Saturday, February 29, 2020

Block compilation - "Fresh" in SBCL 2.0.2

I've just managed to land a feature I've been missing in SBCL for quite some time now called block compilation, enabling whole-program-like optimizations for a dynamic language. There was at least one person back in 2003 who requested such a feature on the sbcl-help mailing list, so it's definitely been a long time coming. My suspicion is that although many of you old-timers have at least heard of this feature, this is one of those old state-of-the-art features in dynamic language implementations lost and forgotten for younger generations to the passage of time...

what is "block compilation" anyway?

Those of you using Lisp, or any dynamic language know one thing: Function calls to global, top-level functions are expensive. Much more expensive than in a statically compiled language. They're slow because of the late-bound nature of top-level defined functions, allowing arbitrary redefinition while the program is running and forcing runtime checks on whether the function is being called with the right number or types of arguments. This type of call is known as a "full call" in Python (the compiler used in CMUCL and SBCL, not to be confused with the programming language), and their calling convention permits the most runtime flexibility.

But there is another type of call available to us: the local call. A local call is the type of call you would see between local functions inside a top-level function, say, a call to a function introduced via anonymous LAMBDAs, LABELs or FLETs in Lisp, or internal defines in Scheme and Python. These calls are more 'static' in the sense that they are treated more like function calls in static languages, being compiled "together" and at the same time as the local functions they reference, allowing them to be optimized at compile-time. For example, argument checking can be done at compile time because the number of arguments of the callee is known at compile time, unlike in the full call case where the function, and hence the number of arguments it takes, can change dynamically at runtime at any point. Additionally, the local call calling convention can allow for passing unboxed values like floats around, as they are put into unboxed registers never used in the full call convention, which must use boxed argument  and return value registers.

Block compilation is simply the compilation mode that turns what would normally be full calls to top-level defined functions into local calls to said functions, by compiling all functions in a unit of code (e.g. a file) together in "block" or "batch" fashion, just as local functions are compiled together in a single top-level form. You can think of the effect of block compilation as transforming all the DEFUNs in a file into one large LABELs form. You can think of it as being a tunable knob that increases or decreases how dynamic or static the compiler should act with the respect to function definitions, by controlling whether function name resolution is early-bound or late-bound in a given block compiled unit of code.

We can achieve block compilation with a file-level granularity in CMUCL and SBCL specifically by specifying the :block-compile keyword to compile-file. Here's an example:

In foo.lisp:
(defun foo (x y)
  (print (bar x y))
  (bar x y))

(defun bar (x y)
  (+ x y))

(defun fact (n)
  (if (zerop n)
      (* n (fact (1- n)))))

> (compile-file "foo.lisp" :block-compile t :entry-points nil)
> (load "foo.fasl")

> (sb-disassem:disassemble-code-component #'foo)

