Auto merge of #84993 - eddyb:cg-ssa-on-demand-blocks, r=nagisa

rustc_codegen_ssa: only create backend `BasicBlock`s as-needed.

Instead of creating one backend (e.g. LLVM) block per MIR block ahead of time, and then deleting the ones that weren't visited, this PR moves to creating the blocks as they're needed (either reached via the RPO visit, or used as the target of a branch from a different block).

As deleting a block was the only `unsafe` builder method (generally we only *create* backend objects, not *remove* them), that's gone now and codegen is overall a bit safer.

The only change in output is the order of LLVM blocks (which AFAIK has no semantic meaning, other than the first block being the entry block). This happens because the blocks are now created due to control-flow edges, rather than MIR block order.

I'm making this a standalone PR because I keep getting wild perf results when I change *anything* in codegen, but if you want to read more about my plans in this area, see https://github.com/rust-lang/rust/pull/84771#issuecomment-830636256 (and https://github.com/rust-lang/rust/pull/84771#issue-628295651 - but that may be a bit outdated).

(You may notice some of the APIs in this PR, like `append_block`, don't help with the future plans - but I didn't want to include the necessary refactors that pass a build around everywhere, in this PR, so it's a small compromise)

r? `@nagisa` `@bjorn3`

4 files changed, 29 insertions, 28 deletions

diff --git a/compiler/rustc_codegen_llvm/src/builder.rs b/compiler/rustc_codegen_llvm/src/builder.rs
index 053cda1e7cc..bc9d99ed4a1 100644
--- a/compiler/rustc_codegen_llvm/src/builder.rs
+++ b/compiler/rustc_codegen_llvm/src/builder.rs
@@ -118,24 +118,16 @@ macro_rules! builder_methods_for_value_instructions {
 }
 
 impl BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
-    fn new_block<'b>(cx: &'a CodegenCx<'ll, 'tcx>, llfn: &'ll Value, name: &'b str) -> Self {
-        let mut bx = Builder::with_cx(cx);
-        let llbb = unsafe {
-            let name = SmallCStr::new(name);
-            llvm::LLVMAppendBasicBlockInContext(cx.llcx, llfn, name.as_ptr())
-        };
-        bx.position_at_end(llbb);
+    fn build(cx: &'a CodegenCx<'ll, 'tcx>, llbb: &'ll BasicBlock) -> Self {
+        let bx = Builder::with_cx(cx);
+        unsafe {
+            llvm::LLVMPositionBuilderAtEnd(bx.llbuilder, llbb);
+        }
         bx
     }
 
-    fn with_cx(cx: &'a CodegenCx<'ll, 'tcx>) -> Self {
-        // Create a fresh builder from the crate context.
-        let llbuilder = unsafe { llvm::LLVMCreateBuilderInContext(cx.llcx) };
-        Builder { llbuilder, cx }
-    }
-
-    fn build_sibling_block(&self, name: &str) -> Self {
-        Builder::new_block(self.cx, self.llfn(), name)
+    fn cx(&self) -> &CodegenCx<'ll, 'tcx> {
+        self.cx
     }
 
     fn llbb(&self) -> &'ll BasicBlock {
@@ -144,12 +136,22 @@ impl BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
 
     fn set_span(&mut self, _span: Span) {}
 
-    fn position_at_end(&mut self, llbb: &'ll BasicBlock) {
+    fn append_block(cx: &'a CodegenCx<'ll, 'tcx>, llfn: &'ll Value, name: &str) -> &'ll BasicBlock {
         unsafe {
-            llvm::LLVMPositionBuilderAtEnd(self.llbuilder, llbb);
+            let name = SmallCStr::new(name);
+            llvm::LLVMAppendBasicBlockInContext(cx.llcx, llfn, name.as_ptr())
         }
     }
 
+    fn append_sibling_block(&mut self, name: &str) -> &'ll BasicBlock {
+        Self::append_block(self.cx, self.llfn(), name)
+    }
+
+    fn build_sibling_block(&mut self, name: &str) -> Self {
+        let llbb = self.append_sibling_block(name);
+        Self::build(self.cx, llbb)
+    }
+
     fn ret_void(&mut self) {
         unsafe {
             llvm::LLVMBuildRetVoid(self.llbuilder);
@@ -1144,14 +1146,6 @@ impl BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         unsafe { llvm::LLVMBuildZExt(self.llbuilder, val, dest_ty, UNNAMED) }
     }
 
-    fn cx(&self) -> &CodegenCx<'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);
     }
@@ -1165,6 +1159,12 @@ impl StaticBuilderMethods for Builder<'a, 'll, 'tcx> {
 }
 
 impl Builder<'a, 'll, 'tcx> {
+    fn with_cx(cx: &'a CodegenCx<'ll, 'tcx>) -> Self {
+        // Create a fresh builder from the crate context.
+        let llbuilder = unsafe { llvm::LLVMCreateBuilderInContext(cx.llcx) };
+        Builder { llbuilder, cx }
+    }
+
     pub fn llfn(&self) -> &'ll Value {
         unsafe { llvm::LLVMGetBasicBlockParent(self.llbb()) }
     }
diff --git a/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs b/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
index afc2bdbfd52..019bf4a09a7 100644
--- a/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
+++ b/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
@@ -223,7 +223,8 @@ fn declare_unused_fn(cx: &CodegenCx<'ll, 'tcx>, def_id: &DefId) -> Instance<'tcx
 
 fn codegen_unused_fn_and_counter(cx: &CodegenCx<'ll, 'tcx>, instance: Instance<'tcx>) {
     let llfn = cx.get_fn(instance);
-    let mut bx = Builder::new_block(cx, llfn, "unused_function");
+    let llbb = Builder::append_block(cx, llfn, "unused_function");
+    let mut bx = Builder::build(cx, llbb);
     let fn_name = bx.get_pgo_func_name_var(instance);
     let hash = bx.const_u64(0);
     let num_counters = bx.const_u32(1);
diff --git a/compiler/rustc_codegen_llvm/src/intrinsic.rs b/compiler/rustc_codegen_llvm/src/intrinsic.rs
index fc6c1abf4af..22d513d66d1 100644
--- a/compiler/rustc_codegen_llvm/src/intrinsic.rs
+++ b/compiler/rustc_codegen_llvm/src/intrinsic.rs
@@ -678,7 +678,8 @@ fn gen_fn<'ll, 'tcx>(
     cx.apply_target_cpu_attr(llfn);
     // FIXME(eddyb) find a nicer way to do this.
     unsafe { llvm::LLVMRustSetLinkage(llfn, llvm::Linkage::InternalLinkage) };
-    let bx = Builder::new_block(cx, llfn, "entry-block");
+    let llbb = Builder::append_block(cx, llfn, "entry-block");
+    let bx = Builder::build(cx, llbb);
     codegen(bx);
     llfn
 }
diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
index 32fdde9b42e..bf66040a7eb 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
+++ b/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>;