Avoid lowering code under dead SwitchInt targets

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -1179,6 +1179,16 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         }
     }
 
+    pub fn codegen_block_as_unreachable(&mut self, bb: mir::BasicBlock) {
+        let llbb = match self.try_llbb(bb) {
+            Some(llbb) => llbb,
+            None => return,
+        };
+        let bx = &mut Bx::build(self.cx, llbb);
+        debug!("codegen_block_as_unreachable({:?})", bb);
+        bx.unreachable();
+    }
+
     fn codegen_terminator(
         &mut self,
         bx: &mut Bx,

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -256,13 +256,22 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // Apply debuginfo to the newly allocated locals.
     fx.debug_introduce_locals(&mut start_bx);
 
+    let reachable_blocks = mir.reachable_blocks_in_mono(cx.tcx(), instance);
+
     // The builders will be created separately for each basic block at `codegen_block`.
     // So drop the builder of `start_llbb` to avoid having two at the same time.
     drop(start_bx);
 
     // Codegen the body of each block using reverse postorder
     for (bb, _) in traversal::reverse_postorder(mir) {
-        fx.codegen_block(bb);
+        if reachable_blocks.contains(bb) {
+            fx.codegen_block(bb);
+        } else {
+            // This may have references to things we didn't monomorphize, so we
+            // don't actually codegen the body. We still create the block so
+            // terminators in other blocks can reference it without worry.
+            fx.codegen_block_as_unreachable(bb);
+        }
     }
 }
 

compiler/rustc_middle/src/mir/mod.rs

@@ -10,7 +10,7 @@ use crate::ty::print::{pretty_print_const, with_no_trimmed_paths};
 use crate::ty::print::{FmtPrinter, Printer};
 use crate::ty::visit::TypeVisitableExt;
 use crate::ty::{self, List, Ty, TyCtxt};
-use crate::ty::{AdtDef, InstanceDef, UserTypeAnnotationIndex};
+use crate::ty::{AdtDef, Instance, InstanceDef, UserTypeAnnotationIndex};
 use crate::ty::{GenericArg, GenericArgsRef};
 
 use rustc_data_structures::captures::Captures;
@@ -29,6 +29,7 @@ pub use rustc_ast::Mutability;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::graph::dominators::Dominators;
+use rustc_index::bit_set::BitSet;
 use rustc_index::{Idx, IndexSlice, IndexVec};
 use rustc_serialize::{Decodable, Encodable};
 use rustc_span::symbol::Symbol;
@@ -642,6 +643,73 @@ impl<'tcx> Body<'tcx> {
         self.injection_phase.is_some()
     }
 
+    /// Finds which basic blocks are actually reachable for a specific
+    /// monomorphization of this body.
+    ///
+    /// This is allowed to have false positives; just because this says a block
+    /// is reachable doesn't mean that's necessarily true. It's thus always
+    /// legal for this to return a filled set.
+    ///
+    /// Regardless, the [`BitSet::domain_size`] of the returned set will always
+    /// exactly match the number of blocks in the body so that `contains`
+    /// checks can be done without worrying about panicking.
+    ///
+    /// The main case this supports is filtering out `if <T as Trait>::CONST`
+    /// bodies that can't be removed in generic MIR, but *can* be removed once
+    /// the specific `T` is known.
+    ///
+    /// This is used in the monomorphization collector as well as in codegen.
+    pub fn reachable_blocks_in_mono(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        instance: Instance<'tcx>,
+    ) -> BitSet<BasicBlock> {
+        if instance.args.non_erasable_generics(tcx, instance.def_id()).next().is_none() {
+            // If it's non-generic, then mir-opt const prop has already run, meaning it's
+            // probably not worth doing any further filtering. So call everything reachable.
+            return BitSet::new_filled(self.basic_blocks.len());
+        }
+
+        let mut set = BitSet::new_empty(self.basic_blocks.len());
+        self.reachable_blocks_in_mono_from(tcx, instance, &mut set, START_BLOCK);
+        set
+    }
+
+    fn reachable_blocks_in_mono_from(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        instance: Instance<'tcx>,
+        set: &mut BitSet<BasicBlock>,
+        bb: BasicBlock,
+    ) {
+        if !set.insert(bb) {
+            return;
+        }
+
+        let data = &self.basic_blocks[bb];
+
+        if let TerminatorKind::SwitchInt { discr: Operand::Constant(constant), targets } =
+            &data.terminator().kind
+        {
+            let env = ty::ParamEnv::reveal_all();
+            let mono_literal = instance.instantiate_mir_and_normalize_erasing_regions(
+                tcx,
+                env,
+                crate::ty::EarlyBinder::bind(constant.const_),
+            );
+            if let Some(bits) = mono_literal.try_eval_bits(tcx, env) {
+                let target = targets.target_for_value(bits);
+                return self.reachable_blocks_in_mono_from(tcx, instance, set, target);
+            } else {
+                bug!("Couldn't evaluate constant {:?} in mono {:?}", constant, instance);
+            }
+        }
+
+        for target in data.terminator().successors() {
+            self.reachable_blocks_in_mono_from(tcx, instance, set, target);
+        }
+    }
+
     /// For a `Location` in this scope, determine what the "caller location" at that point is. This
     /// is interesting because of inlining: the `#[track_caller]` attribute of inlined functions
     /// must be honored. Falls back to the `tracked_caller` value for `#[track_caller]` functions,

compiler/rustc_middle/src/mir/traversal.rs

@@ -1,5 +1,3 @@
-use rustc_index::bit_set::BitSet;
-
 use super::*;
 