; Size: 210 bytes. Origin: #x52E63F90 (segment 1 of 4)        ; (XEP BAR)
; 3F90:       .ENTRY BAR(X Y)                           
                                                              ;  (T T) NUMBER)
; 3FA0:       8F4508           POP QWORD PTR [RBP+8]
; 3FA3:       4883F904         CMP RCX, 4
; 3FA7:       0F85B1000000     JNE L2
; 3FAD:       488D65D0         LEA RSP, [RBP-48]
; 3FB1:       4C8BC2           MOV R8, RDX
; 3FB4:       488BF7           MOV RSI, RDI
; 3FB7:       EB03             JMP L1
; 3FB9: L0:   8F4508           POP QWORD PTR [RBP+8]
; Origin #x52E63FBC (segment 2 of 4)                          ; BAR
; 3FBC: L1:   498B4510         MOV RAX, [R13+16]              ; thread.binding-stack-pointer
; 3FC0:       488945F8         MOV [RBP-8], RAX
; 3FC4:       4C8945D8         MOV [RBP-40], R8
; 3FC8:       488975D0         MOV [RBP-48], RSI
; 3FCC:       498BD0           MOV RDX, R8
3FCF:       488BFE           MOV RDI, RSI
; 3FD2:       E8B9CB29FF       CALL #x52100B90                ; GENERIC-+
; 3FD7:       488B75D0         MOV RSI, [RBP-48]
; 3FDB:       4C8B45D8         MOV R8, [RBP-40]
; 3FDF:       488BE5           MOV RSP, RBP
; 3FE2:       F8               CLC
; 3FE3:       5D               POP RBP
; 3FE4:       C3               RET
; Origin #x52E63FE5 (segment 3 of 4)                          ; (XEP FOO)
; 3FE5:       .SKIP 11
; 3FF0:       .ENTRY FOO(X Y)                           
                                                              ;  (T T) NUMBER)
; 4000:       8F4508           POP QWORD PTR [RBP+8]
; 4003:       4883F904         CMP RCX, 4
; 4007:       7557             JNE L3
; 4009:       488D65D0         LEA RSP, [RBP-48]
; 400D:       488955E8         MOV [RBP-24], RDX
; 4011:       48897DE0         MOV [RBP-32], RDI
; Origin #x52E64015 (segment 4 of 4)                          ; FOO
; 4015:       498B4510         MOV RAX, [R13+16]              ; thread.binding-stack-pointer
; 4019:       488945F0         MOV [RBP-16], RAX
; 401D:       4C8BCD           MOV R9, RBP
; 4020:       488D4424F0       LEA RAX, [RSP-16]
; 4025:       4883EC40         SUB RSP, 64
; 4029:       4C8B45E8         MOV R8, [RBP-24]
; 402D:       488B75E0         MOV RSI, [RBP-32]
; 4031:       4C8908           MOV [RAX], R9
; 4034:       488BE8           MOV RBP, RAX
; 4037:       E87DFFFFFF       CALL L0
; 403C:       4883EC10         SUB RSP, 16
; 4040:       B902000000       MOV ECX, 2
; 4045:       48892C24         MOV [RSP], RBP
; 4049:       488BEC           MOV RBP, RSP
; 404C:       E8F1E163FD       CALL #x504A2242                ; #<FDEFN PRINT>
; 4051:       4C8B45E8         MOV R8, [RBP-24]
; 4055:       488B75E0         MOV RSI, [RBP-32]
; 4059:       E95EFFFFFF       JMP L1
; 405E: L2:   CC10             INT3 16                        ; Invalid argument count trap
; 4060: L3:   CC10             INT3 16                        ; Invalid argument count trap

You can see that FOO and BAR are now compiled into the same component (with local calls), and both have valid external entry points. This improves locality of code quite a bit and still allows calling both FOO and BAR externally from the file (e.g. in the REPL). The only thing that has changed is that within the file foo.lisp, all calls to functions within that file shortcut going through the global fdefinition's external entry points which do all the slow argument checking and boxing. Even FACT is faster because the compiler can recognize the tail recursive local call and directly turn it into a loop. Without block-compilation, the user is licensed to, say, redefine FACT while it is running, which forces the compiler to make the self call into a normal full call to allow redefinition and full argument and return value processing.
But there is one more goody block compilation adds...

the :entry-points keyword

Notice we specified :entry-points nil above. That's telling the compiler to still create external entry points to every function in the file, since we'd like to be able to call them normally from outside the code component (i.e. the compiled compilation unit, here the entire file). Now, those of you who know C know there is a useful way to get the compiler to optimize file-local functions, for example automatically inlining them if they are once-use, and also enforce that the function is not visible externally from the file. This is the static keyword in C. The straightforward analogue when block compiling is the :entry-points keyword. Essentially, it makes the DEFUNs which are not entry-points not have any external entry points, i.e. they are not visible to any functions outside the block compiled unit and so become subject to an assortment of optimizations, For example, if a function with no external entry point is never called in the block-compiled unit, it will just be deleted as dead code. Better yet, if a function is once-use, it will be removed and directly turned into a LET at the call site, essentially acting as inlining with no code size tradeoff and is always an optimization.
So, for example, we have
> (compile-file "test.lisp" :block-compile t :entry-points '(bar fact))
which removes FOO for being unused in the block compiled unit (the file). This is all documented very well in the CMUCL manual section Advanced Compiler Use and Efficiency Hints under "Block compilation". Unfortunately this section (among others) never made it over to SBCL's manual, though it is still 99% accurate for SBCL.

