[llvm] r237876 - AsmPrinter: Compute absolute label difference directly

[David Blaikie]

On Thu, Jun 4, 2015 at 2:11 PM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

>
> > On 2015 Jun 4, at 12:48, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >
> >
> > On Thu, Jun 4, 2015 at 12:22 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> >
> > > On 2015-Jun-04, at 11:48, David Blaikie <dblaikie at gmail.com> wrote:
> > >
> > >
> > >
> > > On Wed, May 20, 2015 at 7:41 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> > > Author: dexonsmith
> > > Date: Wed May 20 21:41:23 2015
> > > New Revision: 237876
> > >
> > > URL: http://llvm.org/viewvc/llvm-project?rev=237876&view=rev
> > > Log:
> > > AsmPrinter: Compute absolute label difference directly
> > >
> > > Create a low-overhead path for `EmitLabelDifference()` that emits a
> > > emits an absolute number when (1) the output is an object stream and
> (2)
> > > the two symbols are in the same data fragment.
> > >
> > > This drops memory usage on Mach-O from 975 MB down to 919 MB (5.8%).
> > > The only call is when `!doesDwarfUseRelocationsAcrossSections()` --
> > > i.e., on Mach-O -- since otherwise an absolute offset from the start of
> > > the section needs a relocation.  (`EmitLabelDifference()` is cheaper on
> > > ELF anyway, since it creates 1 fewer temp symbol, and it gets called
> far
> > > less often.  It's not clear to me if this is even a bottleneck there.)
> > >
> > > (I'm looking at `llc` memory usage on `verify-uselistorder.lto.opt.bc`;
> > > see r236629 for details.)
> > >
> > > Modified:
> > >     llvm/trunk/include/llvm/MC/MCObjectStreamer.h
> > >     llvm/trunk/include/llvm/MC/MCStreamer.h
> > >     llvm/trunk/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
> > >     llvm/trunk/lib/MC/MCObjectStreamer.cpp
> > >
> > > Modified: llvm/trunk/include/llvm/MC/MCObjectStreamer.h
> > > URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/MC/MCObjectStreamer.h?rev=237876&r1=237875&r2=237876&view=diff
> > >
> ==============================================================================
> > > --- llvm/trunk/include/llvm/MC/MCObjectStreamer.h (original)
> > > +++ llvm/trunk/include/llvm/MC/MCObjectStreamer.h Wed May 20 21:41:23
> 2015
> > > @@ -136,6 +136,17 @@ public:
> > >    void EmitZeros(uint64_t NumBytes) override;
> > >    void FinishImpl() override;
> > >
> > > +  /// Emit the absolute difference between two symbols if possible.
> > > +  ///
> > > +  /// Emit the absolute difference between \c Hi and \c Lo, as long
> as we can
> > > +  /// compute it.  Currently, that requires that both symbols are in
> the same
> > > +  /// data fragment.  Otherwise, do nothing and return \c false.
> > > +  ///
> > > +  /// \pre Offset of \c Hi is greater than the offset \c Lo.
> > > +  /// \return true on success.
> > > +  bool emitAbsoluteSymbolDiff(const MCSymbol *Hi, const MCSymbol *Lo,
> > > +                              unsigned Size) override;
> > > +
> > >    bool mayHaveInstructions() const override {
> > >      return getCurrentSectionData()->hasInstructions();
> > >    }
> > >
> > > Modified: llvm/trunk/include/llvm/MC/MCStreamer.h
> > > URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/MC/MCStreamer.h?rev=237876&r1=237875&r2=237876&view=diff
> > >
> ==============================================================================
> > > --- llvm/trunk/include/llvm/MC/MCStreamer.h (original)
> > > +++ llvm/trunk/include/llvm/MC/MCStreamer.h Wed May 20 21:41:23 2015
> > > @@ -652,6 +652,15 @@ public:
> > >                                       unsigned Isa, unsigned
> Discriminator,
> > >                                       StringRef FileName);
> > >
> > > +  /// Emit the absolute difference between two symbols if possible.
> > > +  ///
> > > +  /// \pre Offset of \c Hi is greater than the offset \c Lo.
> > > +  /// \return true on success.
> > > +  virtual bool emitAbsoluteSymbolDiff(const MCSymbol *Hi, const
> MCSymbol *Lo,
> > > +                                      unsigned Size) {
> > > +    return false;
> > > +  }
> > > +
> > >    virtual MCSymbol *getDwarfLineTableSymbol(unsigned CUID);
> > >    virtual void EmitCFISections(bool EH, bool Debug);
> > >    void EmitCFIStartProc(bool IsSimple);
> > >
> > > Modified: llvm/trunk/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
> > > URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/AsmPrinter/AsmPrinter.cpp?rev=237876&r1=237875&r2=237876&view=diff
> > >
> ==============================================================================
> > > --- llvm/trunk/lib/CodeGen/AsmPrinter/AsmPrinter.cpp (original)
> > > +++ llvm/trunk/lib/CodeGen/AsmPrinter/AsmPrinter.cpp Wed May 20
> 21:41:23 2015
> > > @@ -1591,6 +1591,10 @@ void AsmPrinter::EmitInt32(int Value) co
> > >  /// .set if it avoids relocations.
> > >  void AsmPrinter::EmitLabelDifference(const MCSymbol *Hi, const
> MCSymbol *Lo,
> > >                                       unsigned Size) const {
> > >
> > > Is this function only used for DWARF emission? It seems sufficiently
> generally named that that's not obvious & could be surprising for other
> users to find this \/ behavior?
> > >
> >
> > Yes, it's only used for DWARF emission; at least, I looked, and only
> > found DWARF callers.  I'll look into changing the name.  I agree it's
> > confusing.
> >
> > Wonder what we use for label difference in inline asm or other things,
> then... *shrug*
>
> Inline asm doesn't use this, anyway.
>
> However, I just did a more thorough look (I was starting to work on
> the patch) and found a single non-DWARF caller:
> `EHStreamer::emitExceptionTable()`.  I'll fix that pronto; looks
> like I should leave `EmitLabelDifference()` around, create a new
> `emitDwarfLabelDifference()` that eventually calls the former, and
> change all the callers but that one.
>
> >
> >
> > Any suggestions?  Maybe, `emitDwarfLabelDifference()`?
> >
> > Sounds plausible - can we assert that it's in one of the blessed
> sections that MachO doesn't link into the executable, too?
>
> Seems like a good idea, but I don't know how to check that.  Do you?
>

I don't know what logic MachO uses to bless the sections - whether it's
name based or what. If it's just debug_* sections, you should be able to
get the section from the symbol (hmm, only if we've already emitted the
symbol which often/usually isn't the case?) and the name from the section,
etc.
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