Move rustc_mir::transform to rustc_mir_transform.

compiler/rustc_mir_transform/src/instcombine.rs

@@ -0,0 +1,131 @@
+//! Performs various peephole optimizations.
+
+use crate::MirPass;
+use rustc_hir::Mutability;
+use rustc_middle::mir::{
+    BinOp, Body, Constant, LocalDecls, Operand, Place, ProjectionElem, Rvalue, SourceInfo,
+    StatementKind, UnOp,
+};
+use rustc_middle::ty::{self, TyCtxt};
+
+pub struct InstCombine;
+
+impl<'tcx> MirPass<'tcx> for InstCombine {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
+        let ctx = InstCombineContext { tcx, local_decls };
+        for block in basic_blocks.iter_mut() {
+            for statement in block.statements.iter_mut() {
+                match statement.kind {
+                    StatementKind::Assign(box (_place, ref mut rvalue)) => {
+                        ctx.combine_bool_cmp(&statement.source_info, rvalue);
+                        ctx.combine_ref_deref(&statement.source_info, rvalue);
+                        ctx.combine_len(&statement.source_info, rvalue);
+                    }
+                    _ => {}
+                }
+            }
+        }
+    }
+}
+
+struct InstCombineContext<'tcx, 'a> {
+    tcx: TyCtxt<'tcx>,
+    local_decls: &'a LocalDecls<'tcx>,
+}
+
+impl<'tcx, 'a> InstCombineContext<'tcx, 'a> {
+    fn should_combine(&self, source_info: &SourceInfo, rvalue: &Rvalue<'tcx>) -> bool {
+        self.tcx.consider_optimizing(|| {
+            format!("InstCombine - Rvalue: {:?} SourceInfo: {:?}", rvalue, source_info)
+        })
+    }
+
+    /// Transform boolean comparisons into logical operations.
+    fn combine_bool_cmp(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) {
+        match rvalue {
+            Rvalue::BinaryOp(op @ (BinOp::Eq | BinOp::Ne), box (a, b)) => {
+                let new = match (op, self.try_eval_bool(a), self.try_eval_bool(b)) {
+                    // Transform "Eq(a, true)" ==> "a"
+                    (BinOp::Eq, _, Some(true)) => Some(Rvalue::Use(a.clone())),
+
+                    // Transform "Ne(a, false)" ==> "a"
+                    (BinOp::Ne, _, Some(false)) => Some(Rvalue::Use(a.clone())),
+
+                    // Transform "Eq(true, b)" ==> "b"
+                    (BinOp::Eq, Some(true), _) => Some(Rvalue::Use(b.clone())),
+
+                    // Transform "Ne(false, b)" ==> "b"
+                    (BinOp::Ne, Some(false), _) => Some(Rvalue::Use(b.clone())),
+
+                    // Transform "Eq(false, b)" ==> "Not(b)"
+                    (BinOp::Eq, Some(false), _) => Some(Rvalue::UnaryOp(UnOp::Not, b.clone())),
+
+                    // Transform "Ne(true, b)" ==> "Not(b)"
+                    (BinOp::Ne, Some(true), _) => Some(Rvalue::UnaryOp(UnOp::Not, b.clone())),
+
+                    // Transform "Eq(a, false)" ==> "Not(a)"
+                    (BinOp::Eq, _, Some(false)) => Some(Rvalue::UnaryOp(UnOp::Not, a.clone())),
+
+                    // Transform "Ne(a, true)" ==> "Not(a)"
+                    (BinOp::Ne, _, Some(true)) => Some(Rvalue::UnaryOp(UnOp::Not, a.clone())),
+
+                    _ => None,
+                };
+
+                if let Some(new) = new {
+                    if self.should_combine(source_info, rvalue) {
+                        *rvalue = new;
+                    }
+                }
+            }
+
+            _ => {}
+        }
+    }
+
+    fn try_eval_bool(&self, a: &Operand<'_>) -> Option<bool> {
+        let a = a.constant()?;
+        if a.literal.ty().is_bool() { a.literal.try_to_bool() } else { None }
+    }
+
+    /// Transform "&(*a)" ==> "a".
+    fn combine_ref_deref(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) {
+        if let Rvalue::Ref(_, _, place) = rvalue {
+            if let Some((base, ProjectionElem::Deref)) = place.as_ref().last_projection() {
+                if let ty::Ref(_, _, Mutability::Not) =
+                    base.ty(self.local_decls, self.tcx).ty.kind()
+                {
+                    // The dereferenced place must have type `&_`, so that we don't copy `&mut _`.
+                } else {
+                    return;
+                }
+
+                if !self.should_combine(source_info, rvalue) {
+                    return;
+                }
+
+                *rvalue = Rvalue::Use(Operand::Copy(Place {
+                    local: base.local,
+                    projection: self.tcx.intern_place_elems(base.projection),
+                }));
+            }
+        }
+    }
+
+    /// Transform "Len([_; N])" ==> "N".
+    fn combine_len(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) {
+        if let Rvalue::Len(ref place) = *rvalue {
+            let place_ty = place.ty(self.local_decls, self.tcx).ty;
+            if let ty::Array(_, len) = *place_ty.kind() {
+                if !self.should_combine(source_info, rvalue) {
+                    return;
+                }
+
+                let constant =
+                    Constant { span: source_info.span, literal: len.into(), user_ty: None };
+                *rvalue = Rvalue::Use(Operand::Constant(Box::new(constant)));
+            }
+        }
+    }
+}