Auto merge of #129283 - saethlin:unreachable-allocas, r=scottmcm

Don't alloca for unused locals

We already have a concept of mono-unreachable basic blocks; this is primarily useful for ensuring that we do not compile code under an `if false`. But since we never gave locals the same analysis, a large local only used under an `if false` will still have stack space allocated for it.

There are 3 places we traverse MIR during monomorphization: Inside the collector, `non_ssa_locals`, and the walk to generate code. Unfortunately, https://github.com/rust-lang/rust/pull/129283#issuecomment-2297925578 indicates that we cannot afford the expense of tracking reachable locals during the collector's traversal, so we do need at least two mono-reachable traversals. And of course caching is of no help here because the benchmarks that regress are incr-unchanged; they don't do any codegen.

This fixes the second problem in https://github.com/rust-lang/rust/issues/129282, and brings us anther step toward `const if` at home.

compiler/rustc_codegen_ssa/src/mir/analyze.rs

@@ -15,6 +15,7 @@ use crate::traits::*;
 
 pub(crate) fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     fx: &FunctionCx<'a, 'tcx, Bx>,
+    traversal_order: &[mir::BasicBlock],
 ) -> BitSet<mir::Local> {
     let mir = fx.mir;
     let dominators = mir.basic_blocks.dominators();
@@ -24,13 +25,7 @@ pub(crate) fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         .map(|decl| {
             let ty = fx.monomorphize(decl.ty);
             let layout = fx.cx.spanned_layout_of(ty, decl.source_info.span);
-            if layout.is_zst() {
+            if layout.is_zst() { LocalKind::ZST } else { LocalKind::Unused }
-                LocalKind::ZST
-            } else if fx.cx.is_backend_immediate(layout) || fx.cx.is_backend_scalar_pair(layout) {
-                LocalKind::Unused
-            } else {
-                LocalKind::Memory
-            }
         })
         .collect();
 
@@ -44,7 +39,8 @@ pub(crate) fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // If there exists a local definition that dominates all uses of that local,
     // the definition should be visited first. Traverse blocks in an order that
     // is a topological sort of dominance partial order.
-    for (bb, data) in traversal::reverse_postorder(mir) {
+    for bb in traversal_order.iter().copied() {
+        let data = &mir.basic_blocks[bb];
         analyzer.visit_basic_block_data(bb, data);
     }
 
@@ -77,11 +73,22 @@ struct LocalAnalyzer<'a, 'b, 'tcx, Bx: BuilderMethods<'b, 'tcx>> {
 
 impl<'a, 'b, 'tcx, Bx: BuilderMethods<'b, 'tcx>> LocalAnalyzer<'a, 'b, 'tcx, Bx> {
     fn define(&mut self, local: mir::Local, location: DefLocation) {
+        let fx = self.fx;
         let kind = &mut self.locals[local];
+        let decl = &fx.mir.local_decls[local];
         match *kind {
             LocalKind::ZST => {}
             LocalKind::Memory => {}
-            LocalKind::Unused => *kind = LocalKind::SSA(location),
+            LocalKind::Unused => {
+                let ty = fx.monomorphize(decl.ty);
+                let layout = fx.cx.spanned_layout_of(ty, decl.source_info.span);
+                *kind =
+                    if fx.cx.is_backend_immediate(layout) || fx.cx.is_backend_scalar_pair(layout) {
+                        LocalKind::SSA(location)
+                    } else {
+                        LocalKind::Memory
+                    };
+            }
             LocalKind::SSA(_) => *kind = LocalKind::Memory,
         }
     }

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -218,7 +218,8 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 
     fx.per_local_var_debug_info = fx.compute_per_local_var_debug_info(&mut start_bx);
 
-    let memory_locals = analyze::non_ssa_locals(&fx);
+    let traversal_order = traversal::mono_reachable_reverse_postorder(mir, cx.tcx(), instance);
+    let memory_locals = analyze::non_ssa_locals(&fx, &traversal_order);
 
