Fix SSA analysis for value types with PassMode::ByRef

Author: bjorn3

src/abi/mod.rs

@@ -10,7 +10,7 @@ use cranelift_codegen::ir::AbiParam;
 use self::pass_mode::*;
 use crate::prelude::*;
 
-pub use self::returning::codegen_return;
+pub use self::returning::{can_return_to_ssa_var, codegen_return};
 
 // Copied from https://github.com/rust-lang/rust/blob/c2f4c57296f0d929618baed0b0d6eb594abf01eb/src/librustc/ty/layout.rs#L2349
 pub fn fn_sig_for_fn_abi<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) -> ty::PolyFnSig<'tcx> {

src/abi/returning.rs

@@ -5,6 +5,14 @@ fn return_layout<'a, 'tcx>(fx: &mut FunctionCx<'a, 'tcx, impl Backend>) -> TyLay
     fx.layout_of(fx.monomorphize(&fx.mir.local_decls[RETURN_PLACE].ty))
 }
 
+pub fn can_return_to_ssa_var<'tcx>(tcx: TyCtxt<'tcx>, dest_layout: TyLayout<'tcx>) -> bool {
+    match get_pass_mode(tcx, dest_layout) {
+        PassMode::NoPass | PassMode::ByVal(_) => true,
+        // FIXME Make it possible to return ByValPair and ByRef to an ssa var.
+        PassMode::ByValPair(_, _) | PassMode::ByRef => false
+    }
+}
+
 pub fn codegen_return_param(
     fx: &mut FunctionCx<impl Backend>,
     ssa_analyzed: &rustc_index::vec::IndexVec<Local, crate::analyze::SsaKind>,

src/analyze.rs

@@ -30,6 +30,18 @@ pub fn analyze(fx: &FunctionCx<'_, '_, impl Backend>) -> IndexVec<Local, SsaKind
                 _ => {}
             }
         }
+
+        match &bb.terminator().kind {
+            TerminatorKind::Call { destination, .. } => {
+                if let Some((dest_place, _dest_bb)) = destination {
+                    let dest_layout = fx.layout_of(fx.monomorphize(&dest_place.ty(&fx.mir.local_decls, fx.tcx).ty));
+                    if !crate::abi::can_return_to_ssa_var(fx.tcx, dest_layout) {
+                        analyze_non_ssa_place(&mut flag_map, dest_place);
+                    }
+                }
+            }
+            _ => {}
+        }
     }
 
     flag_map