Rollup merge of #162998 - saethlin:dse-ret-is-arg, r=oli-obk

Avoid generating overlapping assignments in DSE

This is a fix for #162997.

Considering we also had #155680, I really wonder if this pass should be using LivenessTransferFunction at all.

Author: JonathanBrouwer

compiler/rustc_mir_transform/src/dead_store_elimination.rs

@@ -63,6 +63,13 @@ fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) -> bool {
                 loc,
             );
 
+            // The logic in LivenessTransferFunction isn't quite what we need; it ignores call
+            // destinations that are just locals because they are killed by the call, which makes
+            // it eligible to be moved-from in the argument list. That's backwards.
+            if !destination.is_indirect() {
+                state.insert(destination.local);
+            }
+
             for (index, arg) in args.iter().map(|a| &a.node).enumerate().rev() {
                 if let Operand::Copy(place) = *arg
                     && !place.is_indirect()

tests/mir-opt/dead-store-elimination/call_arg_copy.move_index.DeadStoreElimination-final.panic-abort.diff

@@ -5,7 +5,7 @@
       let mut _0: ();
 
       bb0: {
-          _1[_2] = passthrough_usize(copy _2) -> [return: bb1, unwind unreachable];
+          _1[_2] = identity::<usize>(copy _2) -> [return: bb1, unwind unreachable];
       }
 
       bb1: {

tests/mir-opt/dead-store-elimination/call_arg_copy.move_index.DeadStoreElimination-final.panic-unwind.diff

@@ -5,7 +5,7 @@
       let mut _0: ();
 
       bb0: {
-          _1[_2] = passthrough_usize(copy _2) -> [return: bb1, unwind continue];
+          _1[_2] = identity::<usize>(copy _2) -> [return: bb1, unwind continue];
       }
 
       bb1: {

tests/mir-opt/dead-store-elimination/call_arg_copy.ret_is_arg.DeadStoreElimination-final.panic-abort.diff

@@ -0,0 +1,16 @@
+- // MIR for `ret_is_arg` before DeadStoreElimination-final
++ // MIR for `ret_is_arg` after DeadStoreElimination-final
+  
+  fn ret_is_arg(_1: [u64; 5]) -> [u64; 5] {
+      let mut _0: [u64; 5];
+  
+      bb0: {
+          _1 = identity::<[u64; 5]>(copy _1) -> [return: bb1, unwind unreachable];
+      }
+  
+      bb1: {
+          _0 = copy _1;
+          return;
+      }
+  }
+  

tests/mir-opt/dead-store-elimination/call_arg_copy.ret_is_arg.DeadStoreElimination-final.panic-unwind.diff

@@ -0,0 +1,16 @@
+- // MIR for `ret_is_arg` before DeadStoreElimination-final
++ // MIR for `ret_is_arg` after DeadStoreElimination-final
+  
+  fn ret_is_arg(_1: [u64; 5]) -> [u64; 5] {
+      let mut _0: [u64; 5];
+  
+      bb0: {
+          _1 = identity::<[u64; 5]>(copy _1) -> [return: bb1, unwind unreachable];
+      }
+  
+      bb1: {
+          _0 = copy _1;
+          return;
+      }
+  }
+  

tests/mir-opt/dead-store-elimination/call_arg_copy.rs

@@ -6,6 +6,7 @@
 #![feature(custom_mir)]
 #![allow(internal_features)]
 
+use std::convert::identity;
 use std::intrinsics::mir::*;
 
 #[inline(never)]
@@ -40,31 +41,43 @@ fn move_packed(packed: Packed) {
     }
 }
 
-#[inline(never)]
-fn passthrough_usize(a: usize) -> usize {
-    a
-}
-
 // EMIT_MIR call_arg_copy.move_index.DeadStoreElimination-final.diff
 #[custom_mir(dialect = "analysis")]
 fn move_index(a: [usize; 10], b: usize) {
     // CHECK-LABEL: fn move_index(
-    // CHECK: = passthrough_usize(copy _2)
+    // CHECK: = identity::<usize>(copy _2)
     mir! {
         {
             // The index is used again after the operand is evaluated to
-            // evaluate the destionation place, so the argument cannot be turned
+            // evaluate the destination place, so the argument cannot be turned
             // into a move.
-            Call(a[b] = passthrough_usize(b), ReturnTo(ret), UnwindContinue())
+            Call(a[b] = identity(b), ReturnTo(ret), UnwindContinue())
         }
         ret = {
             Return()
         }
     }
 }
 
+// EMIT_MIR call_arg_copy.ret_is_arg.DeadStoreElimination-final.diff
+#[custom_mir(dialect = "runtime")]
+fn ret_is_arg(x: [u64; 5]) -> [u64; 5] {
+    // CHECK-LABEL: fn ret_is_arg(_1
+    // CHECK: _1 = identity::<[u64; 5]>(copy _1) ->
+    mir! {
+        {
+            Call(x = identity(x), ReturnTo(bb1), UnwindUnreachable())
+        }
+        bb1 = {
+            RET = x;
+            Return()
+        }
+    }
+}
+
 fn main() {
     move_simple(1);
     move_packed(Packed { x: 0, y: 1 });
     move_index([0; _], 1);
+    ret_is_arg([0; 5]);
 }