Use SmallVec in SwitchTargets

This allows building common SwitchTargets (eg. for `if`s) without
allocation.

Author: jonas-schievink

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -196,7 +196,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         mut bx: Bx,
         discr: &mir::Operand<'tcx>,
         switch_ty: Ty<'tcx>,
-        targets: &SwitchTargets<'tcx>,
+        targets: &SwitchTargets,
     ) {
         let discr = self.codegen_operand(&mut bx, &discr);
         // `switch_ty` is redundant, sanity-check that.

compiler/rustc_middle/src/mir/terminator/mod.rs

@@ -1,6 +1,7 @@
 use crate::mir::interpret::Scalar;
 use crate::ty::{self, Ty, TyCtxt};
 use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
+use smallvec::{smallvec, SmallVec};
 
 use super::{
     AssertMessage, BasicBlock, InlineAsmOperand, Operand, Place, SourceInfo, Successors,
@@ -17,10 +18,10 @@ use std::slice;
 pub use super::query::*;
 
 #[derive(Debug, Clone, TyEncodable, TyDecodable, HashStable, PartialEq)]
-pub struct SwitchTargets<'tcx> {
+pub struct SwitchTargets {
     /// Possible values. The locations to branch to in each case
     /// are found in the corresponding indices from the `targets` vector.
-    values: Cow<'tcx, [u128]>,
+    values: SmallVec<[u128; 1]>,
 
     /// Possible branch sites. The last element of this vector is used
     /// for the otherwise branch, so targets.len() == values.len() + 1
@@ -34,24 +35,24 @@ pub struct SwitchTargets<'tcx> {
     //
     // However we’ve decided to keep this as-is until we figure a case
     // where some other approach seems to be strictly better than other.
-    targets: Vec<BasicBlock>,
+    targets: SmallVec<[BasicBlock; 2]>,
 }
 
-impl<'tcx> SwitchTargets<'tcx> {
+impl SwitchTargets {
     /// Creates switch targets from an iterator of values and target blocks.
     ///
     /// The iterator may be empty, in which case the `SwitchInt` instruction is equivalent to
     /// `goto otherwise;`.
     pub fn new(targets: impl Iterator<Item = (u128, BasicBlock)>, otherwise: BasicBlock) -> Self {
-        let (values, mut targets): (Vec<_>, Vec<_>) = targets.unzip();
+        let (values, mut targets): (SmallVec<_>, SmallVec<_>) = targets.unzip();
         targets.push(otherwise);
         Self { values: values.into(), targets }
     }
 
     /// Builds a switch targets definition that jumps to `then` if the tested value equals `value`,
     /// and to `else_` if not.
-    pub fn static_if(value: &'static [u128; 1], then: BasicBlock, else_: BasicBlock) -> Self {
-        Self { values: Cow::Borrowed(value), targets: vec![then, else_] }
+    pub fn static_if(value: u128, then: BasicBlock, else_: BasicBlock) -> Self {
+        Self { values: smallvec![value], targets: smallvec![then, else_] }
     }
 
     /// Returns the fallback target that is jumped to when none of the values match the operand.
@@ -113,7 +114,7 @@ pub enum TerminatorKind<'tcx> {
         /// FIXME: remove this redundant information. Currently, it is relied on by pretty-printing.
         switch_ty: Ty<'tcx>,
 
-        targets: SwitchTargets<'tcx>,
+        targets: SwitchTargets,
     },
 
     /// Indicates that the landing pad is finished and unwinding should
@@ -295,7 +296,7 @@ impl<'tcx> TerminatorKind<'tcx> {
         TerminatorKind::SwitchInt {
             discr: cond,
             switch_ty: tcx.types.bool,
-            targets: SwitchTargets::static_if(&[0], f, t),
+            targets: SwitchTargets::static_if(0, f, t),
         }
     }
 

compiler/rustc_mir/src/dataflow/framework/direction.rs

@@ -514,7 +514,7 @@ impl Direction for Forward {
 
 struct SwitchIntEdgeEffectApplier<'a, D, F> {
     exit_state: &'a mut D,
-    targets: &'a SwitchTargets<'a>,
+    targets: &'a SwitchTargets,
     propagate: F,
 
     effects_applied: bool,

compiler/rustc_mir/src/transform/simplify_comparison_integral.rs

@@ -227,5 +227,5 @@ struct OptimizationInfo<'tcx> {
     /// Either Eq or Ne
     op: BinOp,
     /// Current targets used in the switch
-    targets: SwitchTargets<'tcx>,
+    targets: SwitchTargets,
 }

compiler/rustc_mir/src/util/elaborate_drops.rs

@@ -760,8 +760,6 @@ where
         let elem_size = Place::from(self.new_temp(tcx.types.usize));
         let len = Place::from(self.new_temp(tcx.types.usize));
 
-        static USIZE_SWITCH_ZERO: &[u128; 1] = &[0];
-
         let base_block = BasicBlockData {
             statements: vec![
                 self.assign(elem_size, Rvalue::NullaryOp(NullOp::SizeOf, ety)),
@@ -774,7 +772,7 @@ where
                     discr: move_(elem_size),
                     switch_ty: tcx.types.usize,
                     targets: SwitchTargets::static_if(
-                        USIZE_SWITCH_ZERO,
+                        0,
                         self.drop_loop_pair(ety, false, len),
                         self.drop_loop_pair(ety, true, len),
                     ),