Auto merge of #107921 - cjgillot:codegen-overflow-check, r=tmiasko

Make codegen choose whether to emit overflow checks

ConstProp and DataflowConstProp currently have a specific code path not to propagate constants when they overflow. This is meant to have the correct behaviour when inlining from a crate with overflow checks (like `core`) into a crate compiled without.

This PR shifts the behaviour change to the `Assert(Overflow*)` MIR terminators: if the crate is compiled without overflow checks, just skip emitting the assertions. This is already what happens with `OverflowNeg`.

This allows ConstProp and DataflowConstProp to transform `CheckedBinaryOp(Add, u8::MAX, 1)` into `const (0, true)`, and let codegen ignore the `true`.

 The interpreter is modified to conform to this behaviour.

Fixes #35310

Author: bors

compiler/rustc_codegen_cranelift/src/base.rs

@@ -347,7 +347,12 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
             }
             TerminatorKind::Assert { cond, expected, msg, target, cleanup: _ } => {
                 if !fx.tcx.sess.overflow_checks() {
-                    if let mir::AssertKind::OverflowNeg(_) = *msg {
+                    let overflow_not_to_check = match msg {
+                        AssertKind::OverflowNeg(..) => true,
+                        AssertKind::Overflow(op, ..) => op.is_checkable(),
+                        _ => false,
+                    };
+                    if overflow_not_to_check {
                         let target = fx.get_block(*target);
                         fx.bcx.ins().jump(target, &[]);
                         continue;
@@ -567,15 +572,7 @@ fn codegen_stmt<'tcx>(
                     let lhs = codegen_operand(fx, &lhs_rhs.0);
                     let rhs = codegen_operand(fx, &lhs_rhs.1);
 
-                    let res = if !fx.tcx.sess.overflow_checks() {
-                        let val =
-                            crate::num::codegen_int_binop(fx, bin_op, lhs, rhs).load_scalar(fx);
-                        let is_overflow = fx.bcx.ins().iconst(types::I8, 0);
-                        CValue::by_val_pair(val, is_overflow, lval.layout())
-                    } else {
-                        crate::num::codegen_checked_int_binop(fx, bin_op, lhs, rhs)
-                    };
-
+                    let res = crate::num::codegen_checked_int_binop(fx, bin_op, lhs, rhs);
                     lval.write_cvalue(fx, res);
                 }
                 Rvalue::UnaryOp(un_op, ref operand) => {

compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs

@@ -493,20 +493,6 @@ fn codegen_regular_intrinsic_call<'tcx>(
             let res = crate::num::codegen_int_binop(fx, bin_op, x, y);
             ret.write_cvalue(fx, res);
         }
-        sym::add_with_overflow | sym::sub_with_overflow | sym::mul_with_overflow => {
-            intrinsic_args!(fx, args => (x, y); intrinsic);
-
-            assert_eq!(x.layout().ty, y.layout().ty);
-            let bin_op = match intrinsic {
-                sym::add_with_overflow => BinOp::Add,
-                sym::sub_with_overflow => BinOp::Sub,
-                sym::mul_with_overflow => BinOp::Mul,
-                _ => unreachable!(),
-            };
-
-            let res = crate::num::codegen_checked_int_binop(fx, bin_op, x, y);
-            ret.write_cvalue(fx, res);
-        }
         sym::saturating_add | sym::saturating_sub => {
             intrinsic_args!(fx, args => (lhs, rhs); intrinsic);
 

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -563,11 +563,13 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         // with #[rustc_inherit_overflow_checks] and inlined from
         // another crate (mostly core::num generic/#[inline] fns),
         // while the current crate doesn't use overflow checks.
-        // NOTE: Unlike binops, negation doesn't have its own
-        // checked operation, just a comparison with the minimum
-        // value, so we have to check for the assert message.
-        if !bx.check_overflow() {
-            if let AssertKind::OverflowNeg(_) = *msg {
+        if !bx.cx().check_overflow() {
+            let overflow_not_to_check = match msg {
+                AssertKind::OverflowNeg(..) => true,
+                AssertKind::Overflow(op, ..) => op.is_checkable(),
+                _ => false,
+            };
+            if overflow_not_to_check {
                 const_cond = Some(expected);
             }
         }

