rustc_codegen_ssa: only create backend BasicBlocks as-needed.

compiler/rustc_codegen_llvm/src/builder.rs

@@ -1148,10 +1148,6 @@ impl BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         self.cx
     }
 
-    unsafe fn delete_basic_block(&mut self, bb: &'ll BasicBlock) {
-        llvm::LLVMDeleteBasicBlock(bb);
-    }
-
     fn do_not_inline(&mut self, llret: &'ll Value) {
         llvm::Attribute::NoInline.apply_callsite(llvm::AttributePlace::Function, llret);
     }

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1079,7 +1079,6 @@ extern "C" {
         Fn: &'a Value,
         Name: *const c_char,
     ) -> &'a BasicBlock;
-    pub fn LLVMDeleteBasicBlock(BB: &BasicBlock);
 
     // Operations on instructions
     pub fn LLVMIsAInstruction(Val: &Value) -> Option<&Value>;

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -68,7 +68,7 @@ impl<'a, 'tcx> TerminatorCodegenHelper<'tcx> {
         target: mir::BasicBlock,
     ) -> (Bx::BasicBlock, bool) {
         let span = self.terminator.source_info.span;
-        let lltarget = fx.blocks[target];
+        let lltarget = fx.llbb(target);
         let target_funclet = fx.cleanup_kinds[target].funclet_bb(target);
         match (self.funclet_bb, target_funclet) {
             (None, None) => (lltarget, false),
@@ -133,13 +133,13 @@ impl<'a, 'tcx> TerminatorCodegenHelper<'tcx> {
         // If there is a cleanup block and the function we're calling can unwind, then
         // do an invoke, otherwise do a call.
         if let Some(cleanup) = cleanup.filter(|_| fn_abi.can_unwind) {
-            let ret_bx = if let Some((_, target)) = destination {
+            let ret_llbb = if let Some((_, target)) = destination {
-                fx.blocks[target]
+                fx.llbb(target)
             } else {
                 fx.unreachable_block()
             };
             let invokeret =
-                bx.invoke(fn_ptr, &llargs, ret_bx, self.llblock(fx, cleanup), self.funclet(fx));
+                bx.invoke(fn_ptr, &llargs, ret_llbb, self.llblock(fx, cleanup), self.funclet(fx));
             bx.apply_attrs_callsite(&fn_abi, invokeret);
 
             if let Some((ret_dest, target)) = destination {
@@ -1205,7 +1205,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 
     // FIXME(eddyb) rename this to `eh_pad_for_uncached`.
     fn landing_pad_for_uncached(&mut self, bb: mir::BasicBlock) -> Bx::BasicBlock {
-        let llbb = self.blocks[bb];
+        let llbb = self.llbb(bb);
         if base::wants_msvc_seh(self.cx.sess()) {
             let funclet;
             let ret_llbb;
@@ -1293,9 +1293,23 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         Bx::new_block(self.cx, self.llfn, name)
     }
 
-    pub fn build_block(&self, bb: mir::BasicBlock) -> Bx {
+    /// Get the backend `BasicBlock` for a MIR `BasicBlock`, either already
+    /// cached in `self.cached_llbbs`, or created on demand (and cached).
+    // FIXME(eddyb) rename `llbb` and other `ll`-prefixed things to use a
+    // more backend-agnostic prefix such as `cg` (i.e. this would be `cgbb`).
+    pub fn llbb(&mut self, bb: mir::BasicBlock) -> Bx::BasicBlock {
+        self.cached_llbbs[bb].unwrap_or_else(|| {
+            // FIXME(eddyb) only name the block if `fewer_names` is `false`.
+            // FIXME(eddyb) create the block directly, without a builder.
+            let llbb = self.new_block(&format!("{:?}", bb)).llbb();
+            self.cached_llbbs[bb] = Some(llbb);
+            llbb
+        })
+    }
+
+    pub fn build_block(&mut self, bb: mir::BasicBlock) -> Bx {
         let mut bx = Bx::with_cx(self.cx);
-        bx.position_at_end(self.blocks[bb]);
+        bx.position_at_end(self.llbb(bb));
         bx
     }
 

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -40,8 +40,11 @@ pub struct FunctionCx<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> {
     /// then later loaded when generating the DIVERGE_BLOCK.
     personality_slot: Option<PlaceRef<'tcx, Bx::Value>>,
 
-    /// A `Block` for each MIR `BasicBlock`
+    /// A backend `BasicBlock` for each MIR `BasicBlock`, created lazily
-    blocks: IndexVec<mir::BasicBlock, Bx::BasicBlock>,
+    /// as-needed (e.g. RPO reaching it or another block branching to it).
+    // FIXME(eddyb) rename `llbbs` and other `ll`-prefixed things to use a
+    // more backend-agnostic prefix such as `cg` (i.e. this would be `cgbbs`).
+    cached_llbbs: IndexVec<mir::BasicBlock, Option<Bx::BasicBlock>>,
 
