Add experimental memory.discard instruction (#882)

Author: bvisness

crates/wasm-encoder/src/core/code.rs

@@ -367,6 +367,7 @@ pub enum Instruction<'a> {
     DataDrop(u32),
     MemoryCopy { src_mem: u32, dst_mem: u32 },
     MemoryFill(u32),
+    MemoryDiscard(u32),
 
     // Numeric instructions.
     I32Const(i32),
@@ -1092,6 +1093,11 @@ impl Encode for Instruction<'_> {
                 sink.push(0x0b);
                 mem.encode(sink);
             }
+            Instruction::MemoryDiscard(mem) => {
+                sink.push(0xfc);
+                sink.push(0x12);
+                mem.encode(sink);
+            }
 
             // Numeric instructions.
             Instruction::I32Const(x) => {

crates/wasm-shrink/src/lib.rs

@@ -234,6 +234,7 @@ impl ShrinkRun {
             sign_extension: true,
             component_model: false,
             floats: true,
+            memory_control: true,
         });
 
         validator.validate_all(wasm)?;

crates/wasm-smith/tests/core.rs

@@ -299,6 +299,7 @@ fn parser_features_from_config(config: &impl Config) -> WasmFeatures {
         floats: true,
         extended_const: false,
         component_model: false,
+        memory_control: false,
     }
 }
 

crates/wasmparser/benches/benchmark.rs

@@ -251,6 +251,7 @@ fn define_benchmarks(c: &mut Criterion) {
             mutable_global: true,
             saturating_float_to_int: true,
             sign_extension: true,
+            memory_control: true,
         })
     }
 

crates/wasmparser/src/binary_reader.rs

@@ -1046,6 +1046,11 @@ impl<'a> BinaryReader<'a> {
                 visitor.visit_table_fill(table)
             }
 
+            0x12 => {
+                let mem = self.read_var_u32()?;
+                visitor.visit_memory_discard(mem)
+            }
+
             _ => bail!(pos, "unknown 0xfc subopcode: 0x{code:x}"),
         })
     }

crates/wasmparser/src/lib.rs

@@ -338,6 +338,11 @@ macro_rules! for_each_operator {
             @reference_types TableGrow { table: u32 } => visit_table_grow
             @reference_types TableSize { table: u32 } => visit_table_size
 
+            // OxFC prefixed operators
+            // memory control (experimental)
+            // https://github.com/WebAssembly/design/issues/1439
+            @memory_control MemoryDiscard { mem: u32 } => visit_memory_discard
+
             // 0xFE prefixed operators
             // threads
             // https://github.com/WebAssembly/threads

crates/wasmparser/src/validator.rs

@@ -241,6 +241,8 @@ pub struct WasmFeatures {
     pub extended_const: bool,
     /// The WebAssembly component model proposal.
     pub component_model: bool,
+    /// The WebAssembly memory control proposal
+    pub memory_control: bool,
 }
 
 impl WasmFeatures {
@@ -277,6 +279,7 @@ impl Default for WasmFeatures {
             memory64: false,
             extended_const: false,
             component_model: false,
+            memory_control: false,
 
             // on-by-default features
             mutable_global: true,

crates/wasmparser/src/validator/operators.rs

@@ -936,6 +936,7 @@ macro_rules! validate_proposal {
     (desc sign_extension) => ("sign extension operations");
     (desc exceptions) => ("exceptions");
     (desc tail_call) => ("tail calls");
+    (desc memory_control) => ("memory control");
 }
 
 impl<'a, T> VisitOperator<'a> for WasmProposalValidator<'_, '_, T>
@@ -3018,6 +3019,12 @@ where
         self.pop_operand(Some(ty))?;
         Ok(())
     }
+    fn visit_memory_discard(&mut self, mem: u32) -> Self::Output {
+        let ty = self.check_memory_index(mem)?;
+        self.pop_operand(Some(ty))?;
+        self.pop_operand(Some(ty))?;
+        Ok(())
+    }
     fn visit_table_init(&mut self, segment: u32, table: u32) -> Self::Output {
         if table > 0 {}
         let table = match self.resources.table_at(table) {

crates/wasmprinter/src/operator.rs

@@ -279,6 +279,12 @@ macro_rules! define_visit {
             $self.memory_index($mem)?;
         }
     );
+    (payload $self:ident MemoryDiscard $mem:ident) => (
+        if $mem != 0 {
+            $self.push_str(" ");
+            $self.memory_index($mem)?;
+        }
+    );
     (payload $self:ident I32Const $val:ident) => (write!($self.result(), " {}", $val)?);
     (payload $self:ident I64Const $val:ident) => (write!($self.result(), " {}", $val)?);
     (payload $self:ident F32Const $val:ident) => (
@@ -362,6 +368,7 @@ macro_rules! define_visit {
     (name MemoryInit) => ("memory.init");
     (name MemoryCopy) => ("memory.copy");
     (name MemoryFill) => ("memory.fill");
+    (name MemoryDiscard) => ("memory.discard");
     (name DataDrop) => ("data.drop");
     (name ElemDrop) => ("elem.drop");
     (name TableInit) => ("table.init");

crates/wast/src/core/expr.rs

@@ -568,6 +568,7 @@ instructions! {
         MemoryInit(MemoryInit<'a>) : [0xfc, 0x08] : "memory.init",
         MemoryCopy(MemoryCopy<'a>) : [0xfc, 0x0a] : "memory.copy",
         MemoryFill(MemoryArg<'a>) : [0xfc, 0x0b] : "memory.fill",
+        MemoryDiscard(MemoryArg<'a>) : [0xfc, 0x12] : "memory.discard",
         DataDrop(Index<'a>) : [0xfc, 0x09] : "data.drop",
         ElemDrop(Index<'a>) : [0xfc, 0x0d] : "elem.drop",
         TableInit(TableInit<'a>) : [0xfc, 0x0c] : "table.init",

fuzz/fuzz_targets/validate.rs

@@ -36,8 +36,9 @@ fuzz_target!(|data: &[u8]| {
         mutable_global: (byte2 & 0b0010_0000) != 0,
         saturating_float_to_int: (byte2 & 0b0100_0000) != 0,
         sign_extension: (byte2 & 0b1000_0000) != 0,
+        memory_control: (byte3 & 0b0000_0001) != 0,
     });
-    let use_maybe_invalid = byte3 & 0b0000_0001 != 0;
+    let use_maybe_invalid = byte3 & 0b0000_0010 != 0;
 
     let wasm = &data[3..];
     if log::log_enabled!(log::Level::Debug) {

tests/local/memory-discard.wat

@@ -0,0 +1,7 @@
+(module
+  (memory i32 2)
+
+  (func
+    (memory.discard (i32.const 0) (i32.const 65536))
+  )
+)

tests/roundtrip.rs

@@ -537,6 +537,7 @@ impl TestState {
             saturating_float_to_int: true,
             sign_extension: true,
             mutable_global: true,
+            memory_control: true,
         };
         for part in test.iter().filter_map(|t| t.to_str()) {
             match part {

tests/snapshots/local/memory-discard.wat.print

@@ -0,0 +1,9 @@
+(module
+  (type (;0;) (func))
+  (func (;0;) (type 0)
+    i32.const 0
+    i32.const 65536
+    memory.discard
+  )
+  (memory (;0;) 2)
+)