[meta] Legalization: don't emit a spurious if true for transforms that always apply;

This enhances readability of the generated legalizer code by replacing
`if true { body }` with `body`.

cranelift/codegen/meta/src/cdsl/instructions.rs

@@ -1169,17 +1169,18 @@ impl InstructionPredicate {
         self
     }
 
-    pub fn rust_predicate(&self, func_str: &str) -> String {
-        match &self.node {
-            Some(root) => root.rust_predicate(func_str),
-            None => "true".into(),
-        }
+    pub fn rust_predicate(&self, func_str: &str) -> Option<String> {
+        self.node.as_ref().map(|root| root.rust_predicate(func_str))
     }
 
-    /// Returns true if the predicate only depends on type parameters (and not on an instruction
-    /// format).
-    pub fn is_type_predicate(&self) -> bool {
-        self.node.as_ref().unwrap().is_type_predicate()
+    /// Returns the type predicate if this is one, or None otherwise.
+    pub fn type_predicate(&self, func_str: &str) -> Option<String> {
+        let node = self.node.as_ref().unwrap();
+        if node.is_type_predicate() {
+            Some(node.rust_predicate(func_str))
+        } else {
+            None
+        }
     }
 
     /// Returns references to all the nodes that are leaves in the condition (i.e. by flattening

cranelift/codegen/meta/src/gen_encodings.rs

@@ -76,9 +76,9 @@ use crate::unique_table::UniqueSeqTable;
 /// The generated code is an `if let` pattern match that falls through if the instruction has an
 /// unexpected format. This should lead to a panic.
 fn emit_instp(instp: &InstructionPredicate, has_func: bool, fmt: &mut Formatter) {
-    if instp.is_type_predicate() {
+    if let Some(type_predicate) = instp.type_predicate("func") {
         fmt.line("let args = inst.arguments(&func.dfg.value_lists);");
-        fmt.line(instp.rust_predicate("func"));
+        fmt.line(type_predicate);
         return;
     }
 
@@ -127,7 +127,7 @@ fn emit_instp(instp: &InstructionPredicate, has_func: bool, fmt: &mut Formatter)
             // Silence dead argument.
             fmt.line("let _ = func;");
         }
-        fmtln!(fmt, "return {};", instp.rust_predicate("func"));
+        fmtln!(fmt, "return {};", instp.rust_predicate("func").unwrap());
     });
     fmtln!(fmt, "}");
 

cranelift/codegen/meta/src/gen_legalizer.rs

@@ -452,7 +452,11 @@ fn gen_transform<'a>(
     if has_extra_constraints {
         // Extra constraints rely on the predicate being a variable that we can rebind as we add
         // more constraint predicates.
-        fmt.multi_line(&format!("let predicate = {};", inst_predicate));
+        if let Some(pred) = &inst_predicate {
+            fmt.multi_line(&format!("let predicate = {};", pred));
+        } else {
+            fmt.line("let predicate = true;");
+        }
     }
 
     // Emit any runtime checks; these will rebind `predicate` emitted right above.
@@ -460,13 +464,7 @@ fn gen_transform<'a>(
         emit_runtime_typecheck(constraint, type_sets, fmt);
     }
 
-    // Guard the actual expansion by `predicate`.
-    if has_extra_constraints {
-        fmt.line("if predicate {");
-    } else {
-        fmt.multi_line(&format!("if {} {{", inst_predicate))
-    }
-    fmt.indent(|fmt| {
+    let do_expand = |fmt: &mut Formatter| {
         // Emit any constants that must be created before use.
         for (name, value) in transform.const_pool.iter() {
             fmtln!(
@@ -538,8 +536,25 @@ fn gen_transform<'a>(
         }
 
         fmt.line("return true;");
+    };
+
+    // Guard the actual expansion by `predicate`.
+    if has_extra_constraints {
+        fmt.line("if predicate {");
+        fmt.indent(|fmt| {
+            do_expand(fmt);
         });
         fmt.line("}");
+    } else if let Some(pred) = &inst_predicate {
+        fmt.multi_line(&format!("if {} {{", pred));
+        fmt.indent(|fmt| {
+            do_expand(fmt);
+        });
+        fmt.line("}");
+    } else {
+        // Unconditional transform (there was no predicate), just emit it.
+        do_expand(fmt);
+    }
 }
 
 fn gen_transform_group<'a>(