cg_llvm: Reuse LLVM-C Comdat support

Migrate `llvm::set_comdat` and `llvm::SetUniqueComdat` to LLVM-C FFI.

Note, now we can call `llvm::set_comdat` only when the target actually
supports adding comdat. As this has no convenient LLVM-C API, we
implement this as `TargetOptions::supports_comdat`.

Co-authored-by: Stuart Cook <Zalathar@users.noreply.github.com>

compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs

@@ -13,6 +13,7 @@ use rustc_middle::mir::coverage::CoverageKind;
 use rustc_middle::ty::Instance;
 use rustc_middle::ty::layout::HasTyCtxt;
 use rustc_target::abi::{Align, Size};
+use rustc_target::spec::HasTargetSpec;
 use tracing::{debug, instrument};
 
 use crate::builder::Builder;
@@ -336,7 +337,9 @@ pub(crate) fn save_func_record_to_mod<'ll, 'tcx>(
     llvm::set_section(llglobal, covfun_section_name);
     // LLVM's coverage mapping format specifies 8-byte alignment for items in this section.
     llvm::set_alignment(llglobal, Align::EIGHT);
-    llvm::set_comdat(cx.llmod, llglobal, func_record_var_name.to_str().unwrap());
+    if cx.target_spec().supports_comdat() {
+        llvm::set_comdat(cx.llmod, llglobal, &func_record_var_name);
+    }
     cx.add_used_global(llglobal);
 }
 

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -787,7 +787,9 @@ fn codegen_msvc_try<'ll>(
         let tydesc = bx.declare_global("__rust_panic_type_info", bx.val_ty(type_info));
         unsafe {
             llvm::LLVMRustSetLinkage(tydesc, llvm::Linkage::LinkOnceODRLinkage);
+            if bx.cx.tcx.sess.target.supports_comdat() {
                 llvm::SetUniqueComdat(bx.llmod, tydesc);
+            }
             llvm::LLVMSetInitializer(tydesc, type_info);
         }
 

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -646,6 +646,7 @@ unsafe extern "C" {
     pub type Attribute;
     pub type Metadata;
     pub type BasicBlock;
+    pub type Comdat;
 }
 #[repr(C)]
 pub struct Builder<'a>(InvariantOpaque<'a>);
@@ -1490,6 +1491,9 @@ unsafe extern "C" {
     pub fn LLVMSetUnnamedAddress(Global: &Value, UnnamedAddr: UnnamedAddr);
 
     pub fn LLVMIsAConstantInt(value_ref: &Value) -> Option<&ConstantInt>;
+
+    pub fn LLVMGetOrInsertComdat(M: &Module, Name: *const c_char) -> &Comdat;
+    pub fn LLVMSetComdat(V: &Value, C: &Comdat);
 }
 
 #[link(name = "llvm-wrapper", kind = "static")]
@@ -2320,7 +2324,6 @@ unsafe extern "C" {
 
     pub fn LLVMRustPositionBuilderAtStart<'a>(B: &Builder<'a>, BB: &'a BasicBlock);
 
-    pub fn LLVMRustSetComdat<'a>(M: &'a Module, V: &'a Value, Name: *const c_char, NameLen: size_t);
     pub fn LLVMRustSetModulePICLevel(M: &Module);
     pub fn LLVMRustSetModulePIELevel(M: &Module);
     pub fn LLVMRustSetModuleCodeModel(M: &Module, Model: CodeModel);

compiler/rustc_codegen_llvm/src/llvm/mod.rs

@@ -178,10 +178,10 @@ pub fn SetFunctionCallConv(fn_: &Value, cc: CallConv) {
 // function.
 // For more details on COMDAT sections see e.g., https://www.airs.com/blog/archives/52
 pub fn SetUniqueComdat(llmod: &Module, val: &Value) {
-    unsafe {
-        let name = get_value_name(val);
-        LLVMRustSetComdat(llmod, val, name.as_ptr().cast(), name.len());
-    }
+    let name_buf = get_value_name(val).to_vec();
+    let name =
+        CString::from_vec_with_nul(name_buf).or_else(|buf| CString::new(buf.into_bytes())).unwrap();
+    set_comdat(llmod, val, &name);
 }
 
 pub fn SetUnnamedAddress(global: &Value, unnamed: UnnamedAddr) {
@@ -251,9 +251,14 @@ pub fn set_alignment(llglobal: &Value, align: Align) {
     }
 }
 
-pub fn set_comdat(llmod: &Module, llglobal: &Value, name: &str) {
+/// Get the `name`d comdat from `llmod` and assign it to `llglobal`.
+///
+/// Inserts the comdat into `llmod` if it does not exist.
+/// It is an error to call this if the target does not support comdat.
+pub fn set_comdat(llmod: &Module, llglobal: &Value, name: &CStr) {
     unsafe {
-        LLVMRustSetComdat(llmod, llglobal, name.as_ptr().cast(), name.len());
+        let comdat = LLVMGetOrInsertComdat(llmod, name.as_ptr());
+        LLVMSetComdat(llglobal, comdat);
     }
 }
 

compiler/rustc_codegen_llvm/src/mono_item.rs

@@ -64,7 +64,9 @@ impl<'tcx> PreDefineCodegenMethods<'tcx> for CodegenCx<'_, 'tcx> {
         unsafe { llvm::LLVMRustSetLinkage(lldecl, base::linkage_to_llvm(linkage)) };
         let attrs = self.tcx.codegen_fn_attrs(instance.def_id());
         base::set_link_section(lldecl, attrs);
-        if linkage == Linkage::LinkOnceODR || linkage == Linkage::WeakODR {
+        if (linkage == Linkage::LinkOnceODR || linkage == Linkage::WeakODR)
+            && self.tcx.sess.target.supports_comdat()
+        {
             llvm::SetUniqueComdat(self.llmod, lldecl);
         }
 

compiler/rustc_target/src/spec/mod.rs

@@ -2514,6 +2514,13 @@ fn add_link_args(link_args: &mut LinkArgs, flavor: LinkerFlavor, args: &[&'stati
     add_link_args_iter(link_args, flavor, args.iter().copied().map(Cow::Borrowed))
 }
 
+impl TargetOptions {
+    pub fn supports_comdat(&self) -> bool {
+        // XCOFF and MachO don't support COMDAT.
+        !self.is_like_aix && !self.is_like_osx
+    }
+}
+
 impl TargetOptions {
     fn link_args(flavor: LinkerFlavor, args: &[&'static str]) -> LinkArgs {
         let mut link_args = LinkArgs::new();