 /// Preorder traversal of a graph.

compiler/rustc_mir_transform/src/lib.rs

@@ -108,6 +108,7 @@ mod check_alignment;
 pub mod simplify;
 mod simplify_branches;
 mod simplify_comparison_integral;
+mod simplify_if_const;
 mod sroa;
 mod uninhabited_enum_branching;
 mod unreachable_prop;
@@ -616,6 +617,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &large_enums::EnumSizeOpt { discrepancy: 128 },
             // Some cleanup necessary at least for LLVM and potentially other codegen backends.
             &add_call_guards::CriticalCallEdges,
+            &simplify_if_const::SimplifyIfConst,
             // Cleanup for human readability, off by default.
             &prettify::ReorderBasicBlocks,
             &prettify::ReorderLocals,

compiler/rustc_mir_transform/src/simplify_if_const.rs

@@ -0,0 +1,75 @@
+//! A pass that simplifies branches when their condition is known.
+
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+
+/// The lowering for `if CONST` produces
+/// ```
+/// _1 = Const(...);
+/// switchInt (move _1)
+/// ```
+/// so this pass replaces that with
+/// ```
+/// switchInt (Const(...))
+/// ```
+/// so that further MIR consumers can special-case it more easily.
+///
+/// Unlike ConstProp, this supports generic constants too, not just concrete ones.
+pub struct SimplifyIfConst;
+
+impl<'tcx> MirPass<'tcx> for SimplifyIfConst {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        for block in body.basic_blocks_mut() {
+            simplify_assign_move_switch(tcx, block);
+        }
+    }
+}
+
+fn simplify_assign_move_switch(tcx: TyCtxt<'_>, block: &mut BasicBlockData<'_>) {
+    let Some(Terminator { kind: TerminatorKind::SwitchInt { discr: switch_desc, .. }, .. }) =
+        &mut block.terminator
+    else {
+        return;
+    };
+
+    let &mut Operand::Move(switch_place) = &mut *switch_desc else { return };
+
+    let Some(switch_local) = switch_place.as_local() else { return };
+
+    let Some(last_statement) = block.statements.last_mut() else { return };
+
+    let StatementKind::Assign(boxed_place_rvalue) = &last_statement.kind else { return };
+
+    let Some(assigned_local) = boxed_place_rvalue.0.as_local() else { return };
+
+    if switch_local != assigned_local {
+        return;
+    }
+
+    if !matches!(boxed_place_rvalue.1, Rvalue::Use(Operand::Constant(_))) {
+        return;
+    }
+
+    let should_optimize = tcx.consider_optimizing(|| {
+        format!(
+            "SimplifyBranches - Assignment: {:?} SourceInfo: {:?}",
+            boxed_place_rvalue, last_statement.source_info
+        )
+    });
+
+    if should_optimize {
+        let Some(last_statement) = block.statements.pop() else {
+            bug!("Somehow the statement disappeared?");
+        };
+
+        let StatementKind::Assign(boxed_place_rvalue) = last_statement.kind else {
+            bug!("Somehow it's not an assignment any more?");
+        };
+
+        let Rvalue::Use(assigned_constant @ Operand::Constant(_)) = boxed_place_rvalue.1 else {
+            bug!("Somehow it's not a use of a constant any more?");
+        };
+
+        *switch_desc = assigned_constant;
+    }
+}

tests/codegen/skip-mono-inside-if-false.rs

@@ -0,0 +1,39 @@
+// compile-flags: -O -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+
+#[no_mangle]
+pub fn demo_for_i32() {
+    generic_impl::<i32>();
+}
+
+// Two important things here:
+// - We replace the "then" block with `unreachable` to avoid linking problems
+// - We neither declare nor define the `big_impl` that said block "calls".
+
+// CHECK-LABEL: ; skip_mono_inside_if_false::generic_impl
+// CHECK: start:
+// CHECK-NEXT: br i1 false, label %[[THEN_BRANCH:bb[0-9]+]], label %[[ELSE_BRANCH:bb[0-9]+]]
+// CHECK: [[ELSE_BRANCH]]:
+// CHECK-NEXT: call skip_mono_inside_if_false::small_impl
+// CHECK: [[THEN_BRANCH]]:
+// CHECK-NEXT: unreachable
+
+fn generic_impl<T>() {
+    trait MagicTrait {
+        const IS_BIG: bool;
+    }
+    impl<T> MagicTrait for T {
+        const IS_BIG: bool = std::mem::size_of::<T>() > 10;
+    }
+    if T::IS_BIG {
+        big_impl::<T>();
+    } else {
+        small_impl::<T>();
+    }
+}
+
+#[inline(never)]
+fn small_impl<T>() {}
+#[inline(never)]
+fn big_impl<T>() {}