Auto merge of #66646 - RalfJung:unwind_to_block, r=oli-obk

refactor goto_block and also add unwind_to_block

r? @oli-obk

src/librustc_mir/const_eval.rs

@@ -323,8 +323,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
         instance: ty::Instance<'tcx>,
         args: &[OpTy<'tcx>],
-        dest: Option<PlaceTy<'tcx>>,
-        ret: Option<mir::BasicBlock>,
+        ret: Option<(PlaceTy<'tcx>, mir::BasicBlock)>,
         _unwind: Option<mir::BasicBlock> // unwinding is not supported in consts
     ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>> {
         debug!("eval_fn_call: {:?}", instance);
@@ -337,8 +336,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
                 // Some functions we support even if they are non-const -- but avoid testing
                 // that for const fn!  We certainly do *not* want to actually call the fn
                 // though, so be sure we return here.
-                return if ecx.hook_panic_fn(instance, args, dest)? {
-                    ecx.goto_block(ret)?; // fully evaluated and done
+                return if ecx.hook_panic_fn(instance, args, ret)? {
                     Ok(None)
                 } else {
                     throw_unsup_format!("calling non-const function `{}`", instance)
@@ -364,8 +362,8 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
         fn_val: !,
         _args: &[OpTy<'tcx>],
-        _dest: Option<PlaceTy<'tcx>>,
-        _ret: Option<mir::BasicBlock>,
+        _ret: Option<(PlaceTy<'tcx>, mir::BasicBlock)>,
+        _unwind: Option<mir::BasicBlock>
     ) -> InterpResult<'tcx> {
         match fn_val {}
     }
@@ -375,11 +373,10 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         span: Span,
         instance: ty::Instance<'tcx>,
         args: &[OpTy<'tcx>],
-        dest: Option<PlaceTy<'tcx>>,
-        _ret: Option<mir::BasicBlock>,
+        ret: Option<(PlaceTy<'tcx>, mir::BasicBlock)>,
         _unwind: Option<mir::BasicBlock>
     ) -> InterpResult<'tcx> {
-        if ecx.emulate_intrinsic(span, instance, args, dest)? {
+        if ecx.emulate_intrinsic(span, instance, args, ret)? {
             return Ok(());
         }
         // An intrinsic that we do not support

src/librustc_mir/interpret/eval_context.rs

@@ -555,6 +555,37 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         }
     }
 
+    /// Jump to the given block.
+    #[inline]
+    pub fn go_to_block(&mut self, target: mir::BasicBlock) {
+        let frame = self.frame_mut();
+        frame.block = Some(target);
+        frame.stmt = 0;
+    }
+
+    /// *Return* to the given `target` basic block.
+    /// Do *not* use for unwinding! Use `unwind_to_block` instead.
+    ///
+    /// If `target` is `None`, that indicates the function cannot return, so we raise UB.
+    pub fn return_to_block(&mut self, target: Option<mir::BasicBlock>) -> InterpResult<'tcx> {
+        if let Some(target) = target {
+            Ok(self.go_to_block(target))
+        } else {
+            throw_ub!(Unreachable)
+        }
+    }
+
+    /// *Unwind* to the given `target` basic block.
+    /// Do *not* use for returning! Use `return_to_block` instead.
+    ///
+    /// If `target` is `None`, that indicates the function does not need cleanup during
+    /// unwinding, and we will just keep propagating that upwards.
+    pub fn unwind_to_block(&mut self, target: Option<mir::BasicBlock>) {
+        let frame = self.frame_mut();
+        frame.block = target;
+        frame.stmt = 0;
+    }
+
     /// Pops the current frame from the stack, deallocating the
     /// memory for allocated locals.
     ///
@@ -630,10 +661,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         if cur_unwinding {
             // Follow the unwind edge.
             let unwind = next_block.expect("Encounted StackPopCleanup::None when unwinding!");
-            let next_frame = self.frame_mut();
-            // If `unwind` is `None`, we'll leave that function immediately again.
-            next_frame.block = unwind;
-            next_frame.stmt = 0;
+            self.unwind_to_block(unwind);
         } else {
             // Follow the normal return edge.
             // Validate the return value. Do this after deallocating so that we catch dangling
@@ -660,7 +688,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
             // Jump to new block -- *after* validation so that the spans make more sense.
             if let Some(ret) = next_block {
-                self.goto_block(ret)?;
+                self.return_to_block(ret)?;
             }
         }
 