compiler/rustc_codegen_ssa/src/mir/intrinsic.rs

@@ -218,9 +218,6 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 args[1].val.unaligned_volatile_store(bx, dst);
                 return;
             }
-            sym::add_with_overflow
-            | sym::sub_with_overflow
-            | sym::mul_with_overflow
             | sym::unchecked_div
             | sym::unchecked_rem
             | sym::unchecked_shl
@@ -232,28 +229,6 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 let ty = arg_tys[0];
                 match int_type_width_signed(ty, bx.tcx()) {
                     Some((_width, signed)) => match name {
-                        sym::add_with_overflow
-                        | sym::sub_with_overflow
-                        | sym::mul_with_overflow => {
-                            let op = match name {
-                                sym::add_with_overflow => OverflowOp::Add,
-                                sym::sub_with_overflow => OverflowOp::Sub,
-                                sym::mul_with_overflow => OverflowOp::Mul,
-                                _ => bug!(),
-                            };
-                            let (val, overflow) =
-                                bx.checked_binop(op, ty, args[0].immediate(), args[1].immediate());
-                            // Convert `i1` to a `bool`, and write it to the out parameter
-                            let val = bx.from_immediate(val);
-                            let overflow = bx.from_immediate(overflow);
-
-                            let dest = result.project_field(bx, 0);
-                            bx.store(val, dest.llval, dest.align);
-                            let dest = result.project_field(bx, 1);
-                            bx.store(overflow, dest.llval, dest.align);
-
-                            return;
-                        }
                         sym::exact_div => {
                             if signed {
                                 bx.exactsdiv(args[0].immediate(), args[1].immediate())

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -652,15 +652,6 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         rhs: Bx::Value,
         input_ty: Ty<'tcx>,
     ) -> OperandValue<Bx::Value> {
-        // This case can currently arise only from functions marked
-        // with #[rustc_inherit_overflow_checks] and inlined from
-        // another crate (mostly core::num generic/#[inline] fns),
-        // while the current crate doesn't use overflow checks.
-        if !bx.cx().check_overflow() {
-            let val = self.codegen_scalar_binop(bx, op, lhs, rhs, input_ty);
-            return OperandValue::Pair(val, bx.cx().const_bool(false));
-        }
-
         let (val, of) = match op {
             // These are checked using intrinsics
             mir::BinOp::Add | mir::BinOp::Sub | mir::BinOp::Mul => {

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -210,19 +210,6 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let out_val = numeric_intrinsic(intrinsic_name, bits, kind);
                 self.write_scalar(out_val, dest)?;
             }
-            sym::add_with_overflow | sym::sub_with_overflow | sym::mul_with_overflow => {
-                let lhs = self.read_immediate(&args[0])?;
-                let rhs = self.read_immediate(&args[1])?;
-                let bin_op = match intrinsic_name {
-                    sym::add_with_overflow => BinOp::Add,
-                    sym::sub_with_overflow => BinOp::Sub,
-                    sym::mul_with_overflow => BinOp::Mul,
-                    _ => bug!(),
-                };
-                self.binop_with_overflow(
-                    bin_op, /*force_overflow_checks*/ true, &lhs, &rhs, dest,
-                )?;
-            }
             sym::saturating_add | sym::saturating_sub => {
                 let l = self.read_immediate(&args[0])?;
                 let r = self.read_immediate(&args[1])?;

compiler/rustc_const_eval/src/interpret/machine.rs

@@ -147,8 +147,9 @@ pub trait Machine<'mir, 'tcx>: Sized {
         true
     }
 
-    /// Whether CheckedBinOp MIR statements should actually check for overflow.
-    fn checked_binop_checks_overflow(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;
+    /// Whether Assert(OverflowNeg) and Assert(Overflow) MIR terminators should actually
+    /// check for overflow.
+    fn ignore_checkable_overflow_assertions(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool;
 
     /// Entry point for obtaining the MIR of anything that should get evaluated.
     /// So not just functions and shims, but also const/static initializers, anonymous
@@ -466,8 +467,8 @@ pub macro compile_time_machine(<$mir: lifetime, $tcx: lifetime>) {
     }
 
     #[inline(always)]
-    fn checked_binop_checks_overflow(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
-        true
+    fn ignore_checkable_overflow_assertions(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
+        false
     }
 