a brief history of block compilation

Now to explain why the word "Fresh" in the title of this post is in quotes. You may be surprised to hear that CMUCL, the progenitor of SBCL, has had this interface to block compilation described above since 1991. Indeed, the Python compiler, which was first started in 1985 by Rob MacLachlan, was designed with the explicit goal of being able to block compile arbitrary amounts of code at once in bulk in the manner described above as a way to close the gap between dynamic and static language implementations. Indeed, the top-level intermediate representation data structure in Python is the COMPONENT, which represents a connected component of the flow graph created by compiling multiple functions together. So, what happened? Why did SBCL not have this feature despite its compiler being designed around it?

the fork

When SBCL was first release and forked off from CMUCL in late 1999, the goal of the system was to make it sanely bootstrappable and more maintainable. Many casualties of CMUCL features occured during this fork for the purpose of getting something working, such as the loss of the bytecode compiler, many extensions, hemlock, numerous backends, and block compilation. Many of these features such as the numerous CMUCL backends were eventually restored, but block compilation was one of those features that was never brought back into the fold, with bitrotted remnants of the interface lying around in the compiler for decades. The processing of DECLAIM and PROCLAIM forms, which were a crucial part of the more fine grained form of block compilation, were revamped entirely to make things more ANSI compatible. In fact, the proclamation level of block compilation of CMUCL has not made it back into SBCL even now for this reason, and it is still unclear whether it would be worth adding this form of elegant block compilation back into SBCL and whether it can be done in a clean, ANSI manner. Perhaps once this feature becomes more well known, people will find the finer granularity form of block compilation useful enough to request (or implement) it.

the revival

Reanimating this bitrotted zombie of a feature was surprisingly easy and difficult at the same time. Because the idea of block compilation was so deeply embedded into the structure of the compiler, most things internally worked right off the bat, sans a few assertions that had crept in after assuming an invariant along the lines of one top-level function definition per code component, which is contrary definitionally to the idea of block compilation. The front-end interface to block compilation was in contrast completely blown away and destroyed, with fundamental macros like DEFUN having been rewritten and the intermediate representation namespace behaving more locally. It took some time to redesign the interface to block compilation to fit with this new framework, and my first attempt last month to land the change ended in a Quicklisp library dropping the compiler into an infinite loop. The cause was a leakage in the intermediate representation which I patched up this month. Now things seem robust enough that it doesn't cause regressions for normal compilation mode.

what's left?

Not all is perfect though, and there are still a few bugs lurking around. For example, block compiling and inlining currently does not interact very well, while the same is not true for CMUCL. There are also probably a few bugs lurking around with respect to ensuring the right policies are in effect and have consistent semantics with block compilation. In addition, as mentioned above, the form-by-form level granularity given by the CMUCL-style (declaim (start-block ...)) ... (declaim (end-block ..)) proclamations are still missing. In fact, the CMUCL compiler sprinkled a few of these block compilation declarations around, and it would be nice if SBCL could block compile some of its own code to ensure maximum efficiency. However, the basic apparatus works, and I hope that as more people rediscover this feature and try it on their own performance-oriented code bases, bug reports and feature requests around block compilation and whole program optimization will develop and things will start maturing very quickly!

Block compilation - "Fresh" in SBCL 2.0.2

I've just managed to land a feature I've been missing in SBCL for quite some time now called block compilation, enabling whole-progr...