Auto merge of rust-lang#120268 - DianQK:otherwise_is_last_variant_switchs, r=<try>

Replace the default branch with an unreachable branch If it is the last variant

Fixes rust-lang#119520.

LLVM currently has limited ability to eliminate dead branches in switches, even with the patch of llvm/llvm-project#73446.

The main reasons are as follows:

- Additional costs are required to calculate the range of values, and there exist many scenarios that cannot be analyzed accurately.
- Matching values by bitwise calculation cannot handle odd branches, nor can it handle values like `-1, 0, 1`. See [SimplifyCFG.cpp#L5424](https://github.com/llvm/llvm-project/blob/llvmorg-17.0.6/llvm/lib/Transforms/Utils/SimplifyCFG.cpp#L5424) and https://llvm.godbolt.org/z/qYMqhvMa8
- The current range information is continuous, even if the metadata for the range is submitted. See [ConstantRange.cpp#L1869-L1870](https://github.com/llvm/llvm-project/blob/llvmorg-17.0.6/llvm/lib/IR/ConstantRange.cpp#L1869-L1870).
- The metadata of the range may be lost in passes such as SROA. See https://rust.godbolt.org/z/e7f87vKMK.

Although we can make improvements, I think it would be more appropriate to put this issue to rustc first. After all, we can easily know the possible values.

Note that we've currently found a slow compilation problem in the presence of unreachable branches. See
llvm/llvm-project#78578.

r? compiler

Author: bors

compiler/rustc_middle/src/mir/terminator.rs

@@ -74,6 +74,17 @@ impl SwitchTargets {
     pub fn target_for_value(&self, value: u128) -> BasicBlock {
         self.iter().find_map(|(v, t)| (v == value).then_some(t)).unwrap_or_else(|| self.otherwise())
     }
+
+    /// Adds a new target to the switch. But You cannot add an already present value.
+    #[inline]
+    pub fn add_target(&mut self, value: u128, bb: BasicBlock) {
+        let value = Pu128(value);
+        if self.values.contains(&value) {
+            bug!("target value {:?} already present", value);
+        }
+        self.values.push(value);
+        self.targets.insert(self.targets.len() - 1, bb);
+    }
 }
 
 pub struct SwitchTargetsIter<'a> {

compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs

@@ -78,6 +78,7 @@ impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
         trace!("UninhabitedEnumBranching starting for {:?}", body.source);
 
         let mut removable_switchs = Vec::new();
+        let mut otherwise_is_last_variant_switchs = Vec::new();
 
         for (bb, bb_data) in body.basic_blocks.iter_enumerated() {
             trace!("processing block {:?}", bb);
@@ -92,8 +93,14 @@ impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
                 tcx.param_env_reveal_all_normalized(body.source.def_id()).and(discriminant_ty),
             );
 
-            let allowed_variants = if let Ok(layout) = layout {
+            let mut allowed_variants = if let Ok(layout) = layout {
                 variant_discriminants(&layout, discriminant_ty, tcx)
+            } else if let Some(variant_range) = discriminant_ty.variant_range(tcx) {
+                variant_range
+                    .map(|variant| {
+                        discriminant_ty.discriminant_for_variant(tcx, variant).unwrap().val
+                    })
+                    .collect()
             } else {
                 continue;
             };
@@ -103,20 +110,29 @@ impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
             let terminator = bb_data.terminator();
             let TerminatorKind::SwitchInt { targets, .. } = &terminator.kind else { bug!() };
 
-            let mut reachable_count = 0;
             for (index, (val, _)) in targets.iter().enumerate() {
-                if allowed_variants.contains(&val) {
-                    reachable_count += 1;
-                } else {
+                if !allowed_variants.remove(&val) {
                     removable_switchs.push((bb, index));
                 }
             }
 
-            if reachable_count == allowed_variants.len() {
+            if allowed_variants.is_empty() {
                 removable_switchs.push((bb, targets.iter().count()));
+            } else if allowed_variants.len() == 1 {
+                #[allow(rustc::potential_query_instability)]
+                let last_variant = *allowed_variants.iter().next().unwrap();
+                otherwise_is_last_variant_switchs.push((bb, last_variant));
             }
         }
 
