deal better with return values when underflowing the substring entry stack

it's now able to run all the way to the end of the label transform, inside the hex transform. the output leaves much to be desired, but it runs.

Force-Push: yes
Change-Id: I7cac39df599e8081fac6b050dfeb34e598bb54eb

2 files changed, 74 insertions, 38 deletions

diff --git a/README.txt b/README.txt
index e3e12c1..384c8f9 100644
--- a/README.txt
+++ b/README.txt
@@ -213,8 +213,20 @@ you're metacircular.)
 
   Although the hex transform doesn't yet work on Forth programs (only programs
 written in Evocation-assembly), if you intend to play around with this you may
-wish to know how to attempt to run it on things. There is not yet a
-recommended way to run the hex transform on evoke.e, but when there is it will
-look like appending most of the Evocation-in-Evocation build steps to the
-hex.hex build steps, swapping them in where hex.e is now.
+wish to know how to attempt to run it on things. The latest draft way to do
+that is:
+
+  $ (cat labels.e elf.e transform.e; echo 's" xyzzy" allocate-string dup 1048576 read-to-buffer'; cat core.e linux.e output.e amd64.e execution-support.e log-load.e dynamic.e input.e interpret.e flow-control.e linux-dynamic.e labels.e elf.e transform.e execution.e; echo 's" pyrzqxgl" allocate-string dup 262144 read-to-buffer '; cat core.e linux.e output.e amd64.e execution-support.e log-load.e; echo pyrzqxgl swap 262144 read-to-buffer; cat core.e linux.e output.e amd64.e execution-support.e log-load.e dynamic.e input.e interpret.e flow-control.e linux-dynamic.e; echo pyrzqxgl; cat evoke.e; echo 'xyzzy s" evoke-source" variable 1024 1024 * allocate s" evoke-binary" variable 1024 1024 * allocate s" evoke-metadata" variable evoke-metadata evoke-binary dup evoke-source 5 roll hex-transform bye ' ) | ./evoke > evoke.hex
+
+  It will likely crash somewhere inside label-loop. You may find it helpful
+to modify transform.e's definition of hex-sys-write-replacement by adding
+the words "sys-write" and "exit" at the start of it, thereby turning it off
+and allowing crash dumps to print.
+
+  It's worth understanding that, although the log-load transform's output
+comes first in the executable, it conceptually depends on the label transform,
+and is unlikely to get anywhere until that's complete. For development
+purposes, it may make sense to comment out the call to output-warm-start in
+evoke.e and set a placeholder value for the warm-start label. Now running it
+will cause failure somewhere inside the log-load transform; get debugging! :)
 
diff --git a/transform.e b/transform.e
index 38cd951..95475c2 100644
--- a/transform.e
+++ b/transform.e
@@ -3311,7 +3311,9 @@ allocate-transformation-state s" transformation-state" variable
 
     substring-entry-stack-depth@ 1+ substring-entry-stack-depth!
   } {
+    ~ We still consume the value, so that our caller doesn't crash.
     drop
+
     ." Substring entry stack overflow." newline
   } if-else ;
 
@@ -3324,6 +3326,9 @@ allocate-transformation-state s" transformation-state" variable
     substring-entry-stack-depth@ 8 * + @
   } {
     ." Substring entry stack underflow." newline
+
+    ~ We still return a value, so that our caller doesn't crash.
+    0
   } if-else ;
 
