Thursday , April 15 2021

Gccemacs: Experiment with native compiled Emacs Lisp, Hacker News


gccemacsis a modified Emacs capable of compiling and running Emacs Lisp as native code in form of re-loadable elf files. As the name suggests this is achieved blending together Emacs and the gcc infrastructure.

The scope of this project is to experiment and discuss the proposed compiler design on theemacs-develmailing list.

I’ve no current plan to have this as a long standing maintained out of tree Emacs fork.

Main tech goals forgccemacsare:

  • show that elisp run-time can be considerably improved by the proposed compiler approach.
  • test for a viable and non disruptive way to have a more capable lisp implementation.

Having elisp self-hosted would enable not only a faster environment but also an easier Emacs to be modified requiring less C code to be written.

The actual code can be found here:

1.1Not a jitter

gccemacsis not a jitter. The compilation output (. Elnfiles) are elf files suitable for being reloaded between different sessions.

I believe this solution has advantages over the jitter approach not having the duty to recompile at every start-up the same code.

This is a key advantage especially for an application sensitive to the start-up time as Emacs is. Moreover more time can be spent for advanced compile time optimizations.

Some complication to have the system re-loadable exists but is worth paying.

A further advantage of this solution is the potential for reducing load-time. Optimizing for load time should be further investigated.


Plugging into the gcc infrastructure is done throughlibgccjit.

Despite what the name suggestlibgccjitis usable not just for making jitters but for any other kind of compiler.

libgccjitcan be seen as a way to drive the gcc in form of shared library using function calls. In other words this is probably the ‘cheapest’ way to write what is technically named a front-end in compiler jargon.



libgccjitwas added with gcc 5.gccemacsshould not be using any recently added entry point therefore in theory any libgccjitshould be fine.BUTgcc 9 was affected by PR 91928 .cgi? id=91928. I fixed it for the trunk and did the back port into the 9 branch so as of revision 276625 also gcc 9 should be functional again. Consider this if you want to try thelibgccjitshipped by your distribution. gcc 8 and earlier should not be affected but I had no time to do any serious test with them.

I suggest a recent gcc 9 if you are going to compile. Please report any success or failure of usinggccemacswith different versions of gcc otherwise.

libgccjitcompilation instructions and doc is here: jit / index.html


Oncelibgccjitis installed a normalautogen.shconfiguremakeshould do the job forgccemacs.

Once compiled, the native compiler tests can be run with:

~ / emacs / src / emacs -batch -l ert -l ~ / emacs / test / src / comp-tests.el -f ert-run-tests-batch-and-exit

(adjust path accordingly).

1.3.3Entry points

gccemacsadds two main entry points for compilation:

  • native-compile
  • native-compile-async

Some special variables influence compilation most notablycomp-speed. This can range from 0 to 3 as in Common Lisp style.

See the doc for details.

Once a compilation unit is compiled the output is an. Elnfile. This similarly to an. Elcand can be loaded byrequireor manually.

A lisp file can be compiled as follows:


Once the compilation completes, the producedfile.elncan be loaded conventionally.

Please note thatfile.elhas to be lexically bound in order to be compiled.

A native compiled functionfooshould then present on adescribe-functionsomething like:

foo is a built-in function in ‘C source code’.

A more common usage is to compile more files or entire folders. The following example will compile asynchronously all the installed packages (the number 4 is equivalent to- j4for make).

(native-compile-async"~ / .emacs.d / elpa /"4 t)

Progress can be monitored into the* Async-native-compile-log *buffer.

1.4A foreword about why optimizing outside gcc

While I’m writing I recall few (I think good) reasons why I decided to replicate some optimizing compiler algorithmics outside GCC:

  • The gcc infrastructure has no knowledge that a call to a primitive (sayFcons) will return an object with certain tag bits set (in this case say is a cons). The only feasible way to inform it would be to use LTO but I’ve some doubts this would be practical for this application.
  • I decided to layout the function frame as an array of lisp objects. This not to have to move and store these whenever a call by ref is emitted. To avoid having the compiler clobber all the function frame every time one of these function calls is emitted, I decided to lift the objects that are not involved by this kind of calls in the equivalent of conventional automatic local variables. To do this I need to propagate the ref property within the control flow graph.
  • Gcc infrastructure has no knowledge of what lisp pure functions are therefore this is a barrier for its propagation.
  • Gcc does not provide help for boxing and unboxing values.
  • The propagation engine can be used for giving warnings and errors at compile time.


gccemacscompilation process can be divided into 3 main phases:

  • byte-compilation

    This is the conventional Emacs byte compilation phase, is needed because the output of the byte compiler is used as input for the following phase.

  • middle-end

    Here the code goes through different transformations. This is implemented intolisp / emacs-lisp / comp.el

  • back-end

    This defines inliner and helper functions and driveslibgccjitfor the final code generation. It is implemented insrc / comp.c.


