Auto merge of #102099 - InnovativeInventor:re-cold-land, r=nikic

Rebased: Mark drop calls in landing pads cold instead of noinline

I noticed that certain inlining optimizations were missing while staring at some compiled code output. I'd like to see this relanded, so I rebased the PR from `@erikdesjardins` (PR #94823).

This PR reapplies https://github.com/rust-lang/rust/pull/92419, which was reverted in https://github.com/rust-lang/rust/pull/94402 due to https://github.com/rust-lang/rust/issues/94390.

Fixes https://github.com/rust-lang/rust/issues/46515, fixes https://github.com/rust-lang/rust/issues/87055.

Update: fixes #97217.

compiler/rustc_codegen_gcc/src/builder.rs

@@ -1420,7 +1420,7 @@ fn cx(&self) -> &CodegenCx<'gcc, 'tcx> {
         self.cx
     }
 
-    fn do_not_inline(&mut self, _llret: RValue<'gcc>) {
+    fn apply_attrs_to_cleanup_callsite(&mut self, _llret: RValue<'gcc>) {
         // FIXME(bjorn3): implement
     }
 

compiler/rustc_codegen_llvm/src/builder.rs

@@ -3,6 +3,7 @@
 use crate::common::Funclet;
 use crate::context::CodegenCx;
 use crate::llvm::{self, AtomicOrdering, AtomicRmwBinOp, BasicBlock, False, True};
+use crate::llvm_util;
 use crate::type_::Type;
 use crate::type_of::LayoutLlvmExt;
 use crate::value::Value;
@@ -1225,9 +1226,16 @@ fn zext(&mut self, val: &'ll Value, dest_ty: &'ll Type) -> &'ll Value {
         unsafe { llvm::LLVMBuildZExt(self.llbuilder, val, dest_ty, UNNAMED) }
     }
 
-    fn do_not_inline(&mut self, llret: &'ll Value) {
-        let noinline = llvm::AttributeKind::NoInline.create_attr(self.llcx);
-        attributes::apply_to_callsite(llret, llvm::AttributePlace::Function, &[noinline]);
+    fn apply_attrs_to_cleanup_callsite(&mut self, llret: &'ll Value) {
+        if llvm_util::get_version() < (17, 0, 2) {
+            // Work around https://github.com/llvm/llvm-project/issues/66984.
+            let noinline = llvm::AttributeKind::NoInline.create_attr(self.llcx);
+            attributes::apply_to_callsite(llret, llvm::AttributePlace::Function, &[noinline]);
+        } else {
+            // Cleanup is always the cold path.
+            let cold_inline = llvm::AttributeKind::Cold.create_attr(self.llcx);
+            attributes::apply_to_callsite(llret, llvm::AttributePlace::Function, &[cold_inline]);
+        }
     }
 }
 

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -213,7 +213,7 @@ fn do_call<Bx: BuilderMethods<'a, 'tcx>>(
                 self.funclet(fx),
             );
             if fx.mir[self.bb].is_cleanup {
-                bx.do_not_inline(invokeret);
+                bx.apply_attrs_to_cleanup_callsite(invokeret);
             }
 
             if let Some((ret_dest, target)) = destination {
@@ -228,11 +228,7 @@ fn do_call<Bx: BuilderMethods<'a, 'tcx>>(
         } else {
             let llret = bx.call(fn_ty, fn_attrs, Some(&fn_abi), fn_ptr, &llargs, self.funclet(fx));
             if fx.mir[self.bb].is_cleanup {
-                // Cleanup is always the cold path. Don't inline
-                // drop glue. Also, when there is a deeply-nested
-                // struct, there are "symmetry" issues that cause
-                // exponential inlining - see issue #41696.
-                bx.do_not_inline(llret);
+                bx.apply_attrs_to_cleanup_callsite(llret);
             }
 
             if let Some((ret_dest, target)) = destination {
@@ -1627,7 +1623,7 @@ fn terminate_block(&mut self, reason: UnwindTerminateReason) -> Bx::BasicBlock {
         let fn_ty = bx.fn_decl_backend_type(&fn_abi);
 
         let llret = bx.call(fn_ty, None, Some(&fn_abi), fn_ptr, &[], funclet.as_ref());
-        bx.do_not_inline(llret);
+        bx.apply_attrs_to_cleanup_callsite(llret);
 
         bx.unreachable();
 

compiler/rustc_codegen_ssa/src/traits/builder.rs

@@ -332,5 +332,5 @@ fn call(
     ) -> Self::Value;
     fn zext(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;
 
