fixes, safeguards and docs for "compile-time arithmetics"

README.md

@@ -196,6 +196,35 @@ Some instructions use bit offsets and widths. Width is directly attached to the
 offsets are attached at the respective arguments after a colon: `(ld16 r0:8 r1:32)`  -
 load 16 bits, starting at offset 32 of `r1`, into `r0`, starting at offset 8. The rest if the register is not affected.
 
+### Compile-time arithmetics
+
+Quite often you will want to do some calculations at compile time, for example to create a constant whose value depends on another,
+or to compose a binary value using bit shifts and masking. This can be done in any place that expects an input operand (`Rd`).
+
+The syntax for this is as follows:
+
+```
+(ld r0 (=add 123 456))
+```
+
+The expressions can be nested. The equals sign is not required in the inner levels, and the output operand must be omitted 
+(the compiler inserts a placeholder register there during evaluation):
+
+```
+(def WIDTH 1024)
+(def HALFW_MAX (=sub (div WIDTH 2) 1))
+```
+
+Almost any instruction can be evaluated this way, excluding instructions that perform IO, work with the screen, objects etc.
+Instructions that are not compile-time evaluation safe will produce a compile error.
+
+There are several limitations to consider:
+
+- Only immediate values (literals) and constant names may be used.
+- Registers, if used, will always read as zero and writes have no effect. (This should also produce an error)
+- Only instructions that take the form `(op Wr ...)` can be used. The `Wr` operand is inserted automatically and must NOT be specified!
+- Conditionals are not allowed inside expressions: branches, conditional suffixes
+
 ### Conditional branches
 
 Conditonal branches are written like this:
@@ -296,7 +325,9 @@ Jumping to a label is always safer than a manual skip.
 ; However, if the register is a global register, then the alias is valid everywhere.
 (sym SYM REG)
 
-; Define a constant. These are valid in the whole program.
+; Define a constant. These are valid in the entire program following the definition point, unless
+; un-defined. Constants are evaluated and assigned at compile time, the program control flow has no
+; effect.
 ; Value must be known at compile time.
 (def CONST VALUE)
 

crsn/src/asm/instr/op.rs

@@ -65,4 +65,11 @@ impl OpTrait for Op {
 
         se
     }
+
+    fn is_compile_time_evaluable(&self) -> bool {
+         match &self.kind {
+            OpKind::BuiltIn(op) => op.is_compile_time_evaluable(),
+            OpKind::Ext(op) => op.is_compile_time_evaluable()
+        }
+    }
 }

crsn/src/asm/parse/parse_data.rs

@@ -3,7 +3,7 @@ use std::convert::TryFrom;
 
 use sexp::{Atom, Sexp, SourcePosition};
 
-use crate::asm::data::{DataDisp, Rd, RdData, reg, Wr, WrData, RdWr};
+use crate::asm::data::{DataDisp, Rd, RdData, reg, Wr, WrData, RdWr, Register};
 use crate::asm::data::literal::{ConstantName, Label, RegisterAlias, Value};
 use crate::asm::error::CrsnError;
 use crate::asm::parse::{ParserContext, parse_instructions};
@@ -75,6 +75,7 @@ pub fn parse_label_str(name: &str, pos: &SourcePosition) -> Result<Label, CrsnEr
 pub fn parse_data_disp(tok: Sexp, pcx: &ParserContext) -> Result<DataDisp, CrsnError> {
     // trace!("parse data: {:?}", tok);
     match tok {
+        // immediates are okay in const eval - no tests needed
         Sexp::Atom(Atom::I(val), _pos) => {
             Ok(DataDisp::Immediate(unsafe { std::mem::transmute(val) }))
         }
@@ -92,10 +93,12 @@ pub fn parse_data_disp(tok: Sexp, pcx: &ParserContext) -> Result<DataDisp, CrsnE
         }
         Sexp::List(list, pos) => {
             if let Some(Sexp::Atom(Atom::S(s), s_pos)) = list.first() {
+                // Const eval
+
                 let parsing_expr = pcx.state.borrow().parsing_expr;
 
                 if !s.starts_with('=') && !parsing_expr {
-                    return Err(CrsnError::Parse(format!("Invalid syntax, did you mean \"={}\"?", s).into(), s_pos.clone()));
+                    return Err(CrsnError::Parse(format!("Invalid syntax, did you mean \"={}\" (a compile-time expression)?", s).into(), s_pos.clone()));
                 }
 
