Auto merge of #33905 - eddyb:mir-overflow, r=nikomatsakis

[MIR] Implement overflow checking

The initial set of changes is from @Aatch's #33255 PR, rebased on master, plus:

Added an `Assert` terminator to MIR, to simplify working with overflow and bounds checks.
With this terminator, error cases can be accounted for directly, instead of looking for lang item calls.
It also keeps the MIR slimmer, with no extra explicit blocks for the actual panic calls.

Warnings can be produced when the `Assert` is known to always panic at runtime, e.g.:
```rust
warning: index out of bounds: the len is 1 but the index is 3
 --> <anon>:1:14
1 |> fn main() { &[std::io::stdout()][3]; }
  |>              ^^^^^^^^^^^^^^^^^^^^^^
```

Generalized the `OperandValue::FatPtr` optimization to any aggregate pair of immediates.
This allows us to generate the same IR for overflow checks as old trans, not something worse.
For example, addition on `i16` calls `llvm.sadd.with.overflow.i16`, which returns `{i16, i1}`.
However, the Rust type `(i16, bool)`, has to be `{i16, i8}`, only an immediate `bool` is `i1`.
But if we split the pair into an `i16` and an `i1`, we can pass them around as such for free.

The latest addition is a rebase of #34054, updated to work for pairs too. Closes #34054, fixes #33873.

Last but not least, the `#[rustc_inherit_overflow_checks]` attribute was introduced to control the
overflow checking behavior of generic or `#[inline]` functions, when translated in another crate.

It is **not** intended to be used by crates other than `libcore`, which is in the unusual position of
being distributed as only an optimized build with no checks, even when used from debug mode.
Before MIR-based translation, this worked out fine, as the decision for overflow was made at
translation time, in the crate being compiled, but MIR stored in `rlib` has to contain the checks.

To avoid always generating the checks and slowing everything down, a decision was made to
use an attribute in the few spots of `libcore` that need it (see #33255 for previous discussion):
* `core::ops::{Add, Sub, Mul, Neg, Shl, Shr}` implementations for integers, which have `#[inline]` methods and can be used in generic abstractions from other crates
* `core::ops::{Add, Sub, Mul, Neg, Shl, Shr}Assign` same as above, for augmented assignment
* `pow` and `abs` methods on integers, which intentionally piggy-back on built-in multiplication and negation, respectively, to get overflow checks
* `core::iter::{Iterator, Chain, Peek}::count` and `core::iter::Enumerate::{next, nth}`, also documented as panicking on overflow, from addition, counting elements of an iterator in an `usize`

src/libcore/iter/iterator.rs

@@ -172,6 +172,7 @@ pub trait Iterator {
     /// assert_eq!(a.iter().count(), 5);
     /// ```
     #[inline]
+    #[rustc_inherit_overflow_checks]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn count(self) -> usize where Self: Sized {
         // Might overflow.

src/libcore/iter/mod.rs

@@ -510,6 +510,7 @@ impl<A, B> Iterator for Chain<A, B> where
     }
 
     #[inline]
+    #[rustc_inherit_overflow_checks]
     fn count(self) -> usize {
         match self.state {
             ChainState::Both => self.a.count() + self.b.count(),
@@ -932,6 +933,7 @@ impl<I> Iterator for Enumerate<I> where I: Iterator {
     ///
     /// Might panic if the index of the element overflows a `usize`.
     #[inline]
+    #[rustc_inherit_overflow_checks]
     fn next(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
         self.iter.next().map(|a| {
             let ret = (self.count, a);
@@ -947,6 +949,7 @@ impl<I> Iterator for Enumerate<I> where I: Iterator {
     }
 
     #[inline]
+    #[rustc_inherit_overflow_checks]
     fn nth(&mut self, n: usize) -> Option<(usize, I::Item)> {
         self.iter.nth(n).map(|a| {
             let i = self.count + n;
@@ -1008,6 +1011,7 @@ impl<I: Iterator> Iterator for Peekable<I> {
     }
 
     #[inline]
+    #[rustc_inherit_overflow_checks]
     fn count(self) -> usize {
         (if self.peeked.is_some() { 1 } else { 0 }) + self.iter.count()
     }