     #[inline(always)]

compiler/rustc_const_eval/src/interpret/operator.rs

@@ -10,13 +10,9 @@ use super::{ImmTy, Immediate, InterpCx, Machine, PlaceTy};
 impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     /// Applies the binary operation `op` to the two operands and writes a tuple of the result
     /// and a boolean signifying the potential overflow to the destination.
-    ///
-    /// `force_overflow_checks` indicates whether overflow checks should be done even when
-    /// `tcx.sess.overflow_checks()` is `false`.
     pub fn binop_with_overflow(
         &mut self,
         op: mir::BinOp,
-        force_overflow_checks: bool,
         left: &ImmTy<'tcx, M::Provenance>,
         right: &ImmTy<'tcx, M::Provenance>,
         dest: &PlaceTy<'tcx, M::Provenance>,
@@ -28,10 +24,6 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             "type mismatch for result of {:?}",
             op,
         );
-        // As per https://github.com/rust-lang/rust/pull/98738, we always return `false` in the 2nd
-        // component when overflow checking is disabled.
-        let overflowed =
-            overflowed && (force_overflow_checks || M::checked_binop_checks_overflow(self));
         // Write the result to `dest`.
         if let Abi::ScalarPair(..) = dest.layout.abi {
             // We can use the optimized path and avoid `place_field` (which might do

compiler/rustc_const_eval/src/interpret/step.rs

@@ -185,9 +185,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let left = self.read_immediate(&self.eval_operand(left, None)?)?;
                 let layout = binop_right_homogeneous(bin_op).then_some(left.layout);
                 let right = self.read_immediate(&self.eval_operand(right, layout)?)?;
-                self.binop_with_overflow(
-                    bin_op, /*force_overflow_checks*/ false, &left, &right, &dest,
-                )?;
+                self.binop_with_overflow(bin_op, &left, &right, &dest)?;
             }
 
             UnaryOp(un_op, ref operand) => {

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -137,8 +137,14 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
 
             Assert { ref cond, expected, ref msg, target, cleanup } => {
+                let ignored = M::ignore_checkable_overflow_assertions(self)
+                    && match msg {
+                        mir::AssertKind::OverflowNeg(..) => true,
+                        mir::AssertKind::Overflow(op, ..) => op.is_checkable(),
+                        _ => false,
+                    };
                 let cond_val = self.read_scalar(&self.eval_operand(cond, None)?)?.to_bool()?;
-                if expected == cond_val {
+                if ignored || expected == cond_val {
                     self.go_to_block(target);
                 } else {
                     M::assert_panic(self, msg, cleanup)?;

compiler/rustc_middle/src/mir/syntax.rs

@@ -671,6 +671,12 @@ pub enum TerminatorKind<'tcx> {
     /// as parameters, and `None` for the destination. Keep in mind that the `cleanup` path is not
     /// necessarily executed even in the case of a panic, for example in `-C panic=abort`. If the
     /// assertion does not fail, execution continues at the specified basic block.
+    ///
+    /// When overflow checking is disabled and this is run-time MIR (as opposed to compile-time MIR
+    /// that is used for CTFE), the following variants of this terminator behave as `goto target`:
+    /// - `OverflowNeg(..)`,
+    /// - `Overflow(op, ..)` if op is a "checkable" operation (add, sub, mul, shl, shr, but NOT
+    /// div or rem).
     Assert {
         cond: Operand<'tcx>,
         expected: bool,
@@ -1103,10 +1109,6 @@ pub enum Rvalue<'tcx> {
 
     /// Same as `BinaryOp`, but yields `(T, bool)` with a `bool` indicating an error condition.
     ///
-    /// When overflow checking is disabled and we are generating run-time code, the error condition
-    /// is false. Otherwise, and always during CTFE, the error condition is determined as described
-    /// below.
-    ///
     /// For addition, subtraction, and multiplication on integers the error condition is set when
     /// the infinite precision result would be unequal to the actual result.
     ///

compiler/rustc_mir_transform/src/const_prop.rs

@@ -15,7 +15,7 @@ use rustc_middle::mir::visit::{
 };
 use rustc_middle::mir::{
     BasicBlock, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, LocalKind, Location,
-    Operand, Place, Rvalue, SourceInfo, Statement, StatementKind, Terminator, TerminatorKind, UnOp,
+    Operand, Place, Rvalue, SourceInfo, Statement, StatementKind, Terminator, TerminatorKind,
     RETURN_PLACE,
 };
 use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
@@ -503,55 +503,6 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
         }
     }
 
