[llvm] r222935 - Simplify some more ownership using forward_list<T> rather than vector<unique_ptr<T>>
David Blaikie
dblaikie at gmail.com
Mon Feb 9 14:27:19 PST 2015
On Thu, Feb 5, 2015 at 11:50 AM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:
>
> > On 2015-Feb-04, at 22:21, Duncan P. N. Exon Smith <dexonsmith at apple.com>
> wrote:
> >
> >>
> >> On 2015 Feb 2, at 11:38, David Blaikie <dblaikie at gmail.com> wrote:
> >>
> >>
> >>
> >> On Mon, Feb 2, 2015 at 11:28 AM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> >>
> >>> On 2015-Feb-02, at 11:00, David Blaikie <dblaikie at gmail.com> wrote:
> >>>
> >>>
> >>>
> >>> On Mon, Jan 19, 2015 at 1:00 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> >>>
> >>>> On 2014-Dec-22, at 14:28, David Blaikie <dblaikie at gmail.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On Fri, Dec 19, 2014 at 4:07 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> >>>>
> >>>>> On 2014-Dec-19, at 16:02, David Blaikie <dblaikie at gmail.com> wrote:
> >>>>>
> >>>>>
> >>>>>
> >>>>> On Fri, Dec 19, 2014 at 2:22 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> >>>>>
> >>>>>> On 2014-Dec-19, at 14:11, David Blaikie <dblaikie at gmail.com> wrote:
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> On Wed, Dec 3, 2014 at 3:57 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> >>>>>> I looked into test/CodeGen/X86/compact-unwind.ll. llvm-mc was
> giving
> >>>>>> different results. Given that you changed `tablegen`, I frankly
> gave
> >>>>>> up on figuring out exactly why.
> >>>>>>
> >>>>>> I went back and reviewed the patch for changed semantics, and took a
> >>>>>> shot in the dark. The `sort()` at the end is different from the
> >>>>>> old `sort()`, because items have been front-loaded instead of back-
> >>>>>> loaded.
> >>>>>>
> >>>>>> The fix is to sort it backwards and then reverse the list. This
> passes
> >>>>>> `check` for me.
> >>>>>>
> >>>>>> Any preferences on this approach, versus using std::list (paying an
> extra pointer per element) and pushing onto the end instead? (hmm, or maybe
> we can keep an iterator to the tail end of the list and just use that as an
> insert hint while still using std::forward_list... - not sure if that's
> worth it)
> >>>>>
> >>>>> I'm not sure which approach makes sense here. How memory constrained
> >>>>> does `tablegen` get? `std::list<>` seems easiest if the extra
> pointer
> >>>>> doesn't matter.
> >>>>>
> >>>>> Welp... figured out why I wasn't seeing those failures you saw.
> std::forward_list<>::sort in libstdc++ (even in 4.9) is unstable (& in fact
> exactly inverted - which was exactly what was required). So your fix
> actually caused exactly the same failures for me as you'd seen from my
> original patch...
> >>>>>
> >>>>> In any case, that means we can't rely on its behavior in LLVM and
> I'll just use std::list<> instead. Lame.
> >>>>>
> >>>>
> >>>> That's a shame :(.
> >>>>
> >>>> 'tis rather.
> >>>>
> >>>> Now I have another mystery, in case anyone's interested in having a
> guess.
> >>>>
> >>>> This change:
> >>>>
> >>>> $ git diff
> >>>> diff --git utils/TableGen/AsmMatcherEmitter.cpp
> utils/TableGen/AsmMatcherEmitter.cpp
> >>>> index 3663de7..cb1f178 100644
> >>>> --- utils/TableGen/AsmMatcherEmitter.cpp
> >>>> +++ utils/TableGen/AsmMatcherEmitter.cpp
> >>>> @@ -613,7 +613,7 @@ public:
> >>>> std::forward_list<ClassInfo> Classes;
> >>>>
> >>>> /// The information on the matchables to match.
> >>>> - std::vector<std::unique_ptr<MatchableInfo>> Matchables;
> >>>> + std::list<std::unique_ptr<MatchableInfo>> Matchables;
> >>>>
> >>>> /// Info for custom matching operands by user defined methods.
> >>>> std::vector<OperandMatchEntry> OperandMatchInfo;
> >>>> @@ -1674,7 +1674,7 @@ static unsigned getConverterOperandID(const
> std::string &Name,
> >>>>
> >>>>
> >>>> static void emitConvertFuncs(CodeGenTarget &Target, StringRef
> ClassName,
> >>>> -
> std::vector<std::unique_ptr<MatchableInfo>> &Infos,
> >>>> +
> std::list<std::unique_ptr<MatchableInfo>> &Infos,
> >>>> raw_ostream &OS) {
> >>>> SetVector<std::string> OperandConversionKinds;
> >>>> SetVector<std::string> InstructionConversionKinds;
> >>>> @@ -2593,10 +2593,9 @@ void AsmMatcherEmitter::run(raw_ostream &OS) {
> >>>> // Sort the instruction table using the partial order on classes.
> We use
> >>>> // stable_sort to ensure that ambiguous instructions are still
> >>>> // deterministically ordered.
