add "_" pattern for explicitly specifying the "discard" destination (may be used for side effects)

asm/src/data/mod.rs

@@ -26,6 +26,8 @@ pub enum DataDisp {
     Register(Register),
     /// Pointer into memory, stored in a numbered register
     RegisterPtr(Register),
+    /// Discard the written value
+    Discard,
 }
 
 impl Display for DataDisp {
@@ -43,6 +45,9 @@ impl Display for DataDisp {
             DataDisp::RegisterPtr(r) => {
                 write!(f, "@{}", r)
             }
+            DataDisp::Discard => {
+                write!(f, "_")
+            }
         }
     }
 }
@@ -64,7 +69,7 @@ impl From<DstDisp> for DataDisp {
             DstDisp::ImmediatePtr(x) => DataDisp::ImmediatePtr(x),
             DstDisp::Register(x) => DataDisp::Register(x),
             DstDisp::RegisterPtr(x) => DataDisp::RegisterPtr(x),
-            DstDisp::Discard => DataDisp::Immediate(0),
+            DstDisp::Discard => DataDisp::Discard,
         }
     }
 }
@@ -91,6 +96,7 @@ impl TryFrom<DataDisp> for SrcDisp {
             DataDisp::ImmediatePtr(x) => Ok(SrcDisp::ImmediatePtr(x)),
             DataDisp::Register(x) => Ok(SrcDisp::Register(x)),
             DataDisp::RegisterPtr(x) => Ok(SrcDisp::RegisterPtr(x)),
+            DataDisp::Discard => Err(AsmError::DiscardAsValue),
         }
     }
 }
@@ -128,6 +134,7 @@ impl TryFrom<DataDisp> for DstDisp {
             DataDisp::ImmediatePtr(x) => Ok(DstDisp::ImmediatePtr(x)),
             DataDisp::Register(x) => Ok(DstDisp::Register(x)),
             DataDisp::RegisterPtr(x) => Ok(DstDisp::RegisterPtr(x)),
+            DataDisp::Discard => Ok(DstDisp::Discard),
         }
     }
 }

asm/src/error.rs

@@ -33,6 +33,8 @@ pub enum AsmError {
     UnevenOperandSize(Mask, Mask),
     #[error("Value provided as output argument")]
     ValueAsOutput,
+    #[error("Discard (_) provided as input argument")]
+    DiscardAsValue,
     #[error("Conditional branch already defined for \"{0}\"")]
     ConditionalAlreadyUsed(Cond),
     #[error("Label \"{0:?}\" not defined")]

asm/src/parse/parse_data.rs

@@ -26,6 +26,10 @@ pub fn parse_data_disp(tok: Option<Sexp>) -> Result<DataDisp, Error> {
             Ok(DataDisp::Immediate(unsafe { std::mem::transmute(*val) }))
         },
         Sexp::Atom(Atom::S(s)) => {
+            if s == "_" {
+                return Ok(DataDisp::Discard);
+            }
+
             if let Some(reference) = s.strip_prefix('@') {
                 if reference.starts_with(|c : char| c.is_ascii_digit()) {
                     let val : u64 = parse_u64(reference)?;

crsn/src/main.rs

@@ -43,6 +43,7 @@ fn main() {
     let program = "
     (
         (main
+            (ld _ arg0)
             (call fibo 5)
             (ld r0 res0)
         )