src/librustc_mir/interpret/intrinsics.rs

@@ -9,8 +9,10 @@ use rustc::ty::layout::{LayoutOf, Primitive, Size};
 use rustc::ty::subst::SubstsRef;
 use rustc::hir::def_id::DefId;
 use rustc::ty::TyCtxt;
-use rustc::mir::BinOp;
-use rustc::mir::interpret::{InterpResult, Scalar, GlobalId, ConstValue};
+use rustc::mir::{
+    self, BinOp,
+    interpret::{InterpResult, Scalar, GlobalId, ConstValue}
+};
 
 use super::{
     Machine, PlaceTy, OpTy, InterpCx, ImmTy,
@@ -91,16 +93,20 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         span: Span,
         instance: ty::Instance<'tcx>,
         args: &[OpTy<'tcx, M::PointerTag>],
-        dest: Option<PlaceTy<'tcx, M::PointerTag>>,
+        ret: Option<(PlaceTy<'tcx, M::PointerTag>, mir::BasicBlock)>,
     ) -> InterpResult<'tcx, bool> {
         let substs = instance.substs;
+        let intrinsic_name = &*self.tcx.item_name(instance.def_id()).as_str();
 
-        // We currently do not handle any diverging intrinsics.
-        let dest = match dest {
-            Some(dest) => dest,
-            None => return Ok(false)
+        // We currently do not handle any intrinsics that are *allowed* to diverge,
+        // but `transmute` could lack a return place in case of UB.
+        let (dest, ret) = match ret {
+            Some(p) => p,
+            None => match intrinsic_name {
+                "transmute" => throw_ub!(Unreachable),
+                _ => return Ok(false),
+            }
         };
-        let intrinsic_name = &*self.tcx.item_name(instance.def_id()).as_str();
 
         match intrinsic_name {
             "caller_location" => {
@@ -268,34 +274,39 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 // exception from the exception.)
                 // This is the dual to the special exception for offset-by-0
                 // in the inbounds pointer offset operation (see the Miri code, `src/operator.rs`).
-                if a.is_bits() && b.is_bits() {
+                //
+                // Control flow is weird because we cannot early-return (to reach the
+                // `go_to_block` at the end).
+                let done = if a.is_bits() && b.is_bits() {
                     let a = a.to_machine_usize(self)?;
                     let b = b.to_machine_usize(self)?;
                     if a == b && a != 0 {
                         self.write_scalar(Scalar::from_int(0, isize_layout.size), dest)?;
-                        return Ok(true);
-                    }
-                }
+                        true
+                    } else { false }
+                } else { false };
 
-                // General case: we need two pointers.
-                let a = self.force_ptr(a)?;
-                let b = self.force_ptr(b)?;
-                if a.alloc_id != b.alloc_id {
-                    throw_ub_format!(
-                        "ptr_offset_from cannot compute offset of pointers into different \
-                        allocations.",
-                    );
+                if !done {
+                    // General case: we need two pointers.
+                    let a = self.force_ptr(a)?;
+                    let b = self.force_ptr(b)?;
+                    if a.alloc_id != b.alloc_id {
+                        throw_ub_format!(
+                            "ptr_offset_from cannot compute offset of pointers into different \
+                            allocations.",
+                        );
+                    }
+                    let usize_layout = self.layout_of(self.tcx.types.usize)?;
+                    let a_offset = ImmTy::from_uint(a.offset.bytes(), usize_layout);
+                    let b_offset = ImmTy::from_uint(b.offset.bytes(), usize_layout);
+                    let (val, _overflowed, _ty) = self.overflowing_binary_op(
+                        BinOp::Sub, a_offset, b_offset,
+                    )?;
+                    let pointee_layout = self.layout_of(substs.type_at(0))?;
+                    let val = ImmTy::from_scalar(val, isize_layout);
+                    let size = ImmTy::from_int(pointee_layout.size.bytes(), isize_layout);
+                    self.exact_div(val, size, dest)?;
                 }
-                let usize_layout = self.layout_of(self.tcx.types.usize)?;
-                let a_offset = ImmTy::from_uint(a.offset.bytes(), usize_layout);
-                let b_offset = ImmTy::from_uint(b.offset.bytes(), usize_layout);
-                let (val, _overflowed, _ty) = self.overflowing_binary_op(
-                    BinOp::Sub, a_offset, b_offset,
-                )?;
-                let pointee_layout = self.layout_of(substs.type_at(0))?;
-                let val = ImmTy::from_scalar(val, isize_layout);
-                let size = ImmTy::from_int(pointee_layout.size.bytes(), isize_layout);
-                self.exact_div(val, size, dest)?;
             }
 
