add crate for custom isa extensions

Signed-off-by: xermicus <cyrill@parity.io>

crates/extensions/src/lib.rs

@@ -0,0 +1,40 @@
+//! Custom RISC-V extension in PolkaVM that are partially supported.
+//! We use inline assembly to emit partially supported instructions.
+
+use inkwell::{context::Context, module::Module, support::LLVMString};
+
+include!(concat!(env!("OUT_DIR"), "/bswap.rs"));
+
+/// Returns a LLVM module containing a `__bswap` function, which
+/// - Takes a `i256` value argument
+/// - Byte swaps it using `rev8` from the `zbb` extension
+/// - Returns the `i256` value
+///
+/// Returns `Error` if the module fails to validate, which should never happen.
+pub fn module<'context>(
+    context: &'context Context,
+    module_name: &str,
+) -> Result<Module<'context>, LLVMString> {
+    let module = context.create_module(module_name);
+
+    module.set_inline_assembly(ASSEMBLY);
+    module.verify()?;
+
+    Ok(module)
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn assembly_contains_rev8_instruction() {
+        assert!(crate::ASSEMBLY.contains("rev8"));
+    }
+
+    #[test]
+    fn module_is_valid() {
+        inkwell::targets::Target::initialize_riscv(&Default::default());
+        let context = inkwell::context::Context::create();
+
+        assert!(crate::module(&context, "polkavm_bswap").is_ok());
+    }
+}