Compute unsizing casts in GVN.

Author: cjgillot

compiler/rustc_const_eval/src/interpret/cast.rs

@@ -416,7 +416,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         }
     }
 
-    fn unsize_into(
+    pub fn unsize_into(
         &mut self,
         src: &OpTy<'tcx, M::Provenance>,
         cast_ty: TyAndLayout<'tcx>,

compiler/rustc_mir_transform/src/gvn.rs

@@ -553,6 +553,16 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                     }
                     value.offset(Size::ZERO, to, &self.ecx).ok()?
                 }
+                CastKind::PointerCoercion(ty::adjustment::PointerCoercion::Unsize) => {
+                    let src = self.evaluated[value].as_ref()?;
+                    let to = self.ecx.layout_of(to).ok()?;
+                    let dest = self.ecx.allocate(to, MemoryKind::Stack).ok()?;
+                    self.ecx.unsize_into(src, to, &dest.clone().into()).ok()?;
+                    self.ecx
+                        .alloc_mark_immutable(dest.ptr().provenance.unwrap().alloc_id())
+                        .ok()?;
+                    dest.into()
+                }
                 _ => return None,
             },
         };

tests/mir-opt/const_prop/slice_len.main.GVN.32bit.panic-abort.diff

@@ -31,16 +31,17 @@
           StorageDead(_3);
           StorageLive(_6);
           _6 = const 1_usize;
-          _7 = Len((*_2));
+-         _7 = Len((*_2));
 -         _8 = Lt(_6, _7);
 -         assert(move _8, "index out of bounds: the length is {} but the index is {}", move _7, _6) -> [success: bb1, unwind unreachable];
-+         _8 = Lt(const 1_usize, _7);
-+         assert(move _8, "index out of bounds: the length is {} but the index is {}", move _7, const 1_usize) -> [success: bb1, unwind unreachable];
++         _7 = const 3_usize;
++         _8 = const true;
++         assert(const true, "index out of bounds: the length is {} but the index is {}", const 3_usize, const 1_usize) -> [success: bb1, unwind unreachable];
       }
 
       bb1: {
 -         _1 = (*_2)[_6];
-+         _1 = (*_2)[1 of 2];
++         _1 = const 2_u32;
           StorageDead(_6);
           StorageDead(_4);
           StorageDead(_2);

tests/mir-opt/const_prop/slice_len.main.GVN.32bit.panic-unwind.diff

@@ -31,16 +31,17 @@
           StorageDead(_3);
           StorageLive(_6);
           _6 = const 1_usize;
-          _7 = Len((*_2));
+-         _7 = Len((*_2));
 -         _8 = Lt(_6, _7);
 -         assert(move _8, "index out of bounds: the length is {} but the index is {}", move _7, _6) -> [success: bb1, unwind continue];
-+         _8 = Lt(const 1_usize, _7);
-+         assert(move _8, "index out of bounds: the length is {} but the index is {}", move _7, const 1_usize) -> [success: bb1, unwind continue];
++         _7 = const 3_usize;
++         _8 = const true;
++         assert(const true, "index out of bounds: the length is {} but the index is {}", const 3_usize, const 1_usize) -> [success: bb1, unwind continue];
       }
 
       bb1: {
 -         _1 = (*_2)[_6];
-+         _1 = (*_2)[1 of 2];
++         _1 = const 2_u32;
           StorageDead(_6);
           StorageDead(_4);
           StorageDead(_2);

tests/mir-opt/const_prop/slice_len.main.GVN.64bit.panic-abort.diff

@@ -31,16 +31,17 @@
           StorageDead(_3);
           StorageLive(_6);
           _6 = const 1_usize;
-          _7 = Len((*_2));
+-         _7 = Len((*_2));
 -         _8 = Lt(_6, _7);
 -         assert(move _8, "index out of bounds: the length is {} but the index is {}", move _7, _6) -> [success: bb1, unwind unreachable];
-+         _8 = Lt(const 1_usize, _7);
-+         assert(move _8, "index out of bounds: the length is {} but the index is {}", move _7, const 1_usize) -> [success: bb1, unwind unreachable];
++         _7 = const 3_usize;
++         _8 = const true;
++         assert(const true, "index out of bounds: the length is {} but the index is {}", const 3_usize, const 1_usize) -> [success: bb1, unwind unreachable];
       }
 
       bb1: {
 -         _1 = (*_2)[_6];
-+         _1 = (*_2)[1 of 2];
++         _1 = const 2_u32;
           StorageDead(_6);
           StorageDead(_4);
           StorageDead(_2);

tests/mir-opt/const_prop/slice_len.main.GVN.64bit.panic-unwind.diff

@@ -31,16 +31,17 @@
           StorageDead(_3);
           StorageLive(_6);
           _6 = const 1_usize;
-          _7 = Len((*_2));
+-         _7 = Len((*_2));
 -         _8 = Lt(_6, _7);
 -         assert(move _8, "index out of bounds: the length is {} but the index is {}", move _7, _6) -> [success: bb1, unwind continue];
-+         _8 = Lt(const 1_usize, _7);
-+         assert(move _8, "index out of bounds: the length is {} but the index is {}", move _7, const 1_usize) -> [success: bb1, unwind continue];
++         _7 = const 3_usize;
++         _8 = const true;
++         assert(const true, "index out of bounds: the length is {} but the index is {}", const 3_usize, const 1_usize) -> [success: bb1, unwind continue];
       }
 
       bb1: {
 -         _1 = (*_2)[_6];
-+         _1 = (*_2)[1 of 2];
++         _1 = const 2_u32;
           StorageDead(_6);
           StorageDead(_4);
           StorageDead(_2);

tests/mir-opt/const_prop/slice_len.rs

@@ -8,9 +8,7 @@ fn main() {
     // CHECK-LABEL: fn main(
     // CHECK: debug a => [[a:_.*]];
     // CHECK: [[slice:_.*]] = const {{.*}} as &[u32] (PointerCoercion(Unsize));
-    // FIXME(cjgillot) simplify Len and projection into unsized slice.
-    // CHECK-NOT: assert(const true,
-    // CHECK: [[a]] = (*[[slice]])[1 of 2];
-    // CHECK-NOT: [[a]] = const 2_u32;
+    // CHECK: assert(const true,
+    // CHECK: [[a]] = const 2_u32;
     let a = (&[1u32, 2, 3] as &[u32])[1];
 }