     // Allocate variable and temp allocas
     let local_values = {
@@ -277,17 +278,20 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // So drop the builder of `start_llbb` to avoid having two at the same time.
     drop(start_bx);
 
-    let reachable_blocks = traversal::mono_reachable_as_bitset(mir, cx.tcx(), instance);
+    let mut unreached_blocks = BitSet::new_filled(mir.basic_blocks.len());
+    // Codegen the body of each reachable block using our reverse postorder list.
+    for bb in traversal_order {
+        fx.codegen_block(bb);
+        unreached_blocks.remove(bb);
+    }
 
-    // Codegen the body of each block using reverse postorder
+    // FIXME: These empty unreachable blocks are *mostly* a waste. They are occasionally
-    for (bb, _) in traversal::reverse_postorder(mir) {
+    // targets for a SwitchInt terminator, but the reimplementation of the mono-reachable
-        if reachable_blocks.contains(bb) {
+    // simplification in SwitchInt lowering sometimes misses cases that
-            fx.codegen_block(bb);
+    // mono_reachable_reverse_postorder manages to figure out.
-        } else {
+    // The solution is to do something like post-mono GVN. But for now we have this hack.
-            // We want to skip this block, because it's not reachable. But we still create
+    for bb in unreached_blocks.iter() {
-            // the block so terminators in other blocks can reference it.
+        fx.codegen_block_as_unreachable(bb);
-            fx.codegen_block_as_unreachable(bb);
-        }
     }
 }
 

compiler/rustc_middle/src/mir/basic_blocks.rs

@@ -71,7 +71,7 @@ impl<'tcx> BasicBlocks<'tcx> {
     #[inline]
     pub fn reverse_postorder(&self) -> &[BasicBlock] {
         self.cache.reverse_postorder.get_or_init(|| {
-            let mut rpo: Vec<_> = Postorder::new(&self.basic_blocks, START_BLOCK).collect();
+            let mut rpo: Vec<_> = Postorder::new(&self.basic_blocks, START_BLOCK, ()).collect();
             rpo.reverse();
             rpo
         })

compiler/rustc_middle/src/mir/mod.rs

@@ -1424,6 +1424,19 @@ impl<'tcx> BasicBlockData<'tcx> {
     pub fn is_empty_unreachable(&self) -> bool {
         self.statements.is_empty() && matches!(self.terminator().kind, TerminatorKind::Unreachable)
     }
+
+    /// Like [`Terminator::successors`] but tries to use information available from the [`Instance`]
+    /// to skip successors like the `false` side of an `if const {`.
+    ///
+    /// This is used to implement [`traversal::mono_reachable`] and
+    /// [`traversal::mono_reachable_reverse_postorder`].
+    pub fn mono_successors(&self, tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) -> Successors<'_> {
+        if let Some((bits, targets)) = Body::try_const_mono_switchint(tcx, instance, self) {
+            targets.successors_for_value(bits)
+        } else {
+            self.terminator().successors()
+        }
+    }
 }
 
 ///////////////////////////////////////////////////////////////////////////

compiler/rustc_middle/src/mir/terminator.rs

@@ -413,6 +413,17 @@ mod helper {
     use super::*;
     pub type Successors<'a> = impl DoubleEndedIterator<Item = BasicBlock> + 'a;
     pub type SuccessorsMut<'a> = impl DoubleEndedIterator<Item = &'a mut BasicBlock> + 'a;
+
+    impl SwitchTargets {
+        /// Like [`SwitchTargets::target_for_value`], but returning the same type as
+        /// [`Terminator::successors`].
+        #[inline]
+        pub fn successors_for_value(&self, value: u128) -> Successors<'_> {
+            let target = self.target_for_value(value);
+            (&[]).into_iter().copied().chain(Some(target))
+        }
+    }
+
     impl<'tcx> TerminatorKind<'tcx> {
         #[inline]
         pub fn successors(&self) -> Successors<'_> {

compiler/rustc_middle/src/mir/traversal.rs

@@ -104,36 +104,46 @@ impl<'a, 'tcx> Iterator for Preorder<'a, 'tcx> {
 /// ```
 ///
 /// A Postorder traversal of this graph is `D B C A` or `D C B A`
-pub struct Postorder<'a, 'tcx> {
+pub struct Postorder<'a, 'tcx, C> {
     basic_blocks: &'a IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
     visited: BitSet<BasicBlock>,
     visit_stack: Vec<(BasicBlock, Successors<'a>)>,
     root_is_start_block: bool,
+    extra: C,
 }
 