             "transmute" => {
@@ -350,6 +361,8 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             _ => return Ok(false),
         }
 
+        self.dump_place(*dest);
+        self.go_to_block(ret);
         Ok(true)
     }
 
@@ -360,7 +373,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         &mut self,
         instance: ty::Instance<'tcx>,
         args: &[OpTy<'tcx, M::PointerTag>],
-        _dest: Option<PlaceTy<'tcx, M::PointerTag>>,
+        _ret: Option<(PlaceTy<'tcx, M::PointerTag>, mir::BasicBlock)>,
     ) -> InterpResult<'tcx, bool> {
         let def_id = instance.def_id();
         if Some(def_id) == self.tcx.lang_items().panic_fn() {

src/librustc_mir/interpret/machine.rs

@@ -141,7 +141,7 @@ pub trait Machine<'mir, 'tcx>: Sized {
     /// Returns either the mir to use for the call, or `None` if execution should
     /// just proceed (which usually means this hook did all the work that the
     /// called function should usually have done). In the latter case, it is
-    /// this hook's responsibility to call `goto_block(ret)` to advance the instruction pointer!
+    /// this hook's responsibility to advance the instruction pointer!
     /// (This is to support functions like `__rust_maybe_catch_panic` that neither find a MIR
     /// nor just jump to `ret`, but instead push their own stack frame.)
     /// Passing `dest`and `ret` in the same `Option` proved very annoying when only one of them
@@ -150,30 +150,28 @@ pub trait Machine<'mir, 'tcx>: Sized {
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
         instance: ty::Instance<'tcx>,
         args: &[OpTy<'tcx, Self::PointerTag>],
-        dest: Option<PlaceTy<'tcx, Self::PointerTag>>,
-        ret: Option<mir::BasicBlock>,
-        unwind: Option<mir::BasicBlock>
+        ret: Option<(PlaceTy<'tcx, Self::PointerTag>, mir::BasicBlock)>,
+        unwind: Option<mir::BasicBlock>,
     ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>>;
 
-    /// Execute `fn_val`.  it is the hook's responsibility to advance the instruction
+    /// Execute `fn_val`.  It is the hook's responsibility to advance the instruction
     /// pointer as appropriate.
     fn call_extra_fn(
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
         fn_val: Self::ExtraFnVal,
         args: &[OpTy<'tcx, Self::PointerTag>],
-        dest: Option<PlaceTy<'tcx, Self::PointerTag>>,
-        ret: Option<mir::BasicBlock>,
+        ret: Option<(PlaceTy<'tcx, Self::PointerTag>, mir::BasicBlock)>,
+        unwind: Option<mir::BasicBlock>,
     ) -> InterpResult<'tcx>;
 
-    /// Directly process an intrinsic without pushing a stack frame.
-    /// If this returns successfully, the engine will take care of jumping to the next block.
+    /// Directly process an intrinsic without pushing a stack frame. It is the hook's
+    /// responsibility to advance the instruction pointer as appropriate.
     fn call_intrinsic(
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
         span: Span,
         instance: ty::Instance<'tcx>,
         args: &[OpTy<'tcx, Self::PointerTag>],
-        dest: Option<PlaceTy<'tcx, Self::PointerTag>>,
-        ret: Option<mir::BasicBlock>,
+        ret: Option<(PlaceTy<'tcx, Self::PointerTag>, mir::BasicBlock)>,
         unwind: Option<mir::BasicBlock>,
     ) -> InterpResult<'tcx>;