src/libcore/num/mod.rs

@@ -1033,7 +1033,7 @@ macro_rules! int_impl {
         /// ```
         #[stable(feature = "rust1", since = "1.0.0")]
         #[inline]
-        #[rustc_no_mir] // FIXME #29769 MIR overflow checking is TBD.
+        #[rustc_inherit_overflow_checks]
         pub fn pow(self, mut exp: u32) -> Self {
             let mut base = self;
             let mut acc = Self::one();
@@ -1075,7 +1075,7 @@ macro_rules! int_impl {
         /// ```
         #[stable(feature = "rust1", since = "1.0.0")]
         #[inline]
-        #[rustc_no_mir] // FIXME #29769 MIR overflow checking is TBD.
+        #[rustc_inherit_overflow_checks]
         pub fn abs(self) -> Self {
             if self.is_negative() {
                 // Note that the #[inline] above means that the overflow
@@ -2061,7 +2061,7 @@ macro_rules! uint_impl {
         /// ```
         #[stable(feature = "rust1", since = "1.0.0")]
         #[inline]
-        #[rustc_no_mir] // FIXME #29769 MIR overflow checking is TBD.
+        #[rustc_inherit_overflow_checks]
         pub fn pow(self, mut exp: u32) -> Self {
             let mut base = self;
             let mut acc = Self::one();

src/libcore/ops.rs

@@ -208,6 +208,7 @@ macro_rules! add_impl {
             type Output = $t;
 
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn add(self, other: $t) -> $t { self + other }
         }
 
@@ -261,6 +262,7 @@ macro_rules! sub_impl {
             type Output = $t;
 
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn sub(self, other: $t) -> $t { self - other }
         }
 
@@ -314,6 +316,7 @@ macro_rules! mul_impl {
             type Output = $t;
 
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn mul(self, other: $t) -> $t { self * other }
         }
 
@@ -511,6 +514,7 @@ macro_rules! neg_impl_core {
             type Output = $t;
 
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn neg(self) -> $t { let $id = self; $body }
         }
 
@@ -788,6 +792,7 @@ macro_rules! shl_impl {
             type Output = $t;
 
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn shl(self, other: $f) -> $t {
                 self << other
             }
@@ -859,6 +864,7 @@ macro_rules! shr_impl {
             type Output = $t;
 
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn shr(self, other: $f) -> $t {
                 self >> other
             }
@@ -923,6 +929,7 @@ macro_rules! add_assign_impl {
         #[stable(feature = "op_assign_traits", since = "1.8.0")]
         impl AddAssign for $t {
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn add_assign(&mut self, other: $t) { *self += other }
         }
     )+)
@@ -967,6 +974,7 @@ macro_rules! sub_assign_impl {
         #[stable(feature = "op_assign_traits", since = "1.8.0")]
         impl SubAssign for $t {
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn sub_assign(&mut self, other: $t) { *self -= other }
         }
     )+)
@@ -1011,6 +1019,7 @@ macro_rules! mul_assign_impl {
         #[stable(feature = "op_assign_traits", since = "1.8.0")]
         impl MulAssign for $t {
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn mul_assign(&mut self, other: $t) { *self *= other }
         }
     )+)
@@ -1275,6 +1284,7 @@ macro_rules! shl_assign_impl {
         #[stable(feature = "op_assign_traits", since = "1.8.0")]
         impl ShlAssign<$f> for $t {
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn shl_assign(&mut self, other: $f) {
                 *self <<= other
             }
@@ -1337,6 +1347,7 @@ macro_rules! shr_assign_impl {
         #[stable(feature = "op_assign_traits", since = "1.8.0")]
         impl ShrAssign<$f> for $t {
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn shr_assign(&mut self, other: $f) {
                 *self >>= other
             }

src/librustc/mir/repr.rs

@@ -10,7 +10,7 @@
 
 use graphviz::IntoCow;
 use middle::const_val::ConstVal;
-use rustc_const_math::{ConstUsize, ConstInt};
+use rustc_const_math::{ConstUsize, ConstInt, ConstMathErr};
 use hir::def_id::DefId;
 use ty::subst::Substs;
 use ty::{self, AdtDef, ClosureSubsts, FnOutput, Region, Ty};
@@ -354,6 +354,16 @@ pub enum TerminatorKind<'tcx> {
         /// Cleanups to be done if the call unwinds.
         cleanup: Option<BasicBlock>
     },
+
+    /// Jump to the target if the condition has the expected value,
+    /// otherwise panic with a message and a cleanup target.
+    Assert {
+        cond: Operand<'tcx>,
+        expected: bool,
+        msg: AssertMessage<'tcx>,
+        target: BasicBlock,
+        cleanup: Option<BasicBlock>
+    }
 }
 
 impl<'tcx> Terminator<'tcx> {
@@ -389,6 +399,8 @@ impl<'tcx> TerminatorKind<'tcx> {
             Drop { ref target, unwind: None, .. } => {
                 slice::ref_slice(target).into_cow()
             }
+            Assert { target, cleanup: Some(unwind), .. } => vec![target, unwind].into_cow(),
+            Assert { ref target, .. } => slice::ref_slice(target).into_cow(),
         }
     }
 
