Add support for user-defined constraints on instructions.

Add support for user-defined constraints on instructions.
These are written as an optional braced-block at the end of the
instruction pattern. When present, they are evaluated when the
instruction is generated and should return true if the values
of the variable fields are OK, false if the generation should
be retried with another random number.

Note that there is not much sanity checking on what we are
being asked to evaluate!

1 files changed, 11 insertions, 0 deletions

diff --git a/thumb.risu b/thumb.risu
index 4406e68..8908a48 100644
--- a/thumb.risu
+++ b/thumb.risu
@@ -20,6 +20,17 @@
 # Field names beginning 'r' are special as they are assumed to be general
 # purpose registers. They get an automatic "cannot be 13 or 15" (sp/pc)
 # constraint.
+# The optional eval-block at the end of the line (which must be
+# enclosed in braces) is a perl statement to be evaluated and which
+# must return true if the generated statement is OK, false if the
+# generator should retry with a fresh random number. It is evaluated
+# in a context where variables with the same names as the defined
+# variable fields are initialised. The intention is that odd cases
+# where you need to apply some sort of constraint to the generated
+# instruction can be handled via this mechanism.
+# NB that there is no sanity checking that you don't do bad things
+# in the eval block, although there is a basic check for syntax
+# errors and and we bail out if the constraint returns failure too often.
 
 #ADD T1 000 1100 rm:3 rn:3 rd:3
 #ADD T3 11101 01 1000 s rn:4 0 imma:3 rd:4 immb:2 type:2 rm:4