-    fn do_not_inline(&mut self, llret: Self::Value);
+    fn apply_attrs_to_cleanup_callsite(&mut self, llret: Self::Value);
 }

tests/assembly/stack-protector/stack-protector-heuristics-effect.rs

@@ -9,6 +9,7 @@
 // [basic] compile-flags: -Z stack-protector=basic
 // [none] compile-flags: -Z stack-protector=none
 // compile-flags: -C opt-level=2 -Z merge-functions=disabled
+// min-llvm-version: 17.0.2
 
 #![crate_type = "lib"]
 
@@ -371,7 +372,7 @@ pub fn unsized_fn_param(s: [u8], l: bool, f: fn([u8])) {
 
 
     // all: __stack_chk_fail
-    // strong: __stack_chk_fail
+    // strong-NOT: __stack_chk_fail
     // basic-NOT: __stack_chk_fail
     // none-NOT: __stack_chk_fail
     // missing-NOT: __stack_chk_fail

tests/codegen/issue-97217.rs

@@ -0,0 +1,22 @@
+// compile-flags: -C opt-level=3
+// ignore-debug: the debug assertions get in the way
+// min-llvm-version: 17.0.2
+#![crate_type = "lib"]
+
+// Regression test for issue 97217 (the following should result in no allocations)
+
+// CHECK-LABEL: @issue97217
+#[no_mangle]
+pub fn issue97217() -> i32 {
+    // drop_in_place should be inlined and never appear
+    // CHECK-NOT: drop_in_place
+
+    // __rust_alloc should be optimized out
+    // CHECK-NOT: __rust_alloc
+
+    let v1 = vec![5, 6, 7];
+    let v1_iter = v1.iter();
+    let total: i32 = v1_iter.sum();
+    println!("{}",total);
+    total
+}

tests/codegen/unwind-landingpad-cold.rs

@@ -0,0 +1,16 @@
+// compile-flags: -Cno-prepopulate-passes
+// needs-unwind
+// min-llvm-version: 17.0.2
+#![crate_type = "lib"]
+
+// This test checks that drop calls in unwind landing pads
+// get the `cold` attribute.
+
+// CHECK-LABEL: @check_cold
+// CHECK: {{(call|invoke) void .+}}drop_in_place{{.+}} [[ATTRIBUTES:#[0-9]+]]
+// CHECK: attributes [[ATTRIBUTES]] = { cold }
+#[no_mangle]
+pub fn check_cold(f: fn(), x: Box<u32>) {
+    // this may unwind
+    f();
+}

tests/codegen/unwind-landingpad-inline.rs

@@ -0,0 +1,39 @@
+// min-llvm-version: 17.0.2
+// compile-flags: -Copt-level=3
+// ignore-debug: the debug assertions get in the way
+#![crate_type = "lib"]
+
+// This test checks that we can inline drop_in_place in
+// unwind landing pads.
+
+// Without inlining, the box pointers escape via the call to drop_in_place,
+// and LLVM will not optimize out the pointer comparison.
+// With inlining, everything should be optimized out.
+// See https://github.com/rust-lang/rust/issues/46515
+// CHECK-LABEL: @check_no_escape_in_landingpad
+// CHECK: start:
+// CHECK-NEXT: __rust_no_alloc_shim_is_unstable
+// CHECK-NEXT: __rust_no_alloc_shim_is_unstable
+// CHECK-NEXT: ret void
+#[no_mangle]
+pub fn check_no_escape_in_landingpad(f: fn()) {
+    let x = &*Box::new(0);
+    let y = &*Box::new(0);
+
+    if x as *const _ == y as *const _ {
+        f();
+    }
+}
+
+// Without inlining, the compiler can't tell that
+// dropping an empty string (in a landing pad) does nothing.
+// With inlining, the landing pad should be optimized out.
+// See https://github.com/rust-lang/rust/issues/87055
+// CHECK-LABEL: @check_eliminate_noop_drop
+// CHECK: call void %g()
+// CHECK-NEXT: ret void
+#[no_mangle]
+pub fn check_eliminate_noop_drop(g: fn()) {
+    let _var = String::new();
+    g();
+}