@@ -413,6 +425,8 @@ impl<'tcx> TerminatorKind<'tcx> {
             Drop { ref mut target, unwind: None, .. } => {
                 vec![target]
             }
+            Assert { ref mut target, cleanup: Some(ref mut unwind), .. } => vec![target, unwind],
+            Assert { ref mut target, .. } => vec![target]
         }
     }
 }
@@ -495,6 +509,26 @@ impl<'tcx> TerminatorKind<'tcx> {
                 }
                 write!(fmt, ")")
             }
+            Assert { ref cond, expected, ref msg, .. } => {
+                write!(fmt, "assert(")?;
+                if !expected {
+                    write!(fmt, "!")?;
+                }
+                write!(fmt, "{:?}, ", cond)?;
+
+                match *msg {
+                    AssertMessage::BoundsCheck { ref len, ref index } => {
+                        write!(fmt, "{:?}, {:?}, {:?}",
+                               "index out of bounds: the len is {} but the index is {}",
+                               len, index)?;
+                    }
+                    AssertMessage::Math(ref err) => {
+                        write!(fmt, "{:?}", err.description())?;
+                    }
+                }
+
+                write!(fmt, ")")
+            }
         }
     }
 
@@ -532,10 +566,21 @@ impl<'tcx> TerminatorKind<'tcx> {
             Drop { unwind: Some(_), .. } => {
                 vec!["return".into_cow(), "unwind".into_cow()]
             }
+            Assert { cleanup: None, .. } => vec!["".into()],
+            Assert { .. } =>
+                vec!["success".into_cow(), "unwind".into_cow()]
         }
     }
 }
 
+#[derive(Clone, Debug, RustcEncodable, RustcDecodable)]
+pub enum AssertMessage<'tcx> {
+    BoundsCheck {
+        len: Operand<'tcx>,
+        index: Operand<'tcx>
+    },
+    Math(ConstMathErr)
+}
 
 ///////////////////////////////////////////////////////////////////////////
 // Statements
@@ -787,6 +832,7 @@ pub enum Rvalue<'tcx> {
     Cast(CastKind, Operand<'tcx>, Ty<'tcx>),
 
     BinaryOp(BinOp, Operand<'tcx>, Operand<'tcx>),
+    CheckedBinaryOp(BinOp, Operand<'tcx>, Operand<'tcx>),
 
     UnaryOp(UnOp, Operand<'tcx>),
 
@@ -880,6 +926,16 @@ pub enum BinOp {
     Gt,
 }
 
+impl BinOp {
+    pub fn is_checkable(self) -> bool {
+        use self::BinOp::*;
+        match self {
+            Add | Sub | Mul | Shl | Shr => true,
+            _ => false
+        }
+    }
+}
+
 #[derive(Copy, Clone, Debug, PartialEq, Eq, RustcEncodable, RustcDecodable)]
 pub enum UnOp {
     /// The `!` operator for logical inversion
@@ -898,6 +954,9 @@ impl<'tcx> Debug for Rvalue<'tcx> {
             Len(ref a) => write!(fmt, "Len({:?})", a),
             Cast(ref kind, ref lv, ref ty) => write!(fmt, "{:?} as {:?} ({:?})", lv, ty, kind),
             BinaryOp(ref op, ref a, ref b) => write!(fmt, "{:?}({:?}, {:?})", op, a, b),
+            CheckedBinaryOp(ref op, ref a, ref b) => {
+                write!(fmt, "Checked{:?}({:?}, {:?})", op, a, b)
+            }
             UnaryOp(ref op, ref a) => write!(fmt, "{:?}({:?})", op, a),
             Box(ref t) => write!(fmt, "Box({:?})", t),
             InlineAsm { ref asm, ref outputs, ref inputs } => {

src/librustc/mir/tcx.rs

@@ -183,6 +183,13 @@ impl<'a, 'gcx, 'tcx> Mir<'tcx> {
                 let rhs_ty = self.operand_ty(tcx, rhs);
                 Some(self.binop_ty(tcx, op, lhs_ty, rhs_ty))
             }
+            Rvalue::CheckedBinaryOp(op, ref lhs, ref rhs) => {
+                let lhs_ty = self.operand_ty(tcx, lhs);
+                let rhs_ty = self.operand_ty(tcx, rhs);
+                let ty = self.binop_ty(tcx, op, lhs_ty, rhs_ty);
+                let ty = tcx.mk_tup(vec![ty, tcx.types.bool]);
+                Some(ty)
+            }
             Rvalue::UnaryOp(_, ref operand) => {
                 Some(self.operand_ty(tcx, operand))
             }

src/librustc/mir/visit.rs

@@ -127,6 +127,11 @@ macro_rules! make_mir_visitor {
                 self.super_terminator_kind(block, kind);
             }
 