 ~ (data start, data length, entry type, string pointer, entry pointer --)
@@ -3762,38 +3767,44 @@ allocate-transformation-state s" transformation-state" variable
 
         pop-substring-entry-stack
 
-        dup hex-output-metadata-entry-type @
-        hex-output-metadata-entry-type-push-substring-decimal = {
-          dup hex-output-metadata-entry-string @ .
-        } if
-
-        dup hex-output-metadata-entry-type @
-        hex-output-metadata-entry-type-push-substring-hex8 = {
-          ." 0x"
-          dup hex-output-metadata-entry-string @ .hex8
-        } if
-
-        dup hex-output-metadata-entry-type @
-        hex-output-metadata-entry-type-push-substring-hex16 = {
-          ." 0x"
-          dup hex-output-metadata-entry-string @ .hex16
-        } if
-
-        dup hex-output-metadata-entry-type @
-        hex-output-metadata-entry-type-push-substring-hex32 = {
-          ." 0x"
-          dup hex-output-metadata-entry-string @ .hex32
-        } if
-
-        dup hex-output-metadata-entry-type @
-        hex-output-metadata-entry-type-push-substring-hex64 = {
-          ." 0x"
-          dup hex-output-metadata-entry-string @ .hex64
-        } if
-
-        dup hex-output-metadata-entry-type @
-        hex-output-metadata-entry-type-push-substring-string = {
-          dup hex-output-metadata-entry-string @ emitstring
+        ~   If we underflowed the stack, we got zero back, so make sure not to
+        ~ crash. A crash here would be a pain to debug because it would be
+        ~ unclear how much of the overall system is working, and it's a
+        ~ complex system.
+        dup {
+          dup hex-output-metadata-entry-type @
+          hex-output-metadata-entry-type-push-substring-decimal = {
+            dup hex-output-metadata-entry-string @ .
+          } if
+
+          dup hex-output-metadata-entry-type @
+          hex-output-metadata-entry-type-push-substring-hex8 = {
+            ." 0x"
+            dup hex-output-metadata-entry-string @ .hex8
+          } if
+
+          dup hex-output-metadata-entry-type @
+          hex-output-metadata-entry-type-push-substring-hex16 = {
+            ." 0x"
+            dup hex-output-metadata-entry-string @ .hex16
+          } if
+
+          dup hex-output-metadata-entry-type @
+          hex-output-metadata-entry-type-push-substring-hex32 = {
+            ." 0x"
+            dup hex-output-metadata-entry-string @ .hex32
+          } if
+
+          dup hex-output-metadata-entry-type @
+          hex-output-metadata-entry-type-push-substring-hex64 = {
+            ." 0x"
+            dup hex-output-metadata-entry-string @ .hex64
+          } if
+
+          dup hex-output-metadata-entry-type @
+          hex-output-metadata-entry-type-push-substring-string = {
+            dup hex-output-metadata-entry-string @ emitstring
+          } if
         } if
 
         ~   Tidy up. We want to treat the next character normally, so we
@@ -3804,11 +3815,16 @@ allocate-transformation-state s" transformation-state" variable
           ~ (input point, first non-word character or 0, word pointer)
           drop
 
+          ~   If we underflow the stack, we'll get zeroes back here. In order
+          ~ to avoid causing more confusing problems later, we make sure to
+          ~ discard the zeroes instead of pushing them. As above, the
+          ~ reasoning is that it's easier to diagnose what's wrong with this
+          ~ subsystem when it runs to completion.
           pop-substring-entry-stack
           pop-substring-entry-stack
           swap
-          push-substring-entry-stack
-          push-substring-entry-stack
+          dup { push-substring-entry-stack } { drop } if-else
+          dup { push-substring-entry-stack } { drop } if-else
 
           ~   We want to ignore the next character, so that this word can be
           ~ used without creating spurious spaces in the output.
@@ -3851,6 +3867,14 @@ allocate-transformation-state s" transformation-state" variable
 ~
 ~ (length to write, base address --)
 : hex-sys-write-replacement
+  ~   In intended operation, the code under transformation is a compilation
+  ~ process, and it won't output anything that isn't executable binary.
+  ~ However, while developing, it's common to have crashes, and it's nice to
+  ~ get the crash dump as ASCII rather than hex.
+  dup is-in-label-loop-buffer not {
+    sys-write exit
+  } if
+
   postprocess-metadata-entries
 
   { over 0 <= } {