Set NumRegisterParameters LLVM module flag to N when -Zregparm=N is set

* Implement the -Zregparm=N flag by setting the NumRegisterParameters LLVM module flag.
* Add assembly tests verifying that the parameters are correctly passed in registers for regparm values 1, 2, and 3, for both LLVM intrinsics and non-builtin function
* Add c_void type to minicore

Author: winstonallo

compiler/rustc_codegen_llvm/src/context.rs

@@ -372,6 +372,15 @@ pub(crate) unsafe fn create_module<'ll>(
         }
     }
 
+    if let Some(regparm_count) = sess.opts.unstable_opts.regparm {
+        llvm::add_module_flag_u32(
+            llmod,
+            llvm::ModuleFlagMergeBehavior::Error,
+            "NumRegisterParameters",
+            regparm_count,
+        );
+    }
+
     if let Some(BranchProtection { bti, pac_ret }) = sess.opts.unstable_opts.branch_protection {
         if sess.target.arch == "aarch64" {
             llvm::add_module_flag_u32(

tests/assembly-llvm/regparm-module-flag.rs

@@ -0,0 +1,68 @@
+// Test the regparm ABI with builtin and non-builtin calls
+// Issue: https://github.com/rust-lang/rust/issues/145271
+//@ add-core-stubs
+//@ assembly-output: emit-asm
+//@ compile-flags: -O --target=i686-unknown-linux-gnu -Crelocation-model=static
+//@ revisions: REGPARM1 REGPARM2 REGPARM3
+//@[REGPARM1] compile-flags: -Zregparm=1
+//@[REGPARM2] compile-flags: -Zregparm=2
+//@[REGPARM3] compile-flags: -Zregparm=3
+//@ needs-llvm-components: x86
+#![feature(no_core)]
+#![no_std]
+#![no_core]
+#![crate_type = "lib"]
+
+extern crate minicore;
+use minicore::*;
+
+unsafe extern "C" {
+    fn memset(p: *mut c_void, val: i32, len: usize) -> *mut c_void;
+    fn non_builtin_memset(p: *mut c_void, val: i32, len: usize) -> *mut c_void;
+}
+
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn entrypoint(len: usize, ptr: *mut c_void, val: i32) -> *mut c_void {
+    // REGPARM1-LABEL: entrypoint
+    // REGPARM1: movl %e{{.*}}, %ecx
+    // REPGARM1: pushl
+    // REGPARM1: calll memset
+
+    // REGPARM2-LABEL: entrypoint
+    // REGPARM2: movl 16(%esp), %edx
+    // REGPARM2: movl %e{{.*}}, (%esp)
+    // REGPARM2: movl %e{{.*}}, %eax
+    // REGPARM2: calll memset
+
+    // REGPARM3-LABEL: entrypoint
+    // REGPARM3: movl %e{{.*}}, %esi
+    // REGPARM3: movl %e{{.*}}, %eax
+    // REGPARM3: movl %e{{.*}}, %ecx
+    // REGPARM3: jmp memset
+    unsafe { memset(ptr, val, len) }
+}
+
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn non_builtin_entrypoint(
+    len: usize,
+    ptr: *mut c_void,
+    val: i32,
+) -> *mut c_void {
+    // REGPARM1-LABEL: non_builtin_entrypoint
+    // REGPARM1: movl %e{{.*}}, %ecx
+    // REGPARM1: pushl
+    // REGPARM1: calll non_builtin_memset
+
+    // REGPARM2-LABEL: non_builtin_entrypoint
+    // REGPARM2: movl 16(%esp), %edx
+    // REGPARM2: movl %e{{.*}}, (%esp)
+    // REGPARM2: movl %e{{.*}}, %eax
+    // REGPARM2: calll non_builtin_memset
+
+    // REGPARM3-LABEL: non_builtin_entrypoint
+    // REGPARM3: movl %e{{.*}}, %esi
+    // REGPARM3: movl %e{{.*}}, %eax
+    // REGPARM3: movl %e{{.*}}, %ecx
+    // REGPARM3: jmp non_builtin_memset
+    unsafe { non_builtin_memset(ptr, val, len) }
+}

tests/auxiliary/minicore.rs

@@ -225,3 +225,10 @@ pub mod mem {
     #[rustc_intrinsic]
     pub unsafe fn transmute<Src, Dst>(src: Src) -> Dst;
 }
+
+#[lang = "c_void"]
+#[repr(u8)]
+pub enum c_void {
+    __variant1,
+    __variant2,
+}