[mir-opt] Introduce a new flag to enable experimental/unsound mir opts

Author: wesleywiser

compiler/rustc_mir/src/transform/copy_prop.rs

@@ -31,9 +31,11 @@ pub struct CopyPropagation;
 
 impl<'tcx> MirPass<'tcx> for CopyPropagation {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, _source: MirSource<'tcx>, body: &mut Body<'tcx>) {
+        let opts = &tcx.sess.opts.debugging_opts;
         // We only run when the MIR optimization level is > 1.
         // This avoids a slow pass, and messing up debug info.
-        if tcx.sess.opts.debugging_opts.mir_opt_level <= 1 {
+        // FIXME(76740): This optimization is buggy and can cause unsoundness.
+        if opts.mir_opt_level <= 1 || !opts.unsound_mir_opts {
             return;
         }
 

compiler/rustc_mir/src/transform/simplify_try.rs

@@ -612,7 +612,7 @@ impl<'a, 'tcx> SimplifyBranchSameOptimizationFinder<'a, 'tcx> {
                     && bb_l.terminator().kind == bb_r.terminator().kind;
                     let statement_check = || {
                         bb_l.statements.iter().zip(&bb_r.statements).try_fold(StatementEquality::TrivialEqual, |acc,(l,r)| {
-                            let stmt_equality = self.statement_equality(*adt_matched_on, &l, bb_l_idx, &r, bb_r_idx, self.tcx.sess.opts.debugging_opts.mir_opt_level);
+                            let stmt_equality = self.statement_equality(*adt_matched_on, &l, bb_l_idx, &r, bb_r_idx, self.tcx);
                             if matches!(stmt_equality, StatementEquality::NotEqual) {
                                 // short circuit
                                 None
@@ -672,7 +672,7 @@ impl<'a, 'tcx> SimplifyBranchSameOptimizationFinder<'a, 'tcx> {
         x_bb_idx: BasicBlock,
         y: &Statement<'tcx>,
         y_bb_idx: BasicBlock,
-        mir_opt_level: usize,
+        tcx: TyCtxt<'tcx>,
     ) -> StatementEquality {
         let helper = |rhs: &Rvalue<'tcx>,
                       place: &Place<'tcx>,
@@ -693,7 +693,7 @@ impl<'a, 'tcx> SimplifyBranchSameOptimizationFinder<'a, 'tcx> {
                 Rvalue::Use(operand) if operand.place() == Some(adt_matched_on) => {
                     // FIXME(76803): This logic is currently broken because it does not take into
                     // account the current discriminant value.
-                    if mir_opt_level > 2 {
+                    if tcx.sess.opts.debugging_opts.unsound_mir_opts {
                         StatementEquality::ConsideredEqual(side_to_choose)
                     } else {
                         StatementEquality::NotEqual

compiler/rustc_session/src/options.rs

@@ -1115,6 +1115,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         `hir,typed` (HIR with types for each node),
         `hir-tree` (dump the raw HIR),
         `mir` (the MIR), or `mir-cfg` (graphviz formatted MIR)"),
+    unsound_mir_opts: bool = (false, parse_bool, [TRACKED],
+        "enable unsound and buggy MIR optimizations (default: no)"),
     unstable_options: bool = (false, parse_bool, [UNTRACKED],
         "adds unstable command line options to rustc interface (default: no)"),
     use_ctors_section: Option<bool> = (None, parse_opt_bool, [TRACKED],

src/test/mir-opt/copy_propagation.rs

@@ -1,3 +1,4 @@
+// compile-flags: -Zunsound-mir-opts
 // EMIT_MIR copy_propagation.test.CopyPropagation.diff
 
 fn test(x: u32) -> u32 {

src/test/mir-opt/copy_propagation.test.CopyPropagation.diff

@@ -2,19 +2,19 @@
 + // MIR for `test` after CopyPropagation
 
   fn test(_1: u32) -> u32 {
-      debug x => _1;                       // in scope 0 at $DIR/copy_propagation.rs:3:9: 3:10
-      let mut _0: u32;                     // return place in scope 0 at $DIR/copy_propagation.rs:3:20: 3:23
-      let _2: u32;                         // in scope 0 at $DIR/copy_propagation.rs:4:9: 4:10
+      debug x => _1;                       // in scope 0 at $DIR/copy_propagation.rs:4:9: 4:10
+      let mut _0: u32;                     // return place in scope 0 at $DIR/copy_propagation.rs:4:20: 4:23
+      let _2: u32;                         // in scope 0 at $DIR/copy_propagation.rs:5:9: 5:10
       scope 1 {
-          debug y => _0;                   // in scope 1 at $DIR/copy_propagation.rs:4:9: 4:10
+          debug y => _0;                   // in scope 1 at $DIR/copy_propagation.rs:5:9: 5:10
       }
 
       bb0: {
-          nop;                             // scope 0 at $DIR/copy_propagation.rs:4:9: 4:10
-          _0 = _1;                         // scope 0 at $DIR/copy_propagation.rs:4:13: 4:14
-          nop;                             // scope 1 at $DIR/copy_propagation.rs:5:5: 5:6
-          nop;                             // scope 0 at $DIR/copy_propagation.rs:6:1: 6:2
-          return;                          // scope 0 at $DIR/copy_propagation.rs:6:2: 6:2
+          nop;                             // scope 0 at $DIR/copy_propagation.rs:5:9: 5:10
+          _0 = _1;                         // scope 0 at $DIR/copy_propagation.rs:5:13: 5:14
+          nop;                             // scope 1 at $DIR/copy_propagation.rs:6:5: 6:6
+          nop;                             // scope 0 at $DIR/copy_propagation.rs:7:1: 7:2
+          return;                          // scope 0 at $DIR/copy_propagation.rs:7:2: 7:2
       }
   }
 

src/test/mir-opt/early_otherwise_branch_68867.rs

@@ -1,5 +1,5 @@
 // ignore-tidy-linelength
-// compile-flags: -Z mir-opt-level=3
+// compile-flags: -Z mir-opt-level=3 -Zunsound-mir-opts
 
 // example from #68867
 type CSSFloat = f32;

src/test/mir-opt/inline/inline-any-operand.rs

@@ -1,4 +1,4 @@
-// compile-flags: -Z span_free_formats
+// compile-flags: -Z span_free_formats -Zunsound-mir-opts
 
 // Tests that MIR inliner works for any operand
 

src/test/mir-opt/inline/inline-closure-borrows-arg.rs

@@ -1,4 +1,4 @@
-// compile-flags: -Z span_free_formats
+// compile-flags: -Z span_free_formats -Zunsound-mir-opts
 
 // Tests that MIR inliner can handle closure arguments,
 // even when (#45894)

src/test/mir-opt/inline/inline-trait-method_2.rs

@@ -1,4 +1,4 @@
-// compile-flags: -Z span_free_formats -Z mir-opt-level=3
+// compile-flags: -Z span_free_formats -Z mir-opt-level=3 -Zunsound-mir-opts
 
 // EMIT_MIR inline_trait_method_2.test2.Inline.after.mir
 fn test2(x: &dyn X) -> bool {

src/test/mir-opt/issue-73223.rs

@@ -1,3 +1,5 @@
+// compile-flags: -Zunsound-mir-opts
+
 fn main() {
     let split = match Some(1) {
         Some(v) => v,