+        for (bb, last_variant) in otherwise_is_last_variant_switchs {
+            let bb_data = &mut body.basic_blocks.as_mut()[bb];
+            let terminator = bb_data.terminator_mut();
+            let TerminatorKind::SwitchInt { targets, .. } = &mut terminator.kind else { bug!() };
+            targets.add_target(last_variant, targets.otherwise());
+            removable_switchs.push((bb, targets.iter().count()));
+        }
+
         if removable_switchs.is_empty() {
             return;
         }

tests/codegen/enum/uninhabited_enum_default_branch.rs

@@ -0,0 +1,24 @@
+// compile-flags: -O
+
+#![crate_type = "lib"]
+
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Int(u32);
+
+const A: Int = Int(201);
+const B: Int = Int(270);
+const C: Int = Int(153);
+
+// CHECK-LABEL: @foo
+// CHECK-SAME: [[TMP0:%.*]])
+// CHECK-NEXT:  start:
+// CHECK-NEXT:    [[TMP1:%.*]] = add i32 [[TMP0]], -201
+// CHECK-NEXT:    [[OR_COND:%.*]] = icmp ult i32 [[TMP1]], 70
+// CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i32 [[TMP0]], 153
+// CHECK-NEXT:    [[SPEC_SELECT:%.*]] = or i1 [[OR_COND]], [[TMP2]]
+// CHECK-NEXT:    ret i1 [[SPEC_SELECT]]
+#[no_mangle]
+pub fn foo(x: Int) -> bool {
+    (x >= A && x <= B)
+        || x == C
+}

tests/mir-opt/funky_arms.float_to_exponential_common.GVN.panic-abort.diff

@@ -69,7 +69,7 @@
           StorageLive(_6);
           _6 = ((*_1).4: std::option::Option<usize>);
           _7 = discriminant(_6);
-          switchInt(move _7) -> [1: bb4, otherwise: bb6];
+          switchInt(move _7) -> [1: bb4, 0: bb6, otherwise: bb9];
       }
 
       bb4: {
@@ -135,5 +135,9 @@
           StorageDead(_6);
           return;
       }
+  
+      bb9: {
+          unreachable;
+      }
   }
 

tests/mir-opt/funky_arms.float_to_exponential_common.GVN.panic-unwind.diff

@@ -69,7 +69,7 @@
           StorageLive(_6);
           _6 = ((*_1).4: std::option::Option<usize>);
           _7 = discriminant(_6);
-          switchInt(move _7) -> [1: bb4, otherwise: bb6];
+          switchInt(move _7) -> [1: bb4, 0: bb6, otherwise: bb9];
       }
 
       bb4: {
@@ -135,5 +135,9 @@
           StorageDead(_6);
           return;
       }
+  
+      bb9: {
+          unreachable;
+      }
   }
 

tests/mir-opt/issues/issue_59352.num_to_digit.PreCodegen.after.panic-abort.mir

@@ -33,21 +33,21 @@ fn num_to_digit(_1: char) -> u32 {
         _3 = &_2;
         StorageLive(_4);
         _4 = discriminant(_2);
-        StorageDead(_3);
-        StorageDead(_2);
-        switchInt(move _4) -> [1: bb2, otherwise: bb7];
+        switchInt(move _4) -> [1: bb2, 0: bb6, otherwise: bb8];
     }
 
     bb2: {
         StorageDead(_4);
+        StorageDead(_3);
+        StorageDead(_2);
         StorageLive(_5);
         _5 = char::methods::<impl char>::to_digit(move _1, const 8_u32) -> [return: bb3, unwind unreachable];
     }
 
     bb3: {
         StorageLive(_6);
         _6 = discriminant(_5);
-        switchInt(move _6) -> [0: bb4, 1: bb5, otherwise: bb6];
+        switchInt(move _6) -> [0: bb4, 1: bb5, otherwise: bb8];
     }
 
     bb4: {
@@ -58,20 +58,22 @@ fn num_to_digit(_1: char) -> u32 {
         _0 = move ((_5 as Some).0: u32);
         StorageDead(_6);
         StorageDead(_5);
-        goto -> bb8;
+        goto -> bb7;
     }
 