                 // start expr parsing mode
@@ -116,6 +119,10 @@ pub fn parse_data_disp(tok: Sexp, pcx: &ParserContext) -> Result<DataDisp, CrsnE
 
                 let op = expr_ops.remove(0);
 
+                if !op.is_compile_time_evaluable() {
+                    return Err(CrsnError::Parse(format!("Instruction {} cannot be used in an expression!", op.to_sexp()).into(), pos.clone()));
+                }
+
                 let mut state_mut = pcx.state.borrow_mut();
                 state_mut.const_eval.clear_all();
 
@@ -144,12 +151,16 @@ pub fn parse_data_disp(tok: Sexp, pcx: &ParserContext) -> Result<DataDisp, CrsnE
 }
 
 /// Parse data disp from a string token (a &str is used so the string can be preprocessed)
-pub fn parse_data_disp_from_str(s: &str, pos: &SourcePosition, pcx: &ParserContext) -> Result<DataDisp, CrsnError> {
+fn parse_data_disp_from_str(s: &str, pos: &SourcePosition, pcx: &ParserContext) -> Result<DataDisp, CrsnError> {
     if s == "_" {
         return Ok(DataDisp::Discard);
     }
 
     if let Some(reference) = s.strip_prefix('@') {
+        if pcx.state.borrow().parsing_expr {
+            return Err(CrsnError::Parse("Object handles cannot be used in expressions!".into(), pos.clone()));
+        }
+
         /* extension constants (pre-defined handles) */
         for p in pcx.parsers {
             if let Some(val) = p.get_constant_value(reference) {
@@ -188,7 +199,7 @@ pub fn parse_data_disp_from_str(s: &str, pos: &SourcePosition, pcx: &ParserConte
             return Ok(DataDisp::Immediate(*val));
         }
 
-        /* register aliases */
+        /* register aliases - no need to check for const_eval, the aliases could never be defined */
         if let Some(val) = pstate.reg_aliases.get(s) {
             return Ok(DataDisp::Register(*val));
         } else if let Some(val) = pstate.global_reg_aliases.get(s) {
@@ -215,6 +226,14 @@ pub fn parse_data_disp_from_str(s: &str, pos: &SourcePosition, pcx: &ParserConte
             }
         }
 
+        if pcx.state.borrow().parsing_expr {
+            if s == "=result" {
+                return Ok(DataDisp::Register(Register::Gen(0)));
+            } else {
+                return Err(CrsnError::Parse("Registers cannot be used in expressions!".into(), pos.clone()));
+            }
+        }
+
         /* register */
         let reg = reg::parse_reg(&s, &pos)?;
 
@@ -231,39 +250,13 @@ pub fn parse_data_disp_from_str(s: &str, pos: &SourcePosition, pcx: &ParserConte
 
 /// Parse immediate value
 pub fn parse_value(tok: Sexp, pcx: &ParserContext) -> Result<(Value, SourcePosition), CrsnError> {
-    match tok {
-        Sexp::Atom(Atom::I(val), pos) => {
-            Ok((unsafe { std::mem::transmute(val) }, pos))
-        }
-        Sexp::Atom(Atom::U(val), pos) => {
-            Ok((val, pos))
-        }
-        Sexp::Atom(Atom::C(val), pos) => {
-            Ok((val as u64, pos))
-        }
-        Sexp::Atom(Atom::QS(_), pos) => {
-            Err(CrsnError::Parse("quoted string not expected here".into(), pos))
-        }
-        Sexp::Atom(Atom::F(val), pos) => {
-            Ok((unsafe { std::mem::transmute(val) }, pos))
-        }
-        Sexp::Atom(Atom::S(s), pos) => {
-            let pstate = pcx.state.borrow();
-            if let Some(val) = pstate.constants.get(&s) {
-                return Ok((*val, pos));
-            }
-
-            /* extension constants */
-            for p in pcx.parsers {
-                if let Some(val) = p.get_constant_value(&s) {
-                    return Ok((val, pos));
-                }
-            }
+    let pos = tok.pos().clone();
+    let parsed = parse_rd(tok, pcx)?;
 