Thegccemacsnative compiler is divided in the following passes:

  • spill-lap:

    The input used for compiling is the internal representation created by the byte compiler (LAP). This is used to get the byte-code before being assembled. This pass is responsible for running the byte compiler end extracting the LAP IR.

  • Limplify:

    The main internal representation used bygccemacsis called LIMPLE (as a tribute to the gcc GIMPLE). This pass is responsible for converting LAP output into LIMPLE. At the base of LIMPLE is the structcomp-mvarrepresenting a meta-variable.

    (cl- defstruct(comp-mvar(: constructormake - comp-mvar))      "A meta-variable being a slot in the meta-stack."     (slot nil: typefixnum            : documentation"Slot number.-1 is a special value and indicates the scratch slot. ")      (id nil: type(ornull number)          : documentation"SSA number when in SSA form.")      (const-vld nil: typeboolean                 : documentation"Valid signal for the following slot.")      (constant nil                : documentation"When const-vld non nil this is used for holdinga value known at compile time. ")      (type nil            : documentation"When non nil indicates the type when known at compiletime. ")      (ref nil: typeboolean           : documentation"When t the m-var is involved in a call where is passed byreference. "))

    As an example the following LIMPLE insn assigns to the frame slot number 9 the symbola(found as third element into the relocation array).

    (setimm #s (comp-mvar 9 1 t 3 nil) 3 a)

    As other example, this is moving the 5th frame slot content into the 2nd frame slot.

    (set #s (comp-mvar 2 6 nil nil nil nil) #s (comp-mvar 5 4 nil nil nil nil))

    At this stage the function gets decomposed into basic blocks being each of these a list of insns.

    In this phase the original push & pop within the byte-vm stack is translated into a sequence of assignments within the frame slots of the function. Every slot into the frame is represented by acomp-mvar. All this spurious moves will be eventually optimized. It is important to highlight that doing this we are moving all the push and pop machinery from the run-time into the compile time.

  • SSA

    This is responsible for porting LIMPLE into SSA form. After this pass everym-vargets a unique id and is assigned only once.

    Contextually also phi functions are placed.

  • Propagate

    This pass iteratively propagates within the control flow graph for each m-var the following properties: value, type and if the m-var will be used for a function call by reference.

    Propagate removes also function calls to pure functions when all the arguments are or become known during propagation.

    NOTE: this is done also by the byte optimizer but propagate has greater chances to succeeds due to the CFG analysis itself.

  • call-optim

    This is responsible for (depending on the optimization level) trying to emit call that do not go through the funcall trampoline.

    Note that after this all calls to subrs can have equal dignity despite the fact that they originally got a dedicate byte-op-code ore not.

    Please note also that at speed 3 the compiler is optimizing also calls within the same compilation unit. This saves from the trampoline cost and allow gcc for further in-lining and propagation but makes this functions non safely re-definable unless the whole compilation unit is recompiled.

  • dead-code

    This tries to remove unnecessary assignments.

    Spurious assignment to non floating frame slots cannot be optimized out by gcc in case the frame is clobbered by function calls with the frame used by reference so is good to remove them.

  • final

    This drives LIMPLE intolibgccjitIR and invokes the compilation. Final is also responsible for:

    • Defining the inline functions that gives gcc visibility on the lisp implementation.
    • Suggesting to them the correct type if available while emitting the function call.
    • Lifting variables from the frame array if these are to be located into the floating frame.

1.6Compiler hints

Having realized it was quite easy to feed the propagation engine with additional information I’ve implemented two entry points.

These are:

  • comp-hint-fixnum
  • comp-hint-cons

As an example this is to promise that the result of the following expression evaluates to a cons:

Propagation engine will use this information to propagate for all the m-vars influenced by the evaluation result of(comp-hint-cons    x)in the control flow graph.

Atcomp-speedsmaller then 3 the compiler hints will translate into assertions verifying the property declared.

These low level primitives could be used to implement something like the CLthe.

1.7Type checks removal

This section applies to the (few) functions exposed to the gcc, namely:car,cdr,setcar,setcdr,1 ,1 -,-(Fnegate).

Atcomp-speed3 type checks will not be emitted if the propagation has proved the type to be the expected one for the called function.

1.8Debugging the compiler

The native compiler can be debugged in several ways but I think the most propaedeutic one is to setcomp-debugto 1. In this way the compiler will depose a.cfile aside the. eln.

This is not a true compilable C file but something sufficiently close to understand what’s going on. Also debug symbols are emitted into the. elntherefore is possible to trap or step into it using gdb.

Note thatcomp-debugset to 1 has a negative impact on compile time.

