Add -Zvirtual-function-elimination flag

Adds the virtual-function-elimination unstable compiler flag and a check
that this flag is only used in combination with -Clto. LLVM can only
apply this optimization with fat LTO.

Author: flip1995

compiler/rustc_interface/src/tests.rs

@@ -797,6 +797,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(unleash_the_miri_inside_of_you, true);
     tracked!(use_ctors_section, Some(true));
     tracked!(verify_llvm_ir, true);
+    tracked!(virtual_function_elimination, true);
     tracked!(wasi_exec_model, Some(WasiExecModel::Reactor));
 
     macro_rules! tracked_no_crate_hash {

compiler/rustc_session/src/options.rs

@@ -1579,6 +1579,9 @@ options! {
         "in general, enable more debug printouts (default: no)"),
     verify_llvm_ir: bool = (false, parse_bool, [TRACKED],
         "verify LLVM IR (default: no)"),
+    virtual_function_elimination: bool = (false, parse_bool, [TRACKED],
+        "enables dead virtual function elimination optimization. \
+        Requires `-Clto[=[fat,yes]]`"),
     wasi_exec_model: Option<WasiExecModel> = (None, parse_wasi_exec_model, [TRACKED],
         "whether to build a wasi command or reactor"),
 

compiler/rustc_session/src/session.rs

@@ -1431,14 +1431,14 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
         );
     }
 
-    // LLVM CFI requires LTO.
-    if sess.is_sanitizer_cfi_enabled() {
-        if sess.opts.cg.lto == config::LtoCli::Unspecified
-            || sess.opts.cg.lto == config::LtoCli::No
-            || sess.opts.cg.lto == config::LtoCli::Thin
-        {
+    // LLVM CFI and VFE both require LTO.
+    if sess.lto() != config::Lto::Fat {
+        if sess.is_sanitizer_cfi_enabled() {
             sess.err("`-Zsanitizer=cfi` requires `-Clto`");
         }
+        if sess.opts.debugging_opts.virtual_function_elimination {
+            sess.err("`-Zvirtual-function-elimination` requires `-Clto`");
+        }
     }
 
     if sess.opts.debugging_opts.stack_protector != StackProtector::None {