Auto merge of #57442 - oli-obk:lazy_const, r=RalfJung

Simplify `ConstValue::ScalarPair`

While looking at #57432 I realized that some of our types for representing constants are very big. This reduces `LazyConst` to 3/4th of its original size and simplifies some code around slices at the same time.

r? @RalfJung

Author: bors

src/librustc/ich/impls_ty.rs

@@ -302,7 +302,7 @@ impl_stable_hash_for!(struct ty::FieldDef {
 impl_stable_hash_for!(
     impl<'tcx> for enum mir::interpret::ConstValue<'tcx> [ mir::interpret::ConstValue ] {
         Scalar(val),
-        ScalarPair(a, b),
+        Slice(a, b),
         ByRef(id, alloc, offset),
     }
 );

src/librustc/mir/interpret/pointer.rs

@@ -76,6 +76,8 @@ pub struct Pointer<Tag=(),Id=AllocId> {
     pub tag: Tag,
 }
 
+static_assert!(POINTER_SIZE: ::std::mem::size_of::<Pointer>() == 16);
+
 /// Produces a `Pointer` which points to the beginning of the Allocation
 impl From<AllocId> for Pointer {
     #[inline(always)]

src/librustc/mir/interpret/value.rs

@@ -22,22 +22,28 @@ pub enum ConstValue<'tcx> {
     /// Not using the enum `Value` to encode that this must not be `Undef`
     Scalar(Scalar),
 
-    /// Used only for *fat pointers* with layout::abi::ScalarPair
+    /// Used only for slices and strings (`&[T]`, `&str`, `*const [T]`, `*mut str`, `Box<str>`, ...)
     ///
-    /// Needed for pattern matching code related to slices and strings.
-    ScalarPair(Scalar, Scalar),
+    /// Empty slices don't necessarily have an address backed by an `AllocId`, thus we also need to
+    /// enable integer pointers. The `Scalar` type covers exactly those two cases. While we could
+    /// create dummy-`AllocId`s, the additional code effort for the conversions doesn't seem worth
+    /// it.
+    Slice(Scalar, u64),
 
     /// An allocation + offset into the allocation.
     /// Invariant: The AllocId matches the allocation.
     ByRef(AllocId, &'tcx Allocation, Size),
 }
 
+#[cfg(target_arch = "x86_64")]
+static_assert!(CONST_SIZE: ::std::mem::size_of::<ConstValue<'static>>() == 40);
+
 impl<'tcx> ConstValue<'tcx> {
     #[inline]
     pub fn try_to_scalar(&self) -> Option<Scalar> {
         match *self {
             ConstValue::ByRef(..) |
-            ConstValue::ScalarPair(..) => None,
+            ConstValue::Slice(..) => None,
             ConstValue::Scalar(val) => Some(val),
         }
     }
@@ -56,17 +62,8 @@ impl<'tcx> ConstValue<'tcx> {
     pub fn new_slice(
         val: Scalar,
         len: u64,
-        cx: &impl HasDataLayout
     ) -> Self {
-        ConstValue::ScalarPair(val, Scalar::Bits {
-            bits: len as u128,
-            size: cx.data_layout().pointer_size.bytes() as u8,
-        })
-    }
-
-    #[inline]
-    pub fn new_dyn_trait(val: Scalar, vtable: Pointer) -> Self {
-        ConstValue::ScalarPair(val, Scalar::Ptr(vtable))
+        ConstValue::Slice(val, len)
     }
 }
 
@@ -90,6 +87,9 @@ pub enum Scalar<Tag=(), Id=AllocId> {
     Ptr(Pointer<Tag, Id>),
 }
 
