Add support for passing -Zregparm option to LLVM

introducing a new command-line option `-Zregparm` to `rustc`, and
modify the code generation phase to pass the `regparm` value to LLVM.

Signed-off-by: Vincenzo Palazzo <[email protected]>

Author: vincenzopalazzo

compiler/rustc_codegen_llvm/src/context.rs

@@ -305,6 +305,20 @@ pub(crate) unsafe fn create_module<'ll>(
         }
     }
 
+    if let Some(regparm) = sess.opts.cg.regparm {
+        if sess.target.arch != "x86" && sess.target.arch != "x86_64" {
+            todo!("regparm is not supported on this target");
+        }
+        unsafe {
+            llvm::LLVMRustAddModuleFlagU32(
+                llmod,
+                llvm::LLVMModFlagBehavior::Override,
+                c"NumRegisterParameters".as_ptr(),
+                regparm,
+            );
+        };
+    }
+
     if let Some(BranchProtection { bti, pac_ret }) = sess.opts.unstable_opts.branch_protection {
         if sess.target.arch == "aarch64" {
             unsafe {

compiler/rustc_session/src/options.rs

@@ -1595,6 +1595,8 @@ options! {
         "compile the program with profiling instrumentation"),
     profile_use: Option<PathBuf> = (None, parse_opt_pathbuf, [TRACKED],
         "use the given `.profdata` file for profile-guided optimization"),
+    regparm: Option<u32> = (None, parse_opt_number, [UNTRACKED],
+        "set the number of registers used for parameter passing"),
     #[rustc_lint_opt_deny_field_access("use `Session::relocation_model` instead of this field")]
     relocation_model: Option<RelocModel> = (None, parse_relocation_model, [TRACKED],
         "control generation of position-independent code (PIC) \