-impl<'a, 'tcx> Postorder<'a, 'tcx> {
+impl<'a, 'tcx, C> Postorder<'a, 'tcx, C>
+where
+    C: Customization<'tcx>,
+{
     pub fn new(
         basic_blocks: &'a IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
         root: BasicBlock,
-    ) -> Postorder<'a, 'tcx> {
+        extra: C,
+    ) -> Postorder<'a, 'tcx, C> {
         let mut po = Postorder {
             basic_blocks,
             visited: BitSet::new_empty(basic_blocks.len()),
             visit_stack: Vec::new(),
             root_is_start_block: root == START_BLOCK,
+            extra,
         };
 
-        let data = &po.basic_blocks[root];
+        po.visit(root);
-
+        po.traverse_successor();
-        if let Some(ref term) = data.terminator {
-            po.visited.insert(root);
-            po.visit_stack.push((root, term.successors()));
-            po.traverse_successor();
-        }
 
         po
     }
 
+    fn visit(&mut self, bb: BasicBlock) {
+        if !self.visited.insert(bb) {
+            return;
+        }
+        let data = &self.basic_blocks[bb];
+        let successors = C::successors(data, self.extra);
+        self.visit_stack.push((bb, successors));
+    }
+
     fn traverse_successor(&mut self) {
         // This is quite a complex loop due to 1. the borrow checker not liking it much
         // and 2. what exactly is going on is not clear
@@ -183,16 +193,15 @@ impl<'a, 'tcx> Postorder<'a, 'tcx> {
         // since we've already visited `E`, that child isn't added to the stack. The last
         // two iterations yield `B` and finally `A` for a final traversal of [E, D, C, B, A]
         while let Some(bb) = self.visit_stack.last_mut().and_then(|(_, iter)| iter.next_back()) {
-            if self.visited.insert(bb) {
+            self.visit(bb);
-                if let Some(term) = &self.basic_blocks[bb].terminator {
-                    self.visit_stack.push((bb, term.successors()));
-                }
-            }
         }
     }
 }
 
-impl<'tcx> Iterator for Postorder<'_, 'tcx> {
+impl<'tcx, C> Iterator for Postorder<'_, 'tcx, C>
+where
+    C: Customization<'tcx>,
+{
     type Item = BasicBlock;
 
     fn next(&mut self) -> Option<BasicBlock> {
@@ -232,6 +241,40 @@ pub fn postorder<'a, 'tcx>(
     reverse_postorder(body).rev()
 }
 
+/// Lets us plug in some additional logic and data into a Postorder traversal. Or not.
+pub trait Customization<'tcx>: Copy {
+    fn successors<'a>(_: &'a BasicBlockData<'tcx>, _: Self) -> Successors<'a>;
+}
+
+impl<'tcx> Customization<'tcx> for () {
+    fn successors<'a>(data: &'a BasicBlockData<'tcx>, _: ()) -> Successors<'a> {
+        data.terminator().successors()
+    }
+}
+
+impl<'tcx> Customization<'tcx> for (TyCtxt<'tcx>, Instance<'tcx>) {
+    fn successors<'a>(
+        data: &'a BasicBlockData<'tcx>,
+        (tcx, instance): (TyCtxt<'tcx>, Instance<'tcx>),
+    ) -> Successors<'a> {
+        data.mono_successors(tcx, instance)
+    }
+}
+
+pub fn mono_reachable_reverse_postorder<'a, 'tcx>(
+    body: &'a Body<'tcx>,
+    tcx: TyCtxt<'tcx>,
+    instance: Instance<'tcx>,
+) -> Vec<BasicBlock> {
+    let mut iter = Postorder::new(&body.basic_blocks, START_BLOCK, (tcx, instance));
+    let mut items = Vec::with_capacity(body.basic_blocks.len());
+    while let Some(block) = iter.next() {
+        items.push(block);
+    }
+    items.reverse();
+    items
+}
+
 /// Returns an iterator over all basic blocks reachable from the `START_BLOCK` in no particular
 /// order.
 ///
@@ -358,14 +401,8 @@ impl<'a, 'tcx> Iterator for MonoReachable<'a, 'tcx> {
 
             let data = &self.body[idx];
 
