[PATCH] [opaque pointer type] bitcode support for explicit type parameter to the load instruction
David Blaikie
dblaikie at gmail.com
Tue Feb 17 22:30:14 PST 2015
On Sun, Feb 15, 2015 at 4:30 PM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:
>
> > On 2015 Feb 15, at 16:17, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >
> >
> > On Sun, Feb 15, 2015 at 3:48 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> >
> > > On 2015 Feb 15, at 10:29, David Blaikie <dblaikie at gmail.com> wrote:
> > >
> > >
> > >
> > > On Sun, Feb 15, 2015 at 10:22 AM, David Blaikie <dblaikie at gmail.com>
> wrote:
> > > Hi rafael, resistor, grosbach, chandlerc,
> > >
> > > I've taken my best guess at this, but I've cargo culted in places & so
> > > explanations/corrections would be great.
> > >
> > > This seems to pass all the tests (check-all, covering clang and llvm)
> so I
> > > believe that pretty well exercises both the backwards compatibility
> and common
> > > (same version) compatibility given the number of checked in bitcode
> files we
> > > already have. Is that a reasonable approach to testing here? Would
> some more
> > > explicit tests be desired?
> >
> > Have a look at the tests in test/Bitcode. Typically we create some
> > IR that would use the old records in some known version (typically the
> > previous release; you should probably be using 3.6?), generate bitcode
> > and check it in. E.g., I wrote test/Bitcode/metadata.3.5.ll to test
> > that bitcode from before the metadata-value split was understood going
> > forward.
> >
> > Worth adding if it's already covered by existing tests? (searching for
> load instructions in test/Bitcode there are maybe 10 files with loads, and
> memInstructions.3.2.ll which seems to have lots of them)
> >
>
> Good point; there is coverage in bitcode. Might be nice to add a test
> anyway, but I'm not really sure whether it's worth it.
>
In the interests of not adding extra burden to the test suite, I think the
existing coverage (demonstrable by removing the extra size condition cases
in the patch) seems sufficient to me - so I think the patch stands as-is
from my perspective, awaiting further review/feedback. (just mentioning
this in case anyone assumed they were waiting for me to update with test
cases)
>
> >
> > > 1) is this the right way to do back-compat in this case (looking at
> the number
> > > of entries in the bitcode record to disambiguate between the old
> schema and
> > > the new?)
> > >
> > > As an aside, this approach doesn't seem to work for GEP since there's
> less structure to its record size (it can contain any number of entries in
> the record). Any ideas on the right approach there? An explicit
> abbreviation (I guess we just version them?) or tag in some way? I'm still
> pretty hazy on the details.
> > >
> >
> > Checking the number of entries seems to be usual; that's how I've been
> > doing it. Another option is to take an integer field that uses fewer
> > than 64-bits and add a bit to indicate before/after the change.
> >
> > For `GEP`, you can introduce a new record code and add an `_OLD` suffix
> > to the names of the old codes.
> >
> > OK, makes sense I think - will have a play around with it.
> >
> > You only really need one new record
> > code; you can add an initial 1-bit field to indicate whether it's
> > inbounds
> >
> > Hmm - what's the tradeoff between that and the existing design using two
> codes, do you think?
>
> I assume `inbounds` was added later so it *required* another record
> code at the time... I doubt it was an explicit design chocie (although
> I could be wrong?).
>
> Having one record seems cleaner to me, but I haven't thought it through
> carefully.
>
fair enough - thanks for the thoughts - should have that one out for review
soon (~tomorrow) then.
>
> >
> > (then it's available for versioning in future upgrades).
> >
> > Not quite sure I follow here. Are you suggesting the bit size of the
> field can be increased in future versions (I would've thought that would
> break parsing? But I guess the bit size is in the abbreviation, so it's
> transparent to the reader who still sees a 64 bit value?
>
> Right: the reader always sees it as a 64-bit value. If/when there's a
> need/use for more bits we can change the abbreviation.
>
Thanks - makes sense & provides some good options for optimal size now
while providing easy backwards compatibility later.
>
> > ), or that the 1 bit field will have some spare bits we can use later?
> >
> >
> > In terms of the abbreviations, AFAICT they're not really accessible
> > on the reader side (except in the low-level "reading the bits"). The
> > abbreviations just change the way the record is encoded. They need to
> > match what's actually written out by the writer, but don't add any
> > semantics [1].
> >
> > [1]: http://llvm.org/docs/BitCodeFormat.html#abbreviations
> >
> > >
> > > 2) I don't quite understand the logarithm logic to choose the encoding
> type of
> > > the type parameter in the abbreviation description, but I found
> another
> > > instruction doing the same thing & it seems to work. Is that the
> right
> > > approach?
