--- gforth/except.fs	2003/03/09 15:16:48	1.7
+++ gforth/except.fs	2006/10/13 17:36:07	1.13
@@ -33,7 +33,59 @@ Defer store-backtrace
 ' noop IS store-backtrace
 \ [THEN]
 
+\ Ok, here's the story about how we get to the native code for the
+\ recovery code in case of a THROW, and why there is all this funny
+\ stuff being compiled by TRY and RECOVER:
+
+\ Upon a THROW, we cannot just return through the ordinary return
+\ address, but have to use a different one, for code after the
+\ RECOVER.  How do we do that, in a way portable between the various
+\ threaded and native code engines?  In particular, how does the
+\ native code engine learn about the address of the native recovery
+\ code?
+
+\ On the Forth level, we can compile only references to threaded code.
+\ The only thing that translates a threaded code address to a native
+\ code address is docol, which is only called with EXECUTE and
+\ friends.  So we start the recovery code with a docol, and invoke it
+\ with PERFORM; the recovery code then rdrops the superfluously
+\ generated return address and continues with the proper recovery
+\ code.
+
+\ At compile time, since we cannot compile a forward reference (to the
+\ recovery code) as a literal (backpatching does not work for
+\ native-code literals), we produce a data cell (wrapped in AHEAD
+\ ... THEN) that we can backpatch, and compile the address of that as
+\ literal.
+
+\ Overall, this leads to the following resulting code:
+
+\   ahead
+\ +><recovery address>-+
+\ | then               |
+\ +-lit                |
+\   (try)              |
+\   ...                |
+\   (recover)          |
+\   ahead              |
+\   docol: <-----------+
+\   rdrop
+\   ...
+\   then
+\   ...
+
+\ !! explain handler on-stack structure
+
+Variable first-throw
+: nothrow ( -- ) \ gforth
+    \G Use this (or the standard sequence @code{['] false catch drop})
+    \G after a @code{catch} or @code{endtry} that does not rethrow;
+    \G this ensures that the next @code{throw} will record a
+    \G backtrace.
+    first-throw on ;
+
 : (try) ( ahandler -- )
+    first-throw on
     r>
     swap >r \ recovery address
     rp@ 'catch >r
@@ -42,12 +94,12 @@ Defer store-backtrace
     lp@ >r
     handler @ >r
     rp@ handler !
-    backtrace-empty on
     >r ;
 
 : try ( compilation  -- orig ; run-time  -- ) \ gforth
     \ !! does not work correctly for gforth-native
-    POSTPONE lit >mark POSTPONE (try) ; immediate compile-only
+    POSTPONE ahead here >r >mark 1 cs-roll POSTPONE then
+    r> POSTPONE literal POSTPONE (try) ; immediate compile-only
 
 : (recover) ( -- )
     \ normal end of try block: restore handler, forget rest
@@ -63,7 +115,10 @@ Defer store-backtrace
 : recover ( compilation  orig1 -- orig2 ; run-time  -- ) \ gforth
     \ !! check using a special tag
     POSTPONE (recover)
-    POSTPONE else ; immediate compile-only
+    POSTPONE else
+    docol: here 0 , 0 , code-address! \ start a colon def 
+    postpone rdrop                    \ drop the return address
+; immediate compile-only
 
 : endtry ( compilation  orig -- ; run-time  -- ) \ gforth
     POSTPONE then ; immediate compile-only
@@ -79,7 +134,10 @@ is catch
 :noname ( y1 .. ym error/0 -- y1 .. ym / z1 .. zn error ) \ exception
     ?DUP IF
 	[ here forthstart 9 cells + ! ]
-	store-backtrace
+	first-throw @ IF
+	    store-backtrace error-stack off
+	    first-throw off
+	THEN
 	handler @ ?dup-0=-IF
 	    >stderr cr ." uncaught exception: " .error cr
 	    2 (bye)
@@ -90,7 +148,23 @@ is catch
 	r> lp!
 	r> fp!
 	r> swap >r sp! drop r>
-	rdrop 'throw
+	rdrop 'throw r> perform
     THEN ;
 is throw
-
+[IFDEF] rethrow
+:noname ( y1 .. ym error/0 -- y1 .. ym / z1 .. zn error ) \ exception
+    ?DUP IF
+	handler @ ?dup-0=-IF
+	    >stderr cr ." uncaught exception: " .error cr
+	    2 (bye)
+\	    quit
+	THEN
+	rp!
+	r> handler !
+	r> lp!
+	r> fp!
+	r> swap >r sp! drop r>
+	rdrop 'throw r> perform
+    THEN ;
+is rethrow
+[THEN]
\ No newline at end of file