+            fn visit_assert_message(&mut self,
+                                    msg: & $($mutability)* AssertMessage<'tcx>) {
+                self.super_assert_message(msg);
+            }
+
             fn visit_rvalue(&mut self,
                             rvalue: & $($mutability)* Rvalue<'tcx>) {
                 self.super_rvalue(rvalue);
@@ -426,6 +431,31 @@ macro_rules! make_mir_visitor {
                         }
                         cleanup.map(|t| self.visit_branch(block, t));
                     }
+
+                    TerminatorKind::Assert { ref $($mutability)* cond,
+                                             expected: _,
+                                             ref $($mutability)* msg,
+                                             target,
+                                             cleanup } => {
+                        self.visit_operand(cond);
+                        self.visit_assert_message(msg);
+                        self.visit_branch(block, target);
+                        cleanup.map(|t| self.visit_branch(block, t));
+                    }
+                }
+            }
+
+            fn super_assert_message(&mut self,
+                                    msg: & $($mutability)* AssertMessage<'tcx>) {
+                match *msg {
+                    AssertMessage::BoundsCheck {
+                        ref $($mutability)* len,
+                        ref $($mutability)* index
+                    } => {
+                        self.visit_operand(len);
+                        self.visit_operand(index);
+                    }
+                    AssertMessage::Math(_) => {}
                 }
             }
 
@@ -461,6 +491,9 @@ macro_rules! make_mir_visitor {
                     }
 
                     Rvalue::BinaryOp(_bin_op,
+                                     ref $($mutability)* lhs,
+                                     ref $($mutability)* rhs) |
+                    Rvalue::CheckedBinaryOp(_bin_op,
                                      ref $($mutability)* lhs,
                                      ref $($mutability)* rhs) => {
                         self.visit_operand(lhs);

src/librustc_borrowck/borrowck/mir/dataflow/mod.rs

@@ -450,13 +450,14 @@ impl<'a, 'tcx: 'a, D> DataflowAnalysis<'a, 'tcx, D>
             repr::TerminatorKind::Return |
             repr::TerminatorKind::Resume => {}
             repr::TerminatorKind::Goto { ref target } |
+            repr::TerminatorKind::Assert { ref target, cleanup: None, .. } |
             repr::TerminatorKind::Drop { ref target, location: _, unwind: None } |
-
             repr::TerminatorKind::DropAndReplace {
                 ref target, value: _, location: _, unwind: None
             } => {
                 self.propagate_bits_into_entry_set_for(in_out, changed, target);
             }
+            repr::TerminatorKind::Assert { ref target, cleanup: Some(ref unwind), .. } |
             repr::TerminatorKind::Drop { ref target, location: _, unwind: Some(ref unwind) } |
             repr::TerminatorKind::DropAndReplace {
                 ref target, value: _, location: _, unwind: Some(ref unwind)

src/librustc_borrowck/borrowck/mir/gather_moves.rs

@@ -595,7 +595,8 @@ fn gather_moves<'a, 'tcx>(mir: &Mir<'tcx>, tcx: TyCtxt<'a, 'tcx, 'tcx>) -> MoveD
                             bb_ctxt.on_operand(SK::Repeat, operand, source),
                         Rvalue::Cast(ref _kind, ref operand, ref _ty) =>
                             bb_ctxt.on_operand(SK::Cast, operand, source),
-                        Rvalue::BinaryOp(ref _binop, ref operand1, ref operand2) => {
+                        Rvalue::BinaryOp(ref _binop, ref operand1, ref operand2) |
+                        Rvalue::CheckedBinaryOp(ref _binop, ref operand1, ref operand2) => {
                             bb_ctxt.on_operand(SK::BinaryOp, operand1, source);
                             bb_ctxt.on_operand(SK::BinaryOp, operand2, source);
                         }
@@ -662,6 +663,22 @@ fn gather_moves<'a, 'tcx>(mir: &Mir<'tcx>, tcx: TyCtxt<'a, 'tcx, 'tcx>) -> MoveD
                 bb_ctxt.on_operand(SK::If, cond, source);
             }
 
+            TerminatorKind::Assert {
+                ref cond, expected: _,
+                ref msg, target: _, cleanup: _
+            } => {
+                // The `cond` is always of (copyable) type `bool`,
+                // so there will never be anything to move.
+                let _ = cond;
+                match *msg {
+                    AssertMessage:: BoundsCheck { ref len, ref index } => {
+                        // Same for the usize length and index in bounds-checking.
+                        let _ = (len, index);
+                    }
+                    AssertMessage::Math(_) => {}
+                }
+            }
+
             TerminatorKind::SwitchInt { switch_ty: _, values: _, targets: _, ref discr } |
             TerminatorKind::Switch { adt_def: _, targets: _, ref discr } => {
                 // The `discr` is not consumed; that is instead