Add DerivedFunction for splitting lane widths and doubling the number of lanes

Certain operations (e.g. x86_packss) will have operands with types like `NxM` but will return results with types like `(N/2)x(M*2)` (halve the lane width, double the number of lanes; maintain the same number of vector bits). This is equivalent to applying two `DerivedFunction`s to the type: `DerivedFunction::HalfWidth` then `DerivedFunction::DoubleVector`. Since there is no easy way to apply multiple `DerivedFunction`s (e.g. most of the logic is one-level deep, 1d5a678124/cranelift/codegen/meta/src/gen_inst.rs (L618-L621)), I added `DerivedFunction::SplitLanes` to do the necessary type conversion.

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

@@ -193,6 +193,24 @@ impl TypeVar {
                     "can't double 256 lanes"
                 );
             }
+            DerivedFunc::SplitLanes => {
+                assert!(
+                    ts.ints.is_empty() || *ts.ints.iter().min().unwrap() > 8,
+                    "can't halve all integer types"
+                );
+                assert!(
+                    ts.floats.is_empty() || *ts.floats.iter().min().unwrap() > 32,
+                    "can't halve all float types"
+                );
+                assert!(
+                    ts.bools.is_empty() || *ts.bools.iter().min().unwrap() > 8,
+                    "can't halve all boolean types"
+                );
+                assert!(
+                    *ts.lanes.iter().max().unwrap() < MAX_LANES,
+                    "can't double 256 lanes"
+                );
+            }
             DerivedFunc::LaneOf | DerivedFunc::AsBool => { /* no particular assertions */ }
         }
 
@@ -227,6 +245,9 @@ impl TypeVar {
     pub fn double_vector(&self) -> TypeVar {
         self.derived(DerivedFunc::DoubleVector)
     }
+    pub fn split_lanes(&self) -> TypeVar {
+        self.derived(DerivedFunc::SplitLanes)
+    }
 
     /// Constrain the range of types this variable can assume to a subset of those in the typeset
     /// ts.
@@ -333,6 +354,7 @@ pub(crate) enum DerivedFunc {
     DoubleWidth,
     HalfVector,
     DoubleVector,
+    SplitLanes,
 }
 
 impl DerivedFunc {
@@ -344,6 +366,7 @@ impl DerivedFunc {
             DerivedFunc::DoubleWidth => "double_width",
             DerivedFunc::HalfVector => "half_vector",
             DerivedFunc::DoubleVector => "double_vector",
+            DerivedFunc::SplitLanes => "split_lanes",
         }
     }
 
@@ -438,6 +461,7 @@ impl TypeSet {
             DerivedFunc::DoubleWidth => self.double_width(),
             DerivedFunc::HalfVector => self.half_vector(),
             DerivedFunc::DoubleVector => self.double_vector(),
+            DerivedFunc::SplitLanes => self.half_width().double_vector(),
         }
     }
 
@@ -577,6 +601,7 @@ impl TypeSet {
             DerivedFunc::DoubleWidth => self.half_width(),
             DerivedFunc::HalfVector => self.double_vector(),
             DerivedFunc::DoubleVector => self.half_vector(),
+            DerivedFunc::SplitLanes => self.half_vector().double_width(),
         }
     }
 

cranelift/codegen/src/ir/instructions.rs

@@ -518,6 +518,9 @@ enum OperandConstraint {
 
     /// This operand is `ctrlType.double_vector()`.
     DoubleVector,
+
+    /// This operand is `ctrlType.split_lanes()`.
+    SplitLanes,
 }
 
 impl OperandConstraint {
@@ -544,6 +547,11 @@ impl OperandConstraint {
                     .expect("invalid type for half_vector"),
             ),
             DoubleVector => Bound(ctrl_type.by(2).expect("invalid type for double_vector")),
+            SplitLanes => Bound(
+                ctrl_type
+                    .split_lanes()
+                    .expect("invalid type for split_lanes"),
+            ),
         }
     }
 }

cranelift/codegen/src/ir/types.rs

@@ -279,6 +279,16 @@ impl Type {
         }
     }
 
+    /// Split the lane width in half and double the number of lanes to maintain the same bit-width.
+    ///
+    /// If this is a scalar type of n bits, it produces a SIMD vector type of (n/2)x2.
+    pub fn split_lanes(self) -> Option<Self> {
+        match self.half_width() {
+            Some(half_width) => half_width.by(2),
+            None => None,
+        }
+    }
+
     /// Index of this type, for use with hash tables etc.
     pub fn index(self) -> usize {
         usize::from(self.0)