-            if let Some((bits, targets)) =
+            let targets = data.mono_successors(self.tcx, self.instance);
-                Body::try_const_mono_switchint(self.tcx, self.instance, data)
+            self.add_work(targets);
-            {
-                let target = targets.target_for_value(bits);
-                self.add_work([target]);
-            } else {
-                self.add_work(data.terminator().successors());
-            }
 
             return Some((idx, data));
         }

src/tools/clippy/clippy_utils/src/mir/mod.rs

@@ -30,7 +30,7 @@ pub fn visit_local_usage(locals: &[Local], mir: &Body<'_>, location: Location) -
         locals.len()
     ];
 
-    traversal::Postorder::new(&mir.basic_blocks, location.block)
+    traversal::Postorder::new(&mir.basic_blocks, location.block, ())
         .collect::<Vec<_>>()
         .into_iter()
         .rev()

tests/codegen/constant-branch.rs

@@ -7,18 +7,19 @@
 // CHECK-LABEL: @if_bool
 #[no_mangle]
 pub fn if_bool() {
-    // CHECK: br label %{{.+}}
+    // CHECK-NOT: br i1
+    // CHECK-NOT: switch
     _ = if true { 0 } else { 1 };
 
-    // CHECK: br label %{{.+}}
     _ = if false { 0 } else { 1 };
 }
 
 // CHECK-LABEL: @if_constant_int_eq
 #[no_mangle]
 pub fn if_constant_int_eq() {
+    // CHECK-NOT: br i1
+    // CHECK-NOT: switch
     let val = 0;
-    // CHECK: br label %{{.+}}
     _ = if val == 0 { 0 } else { 1 };
 
     // CHECK: br label %{{.+}}
@@ -28,23 +29,20 @@ pub fn if_constant_int_eq() {
 // CHECK-LABEL: @if_constant_match
 #[no_mangle]
 pub fn if_constant_match() {
-    // CHECK: br label %{{.+}}
+    // CHECK-NOT: br i1
+    // CHECK-NOT: switch
     _ = match 1 {
         1 => 2,
         2 => 3,
         _ => 4,
     };
 
-    // CHECK: br label %{{.+}}
     _ = match 1 {
         2 => 3,
         _ => 4,
     };
 
-    // CHECK: br label %[[MINUS1:.+]]
     _ = match -1 {
-        // CHECK: [[MINUS1]]:
-        // CHECK: store i32 1
         -1 => 1,
         _ => 0,
     }

tests/codegen/no-alloca-inside-if-false.rs

@@ -0,0 +1,27 @@
+//@ compile-flags: -Cno-prepopulate-passes -Copt-level=0 -Cpanic=abort
+// Check that there's an alloca for the reference and the vector, but nothing else.
+// We use panic=abort because unwinding panics give hint::black_box a cleanup block, which has
+// another alloca.
+
+#![crate_type = "lib"]
+
+#[inline(never)]
+fn test<const SIZE: usize>() {
+    // CHECK-LABEL: no_alloca_inside_if_false::test
+    // CHECK: start:
+    // CHECK-NEXT: alloca [{{12|24}} x i8]
+    // CHECK-NOT: alloca
+    if const { SIZE < 4096 } {
+        let arr = [0u8; SIZE];
+        std::hint::black_box(&arr);
+    } else {
+        let vec = vec![0u8; SIZE];
+        std::hint::black_box(&vec);
+    }
+}
+
+// CHECK-LABEL: @main
+#[no_mangle]
+pub fn main() {
+    test::<8192>();
+}