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

[Duncan P. N. Exon Smith]

> 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.

Any suggestions?  Maybe, `emitDwarfLabelDifference()`?