1.9Quick performance discussion

In order to have something to optimize for and measure we have put together a small collection of elisp (nano and non) benchmarks.

The choice of the benchmarks and their weight is certainly very arbitrary but still… better than nothing.

All benchmarks can be considered u-benchmarks exceptnbodyand dhrystone. Worth nothing that these last two has not been tuned specifically for the native compiler.

On the machine I’m running (i7 – 6600 U) the latest performance figure I’ve got is the following:


name byte-code native-bench native-all native-all vs.
(sec) (sec) (sec) byte-code
30. 44 6. 54 6. 29 4.8x
bubble-no-cons 42. 42 10. 22 10. 20 4.2x
FIBN 39. 36 96 98 2.3x
fibn-rec 20. 64 8. 06 7. 99 2.6x
fibn-tc 19. 6. 33 6. 89 2.8x
inclist-tc 20. 2. 16 2. 18 9.4x
Listlen-TC 17 . 66 0. 66 0. 70 25 .2x
inclist-no-type-hints 45. 57 5. 60 5. 81 7.8x
inclist-type-hints 45. 89 3. 67 3. 70 12. 4x
nbody 112. 99 23. 65 24. 44 4.6x
DHrystone 112. 40 64. 67 47. 12 2.4x
tot 507. 23 148. 52 132 .3 3.8x

Row explanation:

  • byte-codebenchmark running byte compiled (baseline).
  • native-benchbenchmark running native compiled on a non native compiled emacs.
  • native-allbenchmark running with the benchmark itself and all emacs lisp native compiled.

All benchmarks has been compiled withcomp-speed3 while the Emacs lisp for thenative-allrow withcomp-speed2.

Here follows averypreliminary and incomplete performance discussion:

bubblebubble-no-consfibnu-benchmarks shows fair results with no specific tuning.

Recursive benchmarks (suffixed with- recor- tc) shows an enormous uplifts most likely dominated by much faster calling convention. Note that no TCO here is performed.

inclist-no-type-hintsandinclist-type-hintsshows a significant performance uplift just due to type checks being optimized out thanks to the information coming from the type hints.

nbodyis ~ 4.6x faster with no lisp specific optimization triggered. This benchmark calls only primitives (as all previous u-benchs) therefore having all Emacs native compiled does not give any advantage. Probably intra compilation unit call optimization plays a role into the result.

On the other sidedhrystonebenefits quite a lot from the full Emacs compilation having to funcall intostore-substringandstring>.

Hope this indicates a potential for considerable performance benefits.

1. 10Status and verification.

A number of tests is defined incomp-tests.el. This is including a number of micro tests (some of these inherited by Tom Tromey’s jitter) plus a classical bootstrap test were the compiler compiles itself two times and the binary is compared.

A couple of colleagues and me are running in productiongccemacswith all the Emacs lexically scoped lisp compiled plus all the elpa folder for each of our setups.

While I’m writing this I’ve loaded on this instance 18682 native compiled elisp functions loaded from 669.elnfiles.

So far this seems stable.

1. 11Current limitations

The following limitations are not due to design limitations but just the state of a “work in progress”:

  • Only lexically scoped code is compiled.
  • Interactive functions are included into eln files but as byte-compiled.
  • No doc string support for native compiled functions.
  • Limited lisp implementation exposure: Just few function are defined and exposed to gcc:car,cdr,setcar,setcdr,1 ,1 -,-(Fnegate). This functions can be fully in-lined and optimized.
  • Type propagation:

    Very few functions have a return type declared to the compiler (seecomp-known-ret-types). Moreover type propagation can’t relay on the CL equivalent ofsub-type-p.

  • Garbage collection support has to be done:

    Once a. Elnfile is loaded it is never unloaded even if the subrs there defined are redefined.

  • No integration with the build system:

    No native compilation is automatically triggered by the build process.

  • Native compiler is not re-entrant:

    Just top level functions are native compiled, the others (lambda included) are still kept as byte-code.

1. 12Further compiler improvements

  • Better lisp exposure

    As mentioned in the previous paragraph the lisp machinery exposed to the gcc infrastructure is at this point quite limited and should be improved.

  • Unboxing

    Introducing the unboxing mechanism was considered out of scope for this investigation but is certainly possible to extend the current infrastructure to have it. Unboxing would have a big impact on performance. Combined with compiler hints should be possible to generate efficient code closing some more gap with C (in some cases probably all).

13Long term

  • Further improving load time performance using the portable dumper for object de / serialization within elns (currently the print read mechanism is in use).
  • Provide better compile time warning and errors relying on the propagation engine?
  • The real final goal would be to be able to assemble the Emacs image having the native compiled functions replacing all the byte-compiled one out of the box.

1. 14Final word

All of this is very experimental. Take it (at your risk) as what it is, a proof of concept.

Feedback is very welcome.

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