-    fn check_unary_op(&mut self, op: UnOp, arg: &Operand<'tcx>) -> Option<()> {
-        if self.use_ecx(|this| {
-            let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
-            let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
-            Ok(overflow)
-        })? {
-            // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
-            // appropriate to use.
-            assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
-            return None;
-        }
-
-        Some(())
-    }
-
-    fn check_binary_op(
-        &mut self,
-        op: BinOp,
-        left: &Operand<'tcx>,
-        right: &Operand<'tcx>,
-    ) -> Option<()> {
-        let r = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?));
-        let l = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?));
-        // Check for exceeding shifts *even if* we cannot evaluate the LHS.
-        if matches!(op, BinOp::Shr | BinOp::Shl) {
-            let r = r.clone()?;
-            // We need the type of the LHS. We cannot use `place_layout` as that is the type
-            // of the result, which for checked binops is not the same!
-            let left_ty = left.ty(self.local_decls, self.tcx);
-            let left_size = self.ecx.layout_of(left_ty).ok()?.size;
-            let right_size = r.layout.size;
-            let r_bits = r.to_scalar().to_bits(right_size).ok();
-            if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
-                return None;
-            }
-        }
-
-        if let (Some(l), Some(r)) = (&l, &r) {
-            // The remaining operators are handled through `overflowing_binary_op`.
-            if self.use_ecx(|this| {
-                let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
-                Ok(overflow)
-            })? {
-                return None;
-            }
-        }
-        Some(())
-    }
-
     fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) {
         match *operand {
             Operand::Copy(l) | Operand::Move(l) => {
@@ -587,28 +538,6 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
         //   2. Working around bugs in other parts of the compiler
         //        - In this case, we'll return `None` from this function to stop evaluation.
         match rvalue {
-            // Additional checking: give lints to the user if an overflow would occur.
-            // We do this here and not in the `Assert` terminator as that terminator is
-            // only sometimes emitted (overflow checks can be disabled), but we want to always
-            // lint.
-            Rvalue::UnaryOp(op, arg) => {
-                trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
-                self.check_unary_op(*op, arg)?;
-            }
-            Rvalue::BinaryOp(op, box (left, right)) => {
-                trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
-                self.check_binary_op(*op, left, right)?;
-            }
-            Rvalue::CheckedBinaryOp(op, box (left, right)) => {
-                trace!(
-                    "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
-                    op,
-                    left,
-                    right
-                );
-                self.check_binary_op(*op, left, right)?;
-            }
-
             // Do not try creating references (#67862)
             Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
                 trace!("skipping AddressOf | Ref for {:?}", place);
@@ -638,7 +567,10 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             | Rvalue::Cast(..)
             | Rvalue::ShallowInitBox(..)
             | Rvalue::Discriminant(..)
-            | Rvalue::NullaryOp(..) => {}
+            | Rvalue::NullaryOp(..)
+            | Rvalue::UnaryOp(..)
+            | Rvalue::BinaryOp(..)
+            | Rvalue::CheckedBinaryOp(..) => {}
         }
 
         // FIXME we need to revisit this for #67176
@@ -1079,31 +1011,18 @@ impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
         // Do NOT early return in this function, it does some crucial fixup of the state at the end!
         match &mut terminator.kind {
             TerminatorKind::Assert { expected, ref mut cond, .. } => {
-                if let Some(ref value) = self.eval_operand(&cond) {
-                    trace!("assertion on {:?} should be {:?}", value, expected);
-                    let expected = Scalar::from_bool(*expected);
+                if let Some(ref value) = self.eval_operand(&cond)
                     // FIXME should be used use_ecx rather than a local match... but we have
                     // quite a few of these read_scalar/read_immediate that need fixing.
-                    if let Ok(value_const) = self.ecx.read_scalar(&value) {
-                        if expected != value_const {
-                            // Poison all places this operand references so that further code
-                            // doesn't use the invalid value
-                            match cond {
-                                Operand::Move(ref place) | Operand::Copy(ref place) => {
-                                    Self::remove_const(&mut self.ecx, place.local);
-                                }
-                                Operand::Constant(_) => {}
-                            }
-                        } else {
-                            if self.should_const_prop(value) {
-                                *cond = self.operand_from_scalar(
-                                    value_const,
-                                    self.tcx.types.bool,
-                                    source_info.span,
-                                );
-                            }
-                        }
-                    }
+                    && let Ok(value_const) = self.ecx.read_scalar(&value)
+                    && self.should_const_prop(value)
+                {
+                    trace!("assertion on {:?} should be {:?}", value, expected);
+                    *cond = self.operand_from_scalar(
+                        value_const,
+                        self.tcx.types.bool,
+                        source_info.span,
+                    );
                 }
             }
             TerminatorKind::SwitchInt { ref mut discr, .. } => {