> >
> > It probably makes sense. The logic is just saying that, since we have
> > T types (which have been numbered from 0 to T-1), we need lg T bits to
> > encode them. I guess the assumption is that there are usually few
> > types; otherwise, the usual VBR-6 might be more efficient. I don't
> > know how many types we tend to have, but I imagine T will drop pretty
> > substantially with your changes, so if it's being used elsewhere it
> > seems here as well.
> >
> > >
> > > http://reviews.llvm.org/D7655
> > >
> > > Files:
> > > lib/Bitcode/Reader/BitcodeReader.cpp
> > > lib/Bitcode/Writer/BitcodeWriter.cpp
> > >
> > > Index: lib/Bitcode/Reader/BitcodeReader.cpp
> > > ===================================================================
> > > --- lib/Bitcode/Reader/BitcodeReader.cpp
> > > +++ lib/Bitcode/Reader/BitcodeReader.cpp
> > > @@ -3510,21 +3510,32 @@
> > > unsigned OpNum = 0;
> > > Value *Op;
> > > if (getValueTypePair(Record, OpNum, NextValueNo, Op) ||
> > > - OpNum+2 != Record.size())
> > > + (OpNum + 2 != Record.size() && OpNum + 3 != Record.size()))
> > > return Error("Invalid record");
> > >
> > > + Type *Ty = nullptr;
> > > + if (OpNum + 3 == Record.size())
> > > + Ty = getTypeByID(Record[OpNum++]);
> > > +
> > > I = new LoadInst(Op, "", Record[OpNum+1], (1 << Record[OpNum])
> >> 1);
> > > +
> > > + assert(!Ty || Ty == I->getType());
> > > +
> > > InstructionList.push_back(I);
> > > break;
> > > }
> > > case bitc::FUNC_CODE_INST_LOADATOMIC: {
> > > // LOADATOMIC: [opty, op, align, vol, ordering, synchscope]
> > > unsigned OpNum = 0;
> > > Value *Op;
> > > if (getValueTypePair(Record, OpNum, NextValueNo, Op) ||
> > > - OpNum+4 != Record.size())
> > > + (OpNum + 4 != Record.size() && OpNum + 5 != Record.size()))
> > > return Error("Invalid record");
> > >
> > > + Type *Ty = nullptr;
> > > + if (OpNum + 5 == Record.size())
> > > + Ty = getTypeByID(Record[OpNum++]);
> > > +
> > > AtomicOrdering Ordering = GetDecodedOrdering(Record[OpNum+2]);
> > > if (Ordering == NotAtomic || Ordering == Release ||
> > > Ordering == AcquireRelease)
> > > @@ -3535,6 +3546,9 @@
> > >
> > > I = new LoadInst(Op, "", Record[OpNum+1], (1 << Record[OpNum])
> >> 1,
> > > Ordering, SynchScope);
> > > +
> > > + assert(!Ty || Ty == I->getType());
> > > +
> > > InstructionList.push_back(I);
> > > break;
> > > }
> > > Index: lib/Bitcode/Writer/BitcodeWriter.cpp
> > > ===================================================================
> > > --- lib/Bitcode/Writer/BitcodeWriter.cpp
> > > +++ lib/Bitcode/Writer/BitcodeWriter.cpp
> > > @@ -1878,6 +1878,7 @@
> > > if (!PushValueAndType(I.getOperand(0), InstID, Vals, VE)) //
> ptr
> > > AbbrevToUse = FUNCTION_INST_LOAD_ABBREV;
> > > }
> > > + Vals.push_back(VE.getTypeID(I.getType()));
> > > Vals.push_back(Log2_32(cast<LoadInst>(I).getAlignment())+1);
> > > Vals.push_back(cast<LoadInst>(I).isVolatile());
> > > if (cast<LoadInst>(I).isAtomic()) {
> > > @@ -2232,6 +2233,8 @@
> > > BitCodeAbbrev *Abbv = new BitCodeAbbrev();
> > > Abbv->Add(BitCodeAbbrevOp(bitc::FUNC_CODE_INST_LOAD));
> > > Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // Ptr
> > > + Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, // dest ty
> > > + Log2_32_Ceil(VE.getTypes().size()+1)));
> > > Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4)); // Align
> > > Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // volatile
> > > if (Stream.EmitBlockInfoAbbrev(bitc::FUNCTION_BLOCK_ID,
> > >
> > > EMAIL PREFERENCES
> > > http://reviews.llvm.org/settings/panel/emailpreferences/
> > >
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> > >
> > >
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>
>
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