> >>>> - std::stable_sort(Info.Matchables.begin(), Info.Matchables.end(),
> >>>> - [](const std::unique_ptr<MatchableInfo> &a,
> >>>> - const std::unique_ptr<MatchableInfo> &b){
> >>>> - return *a < *b;});
> >>>> + Info.Matchables.sort(
> >>>> + [](const std::unique_ptr<MatchableInfo> &a,
> >>>> + const std::unique_ptr<MatchableInfo> &b) { return *a < *b;
> });
> >>>>
> >>>> DEBUG_WITH_TYPE("instruction_info", {
> >>>> for (const auto &MI : Info.Matchables)
> >>>>
> >>>>
> >>>> Causes observable differences (no test failures, but non-empty diffs)
> in the .inc files in an LLVM build.
> >>>>
> >>>> Anyone want to guess what's going on here? Maybe some partial
> instability... (unlike std::forward_list::sort's completely inverted
> instability)
> >>>
> >>> I can't think of any other explanation, unless there's UB lurking
> >>> somewhere.
> >>>
> >>> Sort of.
> >>>
> >>> I have a theory with some evidence:
> >>>
> >>> If I put this code:
> >>>
> >>> for (auto I = Info.Matchables.begin(), E = Info.Matchables.end(); I
> != E; ++I) {
> >>> for (auto J = I; J != E; ++J) {
> >>> bool greater = **J < **I;
> >>> assert(!greater);
> >>> }
> >>> }
> >>>
> >>> immediately after the call to std::stable_sort, it does actually fail
> (even in the existing code using std::vector). (wouldn't mind someone
> verifying this, just for a sanity check - that I got the comparisons around
> the right way and that the assertion failure reproduces for them too)
> >>>
> >>> So it seems that the MatchableInfo operator< might not provide a
> strict weak ordering.
> >>>
> >>> Do you/does anyone know an easy way for me to find a specific
> contradiction in the ordering so I can understand/fix it?
> >>>
> >>> The assertion only gives me the elements that, somehow, somewhere
> along the way, std::stable_sort thought were <= but they're actually >. I
> want to find the set of elements (preferably the minimal set of elements)
> that indirectly show they are <= so I can understand which part of the
> comparison is buggy.
> >>>
> >
> > Okay, so back to your previous question: it sounds like you're looking
> > for the shortest cycle in a directed graph (where the nodes are
> > `MatchableInfo*` with edges `L -> R` when `MatchableInfo::Less(L, R)`).
>
> The `findShortestCycle()` code in the attached patch should get you what
> you want (although as you can see I've hardly tested it). Get the
> shortest cycle like this:
>
> std::vector<unsigned> Cycle =
> findShortestCycle(Matchables.size(), [&](unsigned T, unsigned H) {
> return MatchableInfo::Less()(Matchables[T], Matchables[H]);
> });
>
Neat - thanks!
Though, as it turns out, within the first attempt or two I couldn't get an
answer from this (maybe the process died before it found the answer).
But since I knew the result was meant to be sorted, I just kept track of
the shortest subsequence with one of the contradictions (Y < X) and, as it
happens, I found a minimal example: X < X + 1 < X + 2 && X + 2 < X.
I'm trying to understand the comparison further now:
X:
Mnemonic: vshl
AsmOperands.size(): 4
AsmOperands[0].Class: MCK_CondCode
AsmOperands[1].Class: MCK__DOT_u64
X + 1:
Mnemonic: vshl
AsmOperands.size(): 4
AsmOperands[0].Class: MCK_CondCode
AsmOperands[1].Class: MCK__DOT_i64
X + 2:
Mnemonic: vshl
AsmOperands.size(): 4
AsmOperands[0].Class.MCK_CondCode
AsmOperands[1].Class.MCK_s8
(MCK__DOT_u64 < MCK__DOT_i64)
(MCK_s8 < MCK__DOT_u64)
(MCK__DOT_i64 < MCK__DOT_s8)
Next, trying to understand the Class ordering cycle here...
>
>
>
> >
> >>> Any ideas?
> >>
> >> I just had a look at the code. `MatchableInfo::operator<()` depends
> >> eventually on `ClassInfo::operator<()`, which includes this logic:
> >>
> >> // This class precedes the RHS if it is a proper subset of the RHS.
> >> if (isSubsetOf(RHS))
> >> return true;
> >>
> >> But the comment doesn't match the implementation of `isSubsetOf()`,
> which
> >> returns `true` even on equal sets. Is the "equal set" case possible,
> >> despite the pointer comparison (`this == &RHS`) above?
> >>
> >> Was worth a shot, but I added an "assert(!RHS.isSubsetOf(*this))" in
> the if before the return and it didn't fire, so that may not be it.
> >>
> >> I don't think there's an immediate asymmetry (where something returns
> true for both A < B and B < A) but something indirect (A < B, B < C but !(A
> < C) - though it could have multiple intermediates... )
> >>
> >>
> >> I audited the rest of the code (but just quickly) and nothing else
> popped
> >> out.
> >
> >
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150209/abcf8019/attachment.html>
More information about the llvm-commits
mailing list