+#[cfg(target_arch = "x86_64")]
+static_assert!(SCALAR_SIZE: ::std::mem::size_of::<Scalar>() == 24);
+
 impl<Tag> fmt::Display for Scalar<Tag> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {

src/librustc/mir/mod.rs

@@ -2702,23 +2702,21 @@ pub fn fmt_const_val(f: &mut impl Write, const_val: ty::Const<'_>) -> fmt::Resul
         return write!(f, "{}", item_path_str(did));
     }
     // print string literals
-    if let ConstValue::ScalarPair(ptr, len) = value {
+    if let ConstValue::Slice(ptr, len) = value {
         if let Scalar::Ptr(ptr) = ptr {
-            if let Scalar::Bits { bits: len, .. } = len {
-                if let Ref(_, &ty::TyS { sty: Str, .. }, _) = ty.sty {
-                    return ty::tls::with(|tcx| {
-                        let alloc = tcx.alloc_map.lock().get(ptr.alloc_id);
-                        if let Some(interpret::AllocKind::Memory(alloc)) = alloc {
-                            assert_eq!(len as usize as u128, len);
-                            let slice =
-                                &alloc.bytes[(ptr.offset.bytes() as usize)..][..(len as usize)];
-                            let s = ::std::str::from_utf8(slice).expect("non utf8 str from miri");
-                            write!(f, "{:?}", s)
-                        } else {
-                            write!(f, "pointer to erroneous constant {:?}, {:?}", ptr, len)
-                        }
-                    });
-                }
+            if let Ref(_, &ty::TyS { sty: Str, .. }, _) = ty.sty {
+                return ty::tls::with(|tcx| {
+                    let alloc = tcx.alloc_map.lock().get(ptr.alloc_id);
+                    if let Some(interpret::AllocKind::Memory(alloc)) = alloc {
+                        assert_eq!(len as usize as u64, len);
+                        let slice =
+                            &alloc.bytes[(ptr.offset.bytes() as usize)..][..(len as usize)];
+                        let s = ::std::str::from_utf8(slice).expect("non utf8 str from miri");
+                        write!(f, "{:?}", s)
+                    } else {
+                        write!(f, "pointer to erroneous constant {:?}, {:?}", ptr, len)
+                    }
+                });
             }
         }
     }

src/librustc/ty/context.rs

@@ -1767,7 +1767,7 @@ macro_rules! nop_list_lift {
         impl<'a, 'tcx> Lift<'tcx> for &'a List<$ty> {
             type Lifted = &'tcx List<$lifted>;
             fn lift_to_tcx<'b, 'gcx>(&self, tcx: TyCtxt<'b, 'gcx, 'tcx>) -> Option<Self::Lifted> {
-                        if self.is_empty() {
+                if self.is_empty() {
                     return Some(List::empty());
                 }
                 if tcx.interners.arena.in_arena(*self as *const _) {

src/librustc/ty/structural_impls.rs

@@ -498,7 +498,7 @@ impl<'a, 'tcx> Lift<'tcx> for ConstValue<'a> {
     fn lift_to_tcx<'b, 'gcx>(&self, tcx: TyCtxt<'b, 'gcx, 'tcx>) -> Option<Self::Lifted> {
         match *self {
             ConstValue::Scalar(x) => Some(ConstValue::Scalar(x)),
-            ConstValue::ScalarPair(x, y) => Some(ConstValue::ScalarPair(x, y)),
+            ConstValue::Slice(x, y) => Some(ConstValue::Slice(x, y)),
             ConstValue::ByRef(x, alloc, z) => Some(ConstValue::ByRef(
                 x, alloc.lift_to_tcx(tcx)?, z,
             )),

src/librustc/ty/sty.rs

@@ -2063,6 +2063,9 @@ pub enum LazyConst<'tcx> {
     Evaluated(Const<'tcx>),
 }
 
+#[cfg(target_arch = "x86_64")]
+static_assert!(LAZY_CONST_SIZE: ::std::mem::size_of::<LazyConst<'static>>() == 56);
+
 impl<'tcx> LazyConst<'tcx> {
     pub fn map_evaluated<R>(self, f: impl FnOnce(Const<'tcx>) -> Option<R>) -> Option<R> {
         match self {
@@ -2089,6 +2092,9 @@ pub struct Const<'tcx> {
     pub val: ConstValue<'tcx>,
 }
 
