ISLE: Build in maximum limit for multi constructor returns (#7526)

Allows users to avoid runaway rules that match too much stuff.

No statistically significant speed up to compilation on sightglass, but good to
have as a safeguard anyways.

Fixes #7500

cranelift/codegen/src/opts.rs

@@ -23,7 +23,16 @@ pub type Range = (usize, usize);
 pub type ValueArray2 = [Value; 2];
 pub type ValueArray3 = [Value; 3];
 
-pub type ConstructorVec<T> = SmallVec<[T; 8]>;
+const MAX_ISLE_RETURNS: usize = 8;
+
+pub type ConstructorVec<T> = SmallVec<[T; MAX_ISLE_RETURNS]>;
+
+impl<T: smallvec::Array> generated_code::Length for SmallVec<T> {
+    #[inline]
+    fn len(&self) -> usize {
+        SmallVec::len(self)
+    }
+}
 
 pub(crate) mod generated_code;
 use generated_code::{ContextIter, IntoContextIter};

cranelift/isle/isle/isle_examples/link/multi_constructor_main.rs

@@ -3,6 +3,8 @@ use multi_constructor::{ContextIter, IntoContextIter};
 
 struct Context;
 
+const MAX_ISLE_RETURNS: usize = 100;
+
 #[derive(Default)]
 struct It {
     i: u32,

cranelift/isle/isle/isle_examples/link/multi_extractor_main.rs

@@ -2,6 +2,8 @@ mod multi_extractor;
 
 use multi_extractor::{ContextIter, IntoContextIter};
 
+const MAX_ISLE_RETURNS: usize = 100;
+
 #[derive(Clone)]
 pub enum A {
     B,

cranelift/isle/isle/src/codegen.rs

@@ -236,6 +236,16 @@ pub trait IntoContextIter {{
     fn into_context_iter(self) -> Self::IntoIter;
 }}
 
+pub trait Length {{
+    fn len(&self) -> usize;
+}}
+
+impl<T> Length for std::vec::Vec<T> {{
+    fn len(&self) -> usize {{
+        std::vec::Vec::len(self)
+    }}
+}}
+
 pub struct ContextIterWrapper<I, C> {{
     iter: I,
     _ctx: std::marker::PhantomData<C>,
@@ -289,6 +299,11 @@ impl<T, E: Extend<T>, C> Extend<T> for ContextIterWrapper<E, C> {{
     fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {{
         self.iter.extend(iter);
     }}
+}}
+impl<L: Length, C> Length for ContextIterWrapper<L, C> {{
+    fn len(&self) -> usize {{
+        self.iter.len()
+    }}
 }}
            "#,
         )
@@ -406,7 +421,7 @@ impl<T, E: Extend<T>, C> Extend<T> for ContextIterWrapper<E, C> {{
             if let ReturnKind::Iterator = sig.ret_kind {
                 writeln!(
                     ctx.out,
-                    "{}    returns: &mut impl Extend<{}>,",
+                    "{}    returns: &mut (impl Extend<{}> + Length),",
                     &ctx.indent, ret
                 )?;
             }
@@ -635,7 +650,14 @@ impl<T, E: Extend<T>, C> Extend<T> for ContextIterWrapper<E, C> {{
                     match ret_kind {
                         ReturnKind::Plain => writeln!(ctx.out, ";")?,
                         ReturnKind::Option => writeln!(ctx.out, ");")?,
-                        ReturnKind::Iterator => writeln!(ctx.out, "));")?,
+                        ReturnKind::Iterator => {
+                            writeln!(ctx.out, "));")?;
+                            writeln!(
+                                ctx.out,
+                                "{}if returns.len() >= MAX_ISLE_RETURNS {{ return; }}",
+                                ctx.indent
+                            )?;
+                        }
                     }
                 }
             }