Auto merge of #120268 - DianQK:otherwise_is_last_variant_switchs, r=oli-obk

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

Fixes #119520. Fixes #110097.

LLVM currently has limited ability to eliminate dead branches in switches, even with the patch of https://github.com/llvm/llvm-project/issues/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
https://github.com/llvm/llvm-project/issues/78578.

r? compiler

compiler/rustc_middle/src/mir/patch.rs

@@ -11,6 +11,8 @@ pub struct MirPatch<'tcx> {
     resume_block: Option<BasicBlock>,
     // Only for unreachable in cleanup path.
     unreachable_cleanup_block: Option<BasicBlock>,
+    // Only for unreachable not in cleanup path.
+    unreachable_no_cleanup_block: Option<BasicBlock>,
     // Cached block for UnwindTerminate (with reason)
     terminate_block: Option<(BasicBlock, UnwindTerminateReason)>,
     body_span: Span,
@@ -27,6 +29,7 @@ impl<'tcx> MirPatch<'tcx> {
             next_local: body.local_decls.len(),
             resume_block: None,
             unreachable_cleanup_block: None,
+            unreachable_no_cleanup_block: None,
             terminate_block: None,
             body_span: body.span,
         };
@@ -43,9 +46,12 @@ impl<'tcx> MirPatch<'tcx> {
             // Check if we already have an unreachable block
             if matches!(block.terminator().kind, TerminatorKind::Unreachable)
                 && block.statements.is_empty()
-                && block.is_cleanup
             {
-                result.unreachable_cleanup_block = Some(bb);
+                if block.is_cleanup {
+                    result.unreachable_cleanup_block = Some(bb);
+                } else {
+                    result.unreachable_no_cleanup_block = Some(bb);
+                }
                 continue;
             }
 
@@ -95,6 +101,23 @@ impl<'tcx> MirPatch<'tcx> {
         bb
     }
 
+    pub fn unreachable_no_cleanup_block(&mut self) -> BasicBlock {
+        if let Some(bb) = self.unreachable_no_cleanup_block {
+            return bb;
+        }
+
+        let bb = self.new_block(BasicBlockData {
+            statements: vec![],
+            terminator: Some(Terminator {
+                source_info: SourceInfo::outermost(self.body_span),
+                kind: TerminatorKind::Unreachable,
+            }),
+            is_cleanup: false,
+        });
+        self.unreachable_no_cleanup_block = Some(bb);
+        bb
+    }
+
     pub fn terminate_block(&mut self, reason: UnwindTerminateReason) -> BasicBlock {
         if let Some((cached_bb, cached_reason)) = self.terminate_block
             && reason == cached_reason

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> {