Add PreservedSymbols from LLVM to LTO.

When building with LTO, builtin functions that are defined but whose calls have not been inserted yet, get internalized.
We need to prevent these symbols from being internalized at LTO time.

Refer to https://reviews.llvm.org/D49434.

compiler/rustc_codegen_llvm/src/back/lto.rs

@@ -39,6 +39,24 @@ pub fn crate_type_allows_lto(crate_type: CrateType) -> bool {
     }
 }
 
+fn get_llvm_preserved_symbols() -> Vec<String> {
+    let mut len = 0;
+    unsafe {
+        let symbols = llvm::LLVMRustPreservedSymbols(&mut len);
+        let symbols: &[*const _] = slice::from_raw_parts(symbols, len);
+        symbols
+            .iter()
+            .filter_map(|&symbol| {
+                if symbol.is_null() {
+                    None
+                } else {
+                    Some(String::from_utf8(CStr::from_ptr(symbol).to_bytes().to_vec()).unwrap())
+                }
+            })
+            .collect()
+    }
+}
+
 fn prepare_lto(
     cgcx: &CodegenContext<LlvmCodegenBackend>,
     diag_handler: &Handler,
@@ -53,8 +71,13 @@ fn prepare_lto(
         Lto::No => panic!("didn't request LTO but we're doing LTO"),
     };
 
+    let llvm_reserved_symbols = get_llvm_preserved_symbols();
+
     let symbol_filter = &|&(ref name, info): &(String, SymbolExportInfo)| {
-        if info.level.is_below_threshold(export_threshold) || info.used {
+        if info.level.is_below_threshold(export_threshold)
+            || info.used
+            || llvm_reserved_symbols.contains(name)
+        {
             Some(CString::new(name.as_str()).unwrap())
         } else {
             None

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2170,6 +2170,7 @@ extern "C" {
     pub fn LLVMRustSetLLVMOptions(Argc: c_int, Argv: *const *const c_char);
     pub fn LLVMRustPrintPasses();
     pub fn LLVMRustSetNormalizedTarget(M: &Module, triple: *const c_char);
+    pub fn LLVMRustPreservedSymbols(len: *mut usize) -> *const *const c_char;
     pub fn LLVMRustRunRestrictionPass(M: &Module, syms: *const *const c_char, len: size_t);
 
     pub fn LLVMRustOpenArchive(path: *const c_char) -> Option<&'static mut Archive>;

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -1043,6 +1043,20 @@ extern "C" void LLVMRustPrintPasses() {
   PB.printPassNames(outs());
 }
 
+// from /~https://github.com/llvm/llvm-project/blob/7021182d6b43de9488ab70de626192ce70b3a4a6/llvm/lib/Object/IRSymtab.cpp#L48-L57
+static const char *PreservedSymbols[] = {
+#define HANDLE_LIBCALL(code, name) name,
+#include "llvm/IR/RuntimeLibcalls.def"
+#undef HANDLE_LIBCALL
+    "__ssp_canary_word",
+    "__stack_chk_guard",
+};
+
+extern "C" const char **LLVMRustPreservedSymbols(size_t *len) {
+  *len = sizeof(PreservedSymbols) / sizeof(PreservedSymbols[0]);
+  return PreservedSymbols;
+}
+
 extern "C" void LLVMRustRunRestrictionPass(LLVMModuleRef M, char **Symbols,
                                            size_t Len) {
   auto PreserveFunctions = [=](const GlobalValue &GV) {

tests/run-make/wasm-spurious-import/main.rs → tests/run-make/wasm-builtins-import/main.rs

@@ -8,7 +8,7 @@ fn my_panic(_info: &core::panic::PanicInfo) -> ! {
 
 #[no_mangle]
 pub fn multer(a: i128, b: i128) -> i128 {
-    // Trigger usage of the __multi3 compiler intrinsic which then leads to an imported
-    // panic function in case of a bug. We verify that no imports exist in our verifier.
+    // Trigger usage of the __multi3 compiler intrinsic which then leads to an imported function
+    // such as panic or __multi3 in case of a bug. We verify that no imports exist in our verifier.
     a * b
 }