     bb6: {
-        unreachable;
+        StorageDead(_4);
+        StorageDead(_3);
+        StorageDead(_2);
+        _0 = const 0_u32;
+        goto -> bb7;
     }
 
     bb7: {
-        StorageDead(_4);
-        _0 = const 0_u32;
-        goto -> bb8;
+        return;
     }
 
     bb8: {
-        return;
+        unreachable;
     }
 }

tests/mir-opt/issues/issue_59352.num_to_digit.PreCodegen.after.panic-unwind.mir

@@ -33,21 +33,21 @@ fn num_to_digit(_1: char) -> u32 {
         _3 = &_2;
         StorageLive(_4);
         _4 = discriminant(_2);
-        StorageDead(_3);
-        StorageDead(_2);
-        switchInt(move _4) -> [1: bb2, otherwise: bb7];
+        switchInt(move _4) -> [1: bb2, 0: bb6, otherwise: bb8];
     }
 
     bb2: {
         StorageDead(_4);
+        StorageDead(_3);
+        StorageDead(_2);
         StorageLive(_5);
         _5 = char::methods::<impl char>::to_digit(move _1, const 8_u32) -> [return: bb3, unwind continue];
     }
 
     bb3: {
         StorageLive(_6);
         _6 = discriminant(_5);
-        switchInt(move _6) -> [0: bb4, 1: bb5, otherwise: bb6];
+        switchInt(move _6) -> [0: bb4, 1: bb5, otherwise: bb8];
     }
 
     bb4: {
@@ -58,20 +58,22 @@ fn num_to_digit(_1: char) -> u32 {
         _0 = move ((_5 as Some).0: u32);
         StorageDead(_6);
         StorageDead(_5);
-        goto -> bb8;
+        goto -> bb7;
     }
 
     bb6: {
-        unreachable;
+        StorageDead(_4);
+        StorageDead(_3);
+        StorageDead(_2);
+        _0 = const 0_u32;
+        goto -> bb7;
     }
 
     bb7: {
-        StorageDead(_4);
-        _0 = const 0_u32;
-        goto -> bb8;
+        return;
     }
 
     bb8: {
-        return;
+        unreachable;
     }
 }

tests/mir-opt/simplify_locals_fixedpoint.foo.SimplifyLocals-final.panic-abort.diff

@@ -23,12 +23,12 @@
           StorageDead(_3);
           StorageDead(_2);
           _5 = discriminant((_1.0: std::option::Option<u8>));
-          switchInt(move _5) -> [1: bb1, otherwise: bb3];
+          switchInt(move _5) -> [1: bb1, 0: bb3, otherwise: bb5];
       }
 
       bb1: {
           _4 = discriminant((_1.1: std::option::Option<T>));
-          switchInt(move _4) -> [0: bb2, otherwise: bb3];
+          switchInt(move _4) -> [0: bb2, 1: bb3, otherwise: bb5];
       }
 
       bb2: {
@@ -46,5 +46,9 @@
           StorageDead(_1);
           return;
       }
+  
+      bb5: {
+          unreachable;
+      }
   }
 

tests/mir-opt/simplify_locals_fixedpoint.foo.SimplifyLocals-final.panic-unwind.diff

@@ -23,12 +23,12 @@
           StorageDead(_3);
           StorageDead(_2);
           _5 = discriminant((_1.0: std::option::Option<u8>));
-          switchInt(move _5) -> [1: bb1, otherwise: bb3];
+          switchInt(move _5) -> [1: bb1, 0: bb3, otherwise: bb5];
       }
 
       bb1: {
           _4 = discriminant((_1.1: std::option::Option<T>));
-          switchInt(move _4) -> [0: bb2, otherwise: bb3];
+          switchInt(move _4) -> [0: bb2, 1: bb3, otherwise: bb5];
       }
 
       bb2: {
@@ -46,5 +46,9 @@
           StorageDead(_1);
           return;
       }
+  
+      bb5: {
+          unreachable;
+      }
   }