From a448837045326d7c33059dc3aa2d1d87529dcf3d Mon Sep 17 00:00:00 2001
From: Joel Wejdenstål <joel.wejdenstal@gmail.com>
Date: Fri, 25 Jul 2025 21:21:42 +0200
Subject: Implement support for explicit tail calls in the MIR block builders
 and the LLVM codegen backend.

---
 compiler/rustc_codegen_llvm/src/builder.rs  | 25 ++++++++++++++++++++++++-
 compiler/rustc_codegen_llvm/src/llvm/ffi.rs | 11 +++++++++++
 2 files changed, 35 insertions(+), 1 deletion(-)

(limited to 'compiler/rustc_codegen_llvm')

diff --git a/compiler/rustc_codegen_llvm/src/builder.rs b/compiler/rustc_codegen_llvm/src/builder.rs
index 0ade9edb0d2..e8d90305829 100644
--- a/compiler/rustc_codegen_llvm/src/builder.rs
+++ b/compiler/rustc_codegen_llvm/src/builder.rs
@@ -15,6 +15,7 @@ use rustc_codegen_ssa::mir::place::PlaceRef;
 use rustc_codegen_ssa::traits::*;
 use rustc_data_structures::small_c_str::SmallCStr;
 use rustc_hir::def_id::DefId;
+use rustc_middle::bug;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
 use rustc_middle::ty::layout::{
     FnAbiError, FnAbiOfHelpers, FnAbiRequest, HasTypingEnv, LayoutError, LayoutOfHelpers,
@@ -24,7 +25,7 @@ use rustc_middle::ty::{self, Instance, Ty, TyCtxt};
 use rustc_sanitizers::{cfi, kcfi};
 use rustc_session::config::OptLevel;
 use rustc_span::Span;
-use rustc_target::callconv::FnAbi;
+use rustc_target::callconv::{FnAbi, PassMode};
 use rustc_target::spec::{HasTargetSpec, SanitizerSet, Target};
 use smallvec::SmallVec;
 use tracing::{debug, instrument};
@@ -1431,6 +1432,28 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         call
     }
 
+    fn tail_call(
+        &mut self,
+        llty: Self::Type,
+        fn_attrs: Option<&CodegenFnAttrs>,
+        fn_abi: &FnAbi<'tcx, Ty<'tcx>>,
+        llfn: Self::Value,
+        args: &[Self::Value],
+        funclet: Option<&Self::Funclet>,
+        instance: Option<Instance<'tcx>>,
+    ) {
+        let call = self.call(llty, fn_attrs, Some(fn_abi), llfn, args, funclet, instance);
+        llvm::LLVMRustSetTailCallKind(call, llvm::TailCallKind::MustTail);
+
+        match &fn_abi.ret.mode {
+            PassMode::Ignore | PassMode::Indirect { .. } => self.ret_void(),
+            PassMode::Direct(_) | PassMode::Pair { .. } => self.ret(call),
+            mode @ PassMode::Cast { .. } => {
+                bug!("Encountered `PassMode::{mode:?}` during codegen")
+            }
+        }
+    }
+
     fn zext(&mut self, val: &'ll Value, dest_ty: &'ll Type) -> &'ll Value {
         unsafe { llvm::LLVMBuildZExt(self.llbuilder, val, dest_ty, UNNAMED) }
     }
diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
index edfb29dd1be..9d4bd70549b 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
@@ -97,6 +97,16 @@ pub(crate) enum ModuleFlagMergeBehavior {
 
 // Consts for the LLVM CallConv type, pre-cast to usize.
 
+#[derive(Copy, Clone, PartialEq, Debug)]
+#[repr(C)]
+#[allow(dead_code)]
+pub(crate) enum TailCallKind {
+    None = 0,
+    Tail = 1,
+    MustTail = 2,
+    NoTail = 3,
+}
+
 /// LLVM CallingConv::ID. Should we wrap this?
 ///
 /// See <https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/IR/CallingConv.h>
@@ -1186,6 +1196,7 @@ unsafe extern "C" {
     pub(crate) safe fn LLVMIsGlobalConstant(GlobalVar: &Value) -> Bool;
     pub(crate) safe fn LLVMSetGlobalConstant(GlobalVar: &Value, IsConstant: Bool);
     pub(crate) safe fn LLVMSetTailCall(CallInst: &Value, IsTailCall: Bool);
+    pub(crate) safe fn LLVMRustSetTailCallKind(CallInst: &Value, Kind: TailCallKind);
 
     // Operations on attributes
     pub(crate) fn LLVMCreateStringAttribute(
-- 
cgit 1.4.1-3-g733a5