:
2.1. If contents are numbers, compare numbers.
   Ints with greater bitwidth are greater. Ints with same bitwidths
   compared by their contents.
2.2. "And so on". Just to avoid discrepancies with comments perhaps
it would be better to read the implementation itself.
3. And again about overall picture. Let's look back at how the ordered set
of constants will look like:
[NFCT], [FCT, "others"], [FCT, pointers], [FCT, vectors]

Now look, what could be inside [FCT, "others"], for example:
[FCT, "others"] =
[
  [double 0.1], [double 1.23],
  [i32 1], [i32 2],
  { double 1.0 },       ; StructTyID, NumElements = 1
  { i32 1 },            ; StructTyID, NumElements = 1
  { double 1, i32 1 },  ; StructTyID, NumElements = 2
  { i32 1, double 1 }   ; StructTyID, NumElements = 2
]

Let's explain the order. Float numbers will be less than integers, just
because of cmpTypes terms: FloatTyID < IntegerTyID.
Floats (with same fltSemantics) are sorted according to their value.
Then you can see integers, and they are, like a floats,
could be easy sorted among each others.
The structures. Structures are grouped at the tail, just because of their
TypeID: StructTyID > IntegerTyID > FloatTyID.
Structures with greater number of elements are greater. Structures with
greater elements going first are greater.
The same logic with vectors, arrays and other possible complex types.

Let's consider another question: what about bitcastable constants?
Could some constants, just because of its bitcastability, join to some
group of "so-called-equal" values with different types, and at the same
time live in another group of constants with equal types and "really"
equal values.

The answer is - no:

If constant A with type TyA is bitcastable to B with type TyB, then:
1. All constants with equal types to TyA, are bitcastable to B. Since
   those should be vectors (if TyA is vector), pointers
   (if TyA is pointer), or else (if TyA equal to TyB), those types should
   be equal to TyB as well.
2. All constants with non-equal, but bitcastable types to TyA, are
   bitcastable to B.
   Once again, just because we allow it to vectors and pointers only.
   This statement could be expanded as below:
2.1. All vectors with equal bitwidth to vector A, has equal bitwidth to
     vector B, and thus bitcastable to B as well.
2.2. All pointers of the same address space, no matter what they point to,
     bitcastable. So if C is pointer, it could be bitcasted to A, and to B.
In another words, for pointers and vectors, we ignore top-level type and
look at their particular properties (bit-width for vectors, and
address space for pointers).
If these properties are equal - compare their contents.