     /// The funclet status of each basic block
     cleanup_kinds: IndexVec<mir::BasicBlock, analyze::CleanupKind>,
@@ -151,17 +154,17 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // Allocate a `Block` for every basic block, except
     // the start block, if nothing loops back to it.
     let reentrant_start_block = !mir.predecessors()[mir::START_BLOCK].is_empty();
-    let block_bxs: IndexVec<mir::BasicBlock, Bx::BasicBlock> = mir
+    let cached_llbbs: IndexVec<mir::BasicBlock, Option<Bx::BasicBlock>> =
-        .basic_blocks()
+        mir.basic_blocks()
-        .indices()
+            .indices()
-        .map(|bb| {
+            .map(|bb| {
-            if bb == mir::START_BLOCK && !reentrant_start_block {
+                if bb == mir::START_BLOCK && !reentrant_start_block {
-                bx.llbb()
+                    Some(bx.llbb())
-            } else {
+                } else {
-                bx.build_sibling_block(&format!("{:?}", bb)).llbb()
+                    None
-            }
+                }
-        })
+            })
-        .collect();
+            .collect();
 
     let mut fx = FunctionCx {
         instance,
@@ -170,7 +173,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         fn_abi,
         cx,
         personality_slot: None,
-        blocks: block_bxs,
+        cached_llbbs,
         unreachable_block: None,
         cleanup_kinds,
         landing_pads: IndexVec::from_elem(None, mir.basic_blocks()),
@@ -245,29 +248,14 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 
     // Branch to the START block, if it's not the entry block.
     if reentrant_start_block {
-        bx.br(fx.blocks[mir::START_BLOCK]);
+        bx.br(fx.llbb(mir::START_BLOCK));
     }
 
-    let rpo = traversal::reverse_postorder(&mir);
-    let mut visited = BitSet::new_empty(mir.basic_blocks().len());
-
     // Codegen the body of each block using reverse postorder
-    for (bb, _) in rpo {
+    // FIXME(eddyb) reuse RPO iterator between `analysis` and this.
-        visited.insert(bb.index());
+    for (bb, _) in traversal::reverse_postorder(&mir) {
         fx.codegen_block(bb);
     }
-
-    // Remove blocks that haven't been visited, or have no
-    // predecessors.
-    for bb in mir.basic_blocks().indices() {
-        // Unreachable block
-        if !visited.contains(bb.index()) {
-            debug!("codegen_mir: block {:?} was not visited", bb);
-            unsafe {
-                bx.delete_basic_block(fx.blocks[bb]);
-            }
-        }
-    }
 }
 
 /// Produces, for each argument, a `Value` pointing at the

compiler/rustc_codegen_ssa/src/traits/builder.rs

@@ -291,6 +291,5 @@ pub trait BuilderMethods<'a, 'tcx>:
     ) -> Self::Value;
     fn zext(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;
 
-    unsafe fn delete_basic_block(&mut self, bb: Self::BasicBlock);
     fn do_not_inline(&mut self, llret: Self::Value);
 }

src/test/codegen/drop.rs

@@ -23,13 +23,16 @@ pub fn droppy() {
 // FIXME(eddyb) the `void @` forces a match on the instruction, instead of the
 // comment, that's `; call core::ptr::drop_in_place::<drop::SomeUniqueName>`
 // for the `v0` mangling, should switch to matching on that once `legacy` is gone.
-// CHECK-NOT: call void @{{.*}}drop_in_place{{.*}}SomeUniqueName
-// CHECK: invoke void @{{.*}}drop_in_place{{.*}}SomeUniqueName
-// CHECK: invoke void @{{.*}}drop_in_place{{.*}}SomeUniqueName
 // CHECK-NOT: invoke void @{{.*}}drop_in_place{{.*}}SomeUniqueName
 // CHECK: call void @{{.*}}drop_in_place{{.*}}SomeUniqueName
 // CHECK: call void @{{.*}}drop_in_place{{.*}}SomeUniqueName
+// CHECK-NOT: call void @{{.*}}drop_in_place{{.*}}SomeUniqueName
+// CHECK: invoke void @{{.*}}drop_in_place{{.*}}SomeUniqueName
+// CHECK-NOT: invoke void @{{.*}}drop_in_place{{.*}}SomeUniqueName
 // CHECK: call void @{{.*}}drop_in_place{{.*}}SomeUniqueName
+// CHECK-NOT: call void @{{.*}}drop_in_place{{.*}}SomeUniqueName
+// CHECK: invoke void @{{.*}}drop_in_place{{.*}}SomeUniqueName
+// CHECK-NOT: invoke void @{{.*}}drop_in_place{{.*}}SomeUniqueName
 // CHECK: call void @{{.*}}drop_in_place{{.*}}SomeUniqueName
 // CHECK-NOT: {{(call|invoke) void @.*}}drop_in_place{{.*}}SomeUniqueName
 // The next line checks for the } that ends the function definition

src/test/codegen/match.rs

@@ -14,13 +14,13 @@ pub fn exhaustive_match(e: E) -> u8 {
 // CHECK-NEXT: i[[TY:[0-9]+]] [[DISCR:[0-9]+]], label %[[A:[a-zA-Z0-9_]+]]
 // CHECK-NEXT: i[[TY:[0-9]+]] [[DISCR:[0-9]+]], label %[[B:[a-zA-Z0-9_]+]]
 // CHECK-NEXT: ]
-// CHECK: [[B]]:
-// CHECK-NEXT: store i8 1, i8* %1, align 1
-// CHECK-NEXT: br label %[[EXIT:[a-zA-Z0-9_]+]]
 // CHECK: [[OTHERWISE]]:
 // CHECK-NEXT: unreachable
 // CHECK: [[A]]:
 // CHECK-NEXT: store i8 0, i8* %1, align 1
+// CHECK-NEXT: br label %[[EXIT:[a-zA-Z0-9_]+]]
+// CHECK: [[B]]:
+// CHECK-NEXT: store i8 1, i8* %1, align 1
 // CHECK-NEXT: br label %[[EXIT:[a-zA-Z0-9_]+]]
     match e {
         E::A => 0,