+#[cfg(target_arch = "x86_64")]
+static_assert!(CONST_SIZE: ::std::mem::size_of::<Const<'static>>() == 48);
+
 impl<'tcx> Const<'tcx> {
     #[inline]
     pub fn from_scalar(

src/librustc_codegen_ssa/mir/operand.rs

@@ -88,21 +88,17 @@ impl<'a, 'tcx: 'a, V: CodegenObject> OperandRef<'tcx, V> {
                 );
                 OperandValue::Immediate(llval)
             },
-            ConstValue::ScalarPair(a, b) => {
-                let (a_scalar, b_scalar) = match layout.abi {
-                    layout::Abi::ScalarPair(ref a, ref b) => (a, b),
+            ConstValue::Slice(a, b) => {
+                let a_scalar = match layout.abi {
+                    layout::Abi::ScalarPair(ref a, _) => a,
                     _ => bug!("from_const: invalid ScalarPair layout: {:#?}", layout)
                 };
                 let a_llval = bx.cx().scalar_to_backend(
                     a,
                     a_scalar,
                     bx.cx().scalar_pair_element_backend_type(layout, 0, true),
                 );
-                let b_llval = bx.cx().scalar_to_backend(
-                    b,
-                    b_scalar,
-                    bx.cx().scalar_pair_element_backend_type(layout, 1, true),
-                );
+                let b_llval = bx.cx().const_usize(b);
                 OperandValue::Pair(a_llval, b_llval)
             },
             ConstValue::ByRef(_, alloc, offset) => {

src/librustc_mir/const_eval.rs

@@ -67,14 +67,11 @@ pub fn op_to_const<'tcx>(
     op: OpTy<'tcx>,
     may_normalize: bool,
 ) -> EvalResult<'tcx, ty::Const<'tcx>> {
-    // We do not normalize just any data.  Only scalar layout and fat pointers.
+    // We do not normalize just any data.  Only scalar layout and slices.
     let normalize = may_normalize
         && match op.layout.abi {
             layout::Abi::Scalar(..) => true,
-            layout::Abi::ScalarPair(..) => {
-                // Must be a fat pointer
-                op.layout.ty.builtin_deref(true).is_some()
-            },
+            layout::Abi::ScalarPair(..) => op.layout.ty.is_slice(),
             _ => false,
         };
     let normalized_op = if normalize {
@@ -103,7 +100,7 @@ pub fn op_to_const<'tcx>(
         Ok(Immediate::Scalar(x)) =>
             ConstValue::Scalar(x.not_undef()?),
         Ok(Immediate::ScalarPair(a, b)) =>
-            ConstValue::ScalarPair(a.not_undef()?, b.not_undef()?),
+            ConstValue::Slice(a.not_undef()?, b.to_usize(ecx)?),
     };
     Ok(ty::Const { val, ty: op.layout.ty })
 }

src/librustc_mir/hair/constant.rs

@@ -35,13 +35,13 @@ crate fn lit_to_const<'a, 'gcx, 'tcx>(
         LitKind::Str(ref s, _) => {
             let s = s.as_str();
             let id = tcx.allocate_bytes(s.as_bytes());
-            ConstValue::new_slice(Scalar::Ptr(id.into()), s.len() as u64, &tcx)
+            ConstValue::new_slice(Scalar::Ptr(id.into()), s.len() as u64)
         },
         LitKind::Err(ref s) => {
             let s = s.as_str();
             let id = tcx.allocate_bytes(s.as_bytes());
             return Ok(ty::Const {
-                val: ConstValue::new_slice(Scalar::Ptr(id.into()), s.len() as u64, &tcx),
+                val: ConstValue::new_slice(Scalar::Ptr(id.into()), s.len() as u64),
                 ty: tcx.types.err,
             });
         },