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.
Any ideas?
> This patch looks obviously correct to me :/.
>
> > (obviously, once I've done this - I'll then remove the unique_ptr stuff
> here, and just have a list<MatchableInfo>)
> >
> > >
> > > >
> > > > Also - did you happen to verify that your change causes the diffs on
> the AsmMatcher.inc files you mentioned to go away entirely, or were there
> other remaining changes I should hunt for/verify?
> > > >
> > >
> > > There were some other minor diffs that might be worth looking at.
> > >
> > > IIRC, PR21735 describes them.
> > >
> > > > I don't know why this is platform-specific... I can't think of why
> > > > that makes sense.
> > > >
> > > > Yeah, that's still rather confusing.
> > > >
> > >
> >
>
>
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