Rollup merge of #157220 - scottmcm:less-from-packed-pair, r=dianqk

cg_ssa: a bit less `immediate_or_packed_pair`

This is one of the things that made cg_clif not use cg_ssa, IIRC, so let's take the opportunities to avoid it where we can.

r? codegen

Author: JonathanBrouwer

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -1737,7 +1737,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     op.store_with_annotation(bx, scratch);
                     (scratch.val.llval, scratch.val.align, true)
                 }
-                _ => (op.immediate_or_packed_pair(bx), arg.layout.align.abi, false),
+                PassMode::Direct(_) => (op.immediate(), arg.layout.align.abi, false),
+                PassMode::Ignore | PassMode::Pair(..) => unreachable!("handled above"),
             },
             Ref(op_place_val) => match arg.mode {
                 PassMode::Indirect { attrs, on_stack, .. } => {

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -499,9 +499,12 @@ fn arg_local_refs<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
                     PassMode::Direct(_) => {
                         let llarg = bx.get_param(llarg_idx);
                         llarg_idx += 1;
-                        return local(OperandRef::from_immediate_or_packed_pair(
-                            bx, llarg, arg.layout,
-                        ));
+                        debug_assert!(bx.is_backend_immediate(arg.layout));
+                        return local(OperandRef {
+                            val: OperandValue::Immediate(llarg),
+                            layout: arg.layout,
+                            move_annotation: None,
+                        });
                     }
                     PassMode::Pair(..) => {
                         let (a, b) = (bx.get_param(llarg_idx), bx.get_param(llarg_idx + 1));

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -306,6 +306,9 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
 
     /// If this operand is a `Pair`, we return an aggregate with the two values.
     /// For other cases, see `immediate`.
+    ///
+    /// Note: The use of this is discouraged outside cg_llvm, as some other backends
+    /// don't natively support packing multiple things into one like this.
     pub fn immediate_or_packed_pair<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
         self,
         bx: &mut Bx,
@@ -324,6 +327,9 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
     }
 
     /// If the type is a pair, we return a `Pair`, otherwise, an `Immediate`.
+    ///
+    /// Note: The use of this is discouraged outside cg_llvm, as some other backends
+    /// don't natively support packing multiple things into one like this.
     pub fn from_immediate_or_packed_pair<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
         bx: &mut Bx,
         llval: V,