pnkfelix
commented
9 years ago Its quite possible that I made a mistake in my reading of:
http://www.ccs.neu.edu/home/lth/larceny/notes/note13-malcode.html
(Update: an earlier version of this comment had some mistakes its reasoning, namely a fencepost error in my thinking about what REGr was supposed to denote.)
In particular, we have:
lambda x,n,doc
If n < r, loads RESULT with the procedure formed from the
code x and the values of registers REG0-REGn. If n >= r,
loads RESULT with the procedure formed from the values of
registers REG0-REG{r-1} and the first n-r+1 values taken
from the list in REGr. The code x consists of a bytevector
of pure code and a vector of constants. The documentation
is used only for debugging.So, okay, there's a threshold value r. That sounds like it could correspond to that magic number 31 in @WillClinger's description. From the text above, it seems like it should corresponds to the number of (software) registers on the target, since it talks about loading a list into REGr, which would imply to me that r should be 31.
On further investigation, we find:
Implementation-specific parameters.
The highest numbered general register is REGr, where r = R-1.
See the end of this file for implementation notes, notably the
values of R, MINOFFSET, MAXOFFSET, MINIMM, MAXIMM, MAXSLOT, and MAXRIB.So, hmm, the value of r is actually R-1 according to the docs
But then what is R at the bottom of the file:
Implementation notes.
Minimal parameter values.
R ? (Certainly at least 3, or op3 won't work)
...
SPARC implementation.
R 32
Standard-C implementation.
R 32
...
This makes it seem like R is the number of software registers, while r is R - 1 (i.e. the index of the final software register, i.e. the magic number 31).
(Second update: still more errors corrected.)
Anyway, looking again at the documentation for the lambda macscheme instruction, note that the documented ranges are n < r for the non-list case and n >= r for the list case.
So that implies to me that IAssassin is following the documented semantics, even though that may not be what Twobit actually implements. (That was wrong.)
IAssassin is not following the documented semantics; it appears to be assuming that when n == r that it should copy the value directly from REGr, rather than reading it as the first element of a list stored in REGr. That explains the semantics observed in this ticket: we are creating the singleton list and storing it in REGr, but then IAssassin is treating that list itself as the value of the final free variable.
(Incidentally, is there are good reason to use n < r/n >= r rather than n <= r/n > r ? It seems like the latter semantics would avoid a cons in the case where n == r... but maybe the former semantics has advantages elsewhere.)
(I would be curious to know if this same bug exhibits itself on the Petit/C or Petit/NASM backend, especially since the IAssassin backend was largely directly ported from the Petit/NASM backend, apart from a few opcodes that needed special treatment due to its non-Petit nature.)
WillClinger
lars-t-hansen
In current IAssassin versions of Larceny:
(begin (define (foo a b c d e f g h i j k l m n o p q r s t u v w x y z a27 a28 a29 a30 a31) (lambda () (list a b c d e f g h i j k l m n o p q r s t u v w x y z a27 a28 a29 a30 a31))) ((foo 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31)))evaluates to
When one more argument is added to both
fooand to the lambda expression, the output is correct.This is a bug in the code generator, but I'm not sure whether it's a bug in Twobit or in the IAssassin back end. Twobit produces this MacScheme machine code:
.entry 0,#t save/stores 32,(0),(0) lambda *,0 setreg 3 setglbl foo const/setreg 31,3 global foo setstk 32 const 1 setstk 31 const 2 setstk 30 const 3 setstk 29 const 4 setstk 28 const 5 setstk 27 const 6 setstk 26 const 7 setstk 25 const 8 setstk 24 const 9 setstk 23 const 10 setstk 22 const 11 setstk 21 const 12 setstk 20 const 13 setstk 19 const 14 setstk 18 const 15 setstk 17 const 16 setstk 16 const 17 setstk 15 const 18 setstk 14 const 19 setstk 13 const 20 setstk 12 const 21 setstk 11 const 22 setstk 10 const 23 setstk 9 const 24 setstk 8 const 25 setstk 7 const 26 setstk 6 const 27 setstk 5 const 28 setstk 4 const 29 setstk 3 const 30 setstk 2 reg 3,.T126 setstk 1 const () setreg 31 stack 1 op2 cons,31 setreg 31 load 30,2 load 29,3 load 28,4 load 27,5 load 26,6 load 25,7 load 24,8 load 23,9 load 22,10 load 21,11 load 20,12 load 19,13 load 18,14 load 