-            Err(CrsnError::Parse(format!("unknown constant: {}", s).into(), pos))
-        }
-        Sexp::List(_, pos) => {
-            Err(CrsnError::Parse("expected a value".into(), pos))
+    match parsed {
+        Rd(RdData::Immediate(value)) => Ok((value, pos)),
+        _ => {
+            Err(CrsnError::Parse("expected an immediate value".into(), pos))
         }
     }
 }

crsn/src/asm/parse/parse_instr.rs

@@ -95,7 +95,7 @@ pub fn parse_instructions(items: impl Iterator<Item=Sexp>, pos: &SourcePosition,
 
         if parsing_expr {
             // hackity hack for const expr eval
-            arg_parser.prepend(atom_s("r0"));
+            arg_parser.prepend(atom_s("=result"));
         }
 
         if let Some(op) = parse_op(name.as_str(), arg_parser, &namepos)? {

crsn/src/builtin/exec.rs

@@ -204,6 +204,13 @@ impl OpTrait for BuiltinOp {
         Ok(res)
     }
 
+    fn is_compile_time_evaluable(&self) -> bool {
+        match self {
+            BuiltinOp::Load { .. } | BuiltinOp::LoadBits { .. } => true,
+            _ => false
+        }
+    }
+
     fn to_sexp(&self) -> Sexp {
         super::parse::to_sexp(self)
     }

crsn/src/module/mod.rs

@@ -48,6 +48,13 @@ pub trait OpTrait: Debug + Send + Sync + 'static {
 
     /// Turn into an S-expression that produces this instruction when parsed
     fn to_sexp(&self) -> Sexp;
+
+    /// Check if an operation is safe to use in compile-time arithmetics - has no side effects
+    /// outside the input and output registers (and flags), has the form (op Wr ...), and does not
+    /// use object handles or extension data.
+    fn is_compile_time_evaluable(&self) -> bool {
+        false
+    }
 }
 
 /// CRSN initializer object.

crsn_arith/src/exec.rs

@@ -757,6 +757,15 @@ impl OpTrait for ArithOp {
             ArithOp::FloatHypAcot { dst, a } => to_sexp_1_or_2("facoth", dst, a),
         }
     }
+
+    fn is_compile_time_evaluable(&self) -> bool {
+        match self {
+            ArithOp::Test { .. } | ArithOp::Compare { .. } | ArithOp::RangeTest { .. }
+            | ArithOp::FloatTest { .. } | ArithOp::FloatCompare { .. } | ArithOp::FloatRangeTest { .. } => false,
+            // rng is allowed... makes little sense, but it also has no side effects
+            _ => true, // a bold claim...
+        }
+    }
 }
 
 fn to_sexp_2_or_3(name: &str, dst: &Wr, a: &Rd, b: &Rd) -> Sexp {

examples/expr.csn

@@ -1,12 +1,15 @@
 (
-    (def FOO 14)
+    (def A 10)
+    (def FOO (=add A (sub 10 5)))
+
+    ; FOO is 15
 
     (ld r0 (=add 14 1))
     (cmp r0 15 (ne? (fault "1")))
 
     (ld r0 (=add FOO 1))
-    (cmp r0 15 (ne? (fault "2")))
+    (cmp r0 16 (ne? (fault "2")))
 
     (ld r0 (=add (sub FOO 2) 3))
-    (cmp r0 15 (ne? (fault "3")))
+    (cmp r0 16 (ne? (fault "3")))
 )