Pick argument and return types based on opcode constraints (#5947)

* Pick argument and return types based on opcode constraints

Co-authored-by: Jamey Sharp <jsharp@fastly.com>

* Lazily build the OPCODE_SIGNATURES list

* Skip unsupported isplit/iconcat cases

* Add an issue reference for the isplit/iconcat exemption

* Refactor the deny lists to use exceptions!, and remove redundant entries

---------

Co-authored-by: Jamey Sharp <jsharp@fastly.com>

Cargo.lock

@@ -651,6 +651,7 @@ dependencies = [
  "arbitrary",
  "cranelift",
  "cranelift-native",
+ "once_cell",
  "target-lexicon",
 ]
 

cranelift/fuzzgen/Cargo.toml

@@ -16,4 +16,5 @@ cranelift-native = { workspace = true }
 
 anyhow = { workspace = true }
 arbitrary = "1.0.0"
+once_cell = { workspace = true }
 target-lexicon = { workspace = true, features = ["std"] }

cranelift/fuzzgen/src/cranelift_arbitrary.rs

@@ -23,23 +23,26 @@ pub trait CraneliftArbitrary {
     fn datavalue(&mut self, ty: Type) -> Result<DataValue>;
 }
 
+/// Returns the set of types that are valid for value generation, dependent on architecture.
+pub fn types_for_architecture(architecture: Architecture) -> &'static [Type] {
+    // TODO: It would be nice if we could get these directly from cranelift
+    // TODO: RISCV does not support SIMD yet
+    let supports_simd = !matches!(architecture, Architecture::Riscv64(_));
+    if supports_simd {
+        &[
+            I8, I16, I32, I64, I128, // Scalar Integers
+            F32, F64, // Scalar Floats
+            I8X16, I16X8, I32X4, I64X2, // SIMD Integers
+            F32X4, F64X2, // SIMD Floats
+        ]
+    } else {
+        &[I8, I16, I32, I64, I128, F32, F64]
+    }
+}
+
 impl<'a> CraneliftArbitrary for &mut Unstructured<'a> {
     fn _type(&mut self, architecture: Architecture) -> Result<Type> {
-        // TODO: It would be nice if we could get these directly from cranelift
-        // TODO: RISCV does not support SIMD yet
-        let supports_simd = !matches!(architecture, Architecture::Riscv64(_));
-        let choices = if supports_simd {
-            &[
-                I8, I16, I32, I64, I128, // Scalar Integers
-                F32, F64, // Scalar Floats
-                I8X16, I16X8, I32X4, I64X2, // SIMD Integers
-                F32X4, F64X2, // SIMD Floats
-            ][..]
-        } else {
-            &[I8, I16, I32, I64, I128, F32, F64][..]
-        };
-
-        Ok(*self.choose(choices)?)
+        Ok(*self.choose(types_for_architecture(architecture))?)
     }
 
     fn callconv(&mut self) -> Result<CallConv> {