17,15 load 16,16 load 15,17 load 14,18 load 13,19 load 12,20 load 11,21 load 10,22 load 9,23 load 8,24 load 7,25 load 6,26 load 5,27 load 4,28 load 3,29 load 2,30 load 1,31 stack 32 setrtn/invoke 31 .align 4 L1006 .cont load 0,0 setreg 2 pop 32 invoke 0 .end .entry 1,#f .proc args= 31 save/stores 32,(0 1),(0 2) movereg 31,1 reg/op1/setreg car,1,ebx setstk 1 reg/op1/setreg cdr,1,ebx load 1,1 stack 2 setstk 32 reg 2,|.b\|1| setstk 31 reg 3,|.c\|1| setstk 30 reg 4,|.d\|1| setstk 29 reg 5,|.e\|1| setstk 28 reg 6,|.f\|1| setstk 27 reg 7,|.g\|1| setstk 26 reg 8,|.h\|1| setstk 25 reg 9,|.i\|1| setstk 24 reg 10,|.j\|1| setstk 23 reg 11,|.k\|1| setstk 22 reg 12,|.l\|1| setstk 21 reg 13,|.m\|1| setstk 20 reg 14,|.n\|1| setstk 19 reg 15,|.o\|1| setstk 18 reg 16,|.p\|1| setstk 17 reg 17,|.q\|1| setstk 16 reg 18,|.r\|1| setstk 15 reg 19,|.s\|1| setstk 14 reg 20,|.t\|1| setstk 13 reg 21,|.u\|1| setstk 12 reg 22,|.v\|1| setstk 11 reg 23,|.w\|1| setstk 10 reg 24,|.x\|1| setstk 9 reg 25,|.y\|1| setstk 8 reg 26,|.z\|1| setstk 7 reg 27,|.a27\|1| setstk 6 reg 28,|.a28\|1| setstk 5 reg 29,|.a29\|1| setstk 4 reg 30,|.a30\|1| setstk 3 reg 1,.T93 setstk 1 const () setreg 31 stack 1 op2 cons,31 setreg 31 load 30,3 load 29,4 load 28,5 load 27,6 load 26,7 load 25,8 load 24,9 load 23,10 load 22,11 load 21,12 load 20,13 load 19,14 load 18,15 load 17,16 load 16,17 load 15,18 load 14,19 load 13,20 load 12,21 load 11,22 load 10,23 load 9,24 load 8,25 load 7,26 load 6,27 load 5,28 load 4,29 load 3,30 load 2,31 load 1,32 pop 32 .align 4 L1001 .proc .proc-doc #(foo #f 31 #f #f (a b c d e f g h i j k l m n o p q r s t u v w x y z a27 a28 a29 a30 a31)) save/stores 2,(0 1),(0 2) movereg 31,1 reg/op1/setreg car,1,ebx setstk 1 reg/op1/setreg cdr,1,ebx const () setreg 31 stack 1 op2 cons,31 setreg 31 load 1,2 lambda *,31 pop 2 return .end .entry 2,#f .proc args= 0 const/setreg (),3 lexical 0,31,|.a31\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,30,|.a30\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,29,|.a29\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,28,|.a28\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,27,|.a27\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,26,|.z\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,25,|.y\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,24,|.x\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,23,|.w\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,22,|.v\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,21,|.u\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,20,|.t\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,19,|.s\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,18,|.r\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,17,|.q\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,16,|.p\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,15,|.o\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,14,|.n\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,13,|.m\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,12,|.l\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,11,|.k\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,10,|.j\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,9,|.i\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,8,|.h\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,7,|.g\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,6,|.f\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,5,|.e\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,4,|.d\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,3,|.c\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,2,|.b\|3| reg/op2/setreg cons,ebx,3,3 lexical 0,1,|.a\|3| op2 cons,3 return .endExplanation:
With the current IAssassin (IA32) version of Larceny, the Twobit compiler generates code for an abstract MacScheme machine with 32 registers, of which 31 are used for arguments. The
lambdainstruction creates a procedure that closes over the values in some number of those registers. When closing over less than 31 values, the semantics is straightforward. When closing over 32 or more,reg31holds a list of all values to close over past the first 30 (not counting the contents ofreg0). When closing over exactly 31 registers, there are two reasonable semantics. Evidently Twobit is using one of the two reasonable semantics, but the IAssassin assembler is using the other.I don't know which is right. I'll have to consult the MacScheme machine documentation.