Auto merge of #105582 - saethlin:instcombine-assert-inhabited, r=cjgillot

InstCombine away intrinsic validity assertions

This optimization (currently) fires 246 times on the standard library. It seems to fire hardly at all on the big crates in the benchmark suite. Interesting.

compiler/rustc_mir_transform/src/instcombine.rs

@@ -6,7 +6,8 @@ use rustc_middle::mir::{
     BinOp, Body, Constant, ConstantKind, LocalDecls, Operand, Place, ProjectionElem, Rvalue,
     SourceInfo, Statement, StatementKind, Terminator, TerminatorKind, UnOp,
 };
-use rustc_middle::ty::{self, TyCtxt};
+use rustc_middle::ty::{self, layout::TyAndLayout, ParamEnv, ParamEnvAnd, SubstsRef, Ty, TyCtxt};
+use rustc_span::symbol::{sym, Symbol};
 
 pub struct InstCombine;
 
@@ -16,7 +17,11 @@ impl<'tcx> MirPass<'tcx> for InstCombine {
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
-        let ctx = InstCombineContext { tcx, local_decls: &body.local_decls };
+        let ctx = InstCombineContext {
+            tcx,
+            local_decls: &body.local_decls,
+            param_env: tcx.param_env_reveal_all_normalized(body.source.def_id()),
+        };
         for block in body.basic_blocks.as_mut() {
             for statement in block.statements.iter_mut() {
                 match statement.kind {
@@ -33,6 +38,10 @@ impl<'tcx> MirPass<'tcx> for InstCombine {
                 &mut block.terminator.as_mut().unwrap(),
                 &mut block.statements,
             );
+            ctx.combine_intrinsic_assert(
+                &mut block.terminator.as_mut().unwrap(),
+                &mut block.statements,
+            );
         }
     }
 }
@@ -40,6 +49,7 @@ impl<'tcx> MirPass<'tcx> for InstCombine {
 struct InstCombineContext<'tcx, 'a> {
     tcx: TyCtxt<'tcx>,
     local_decls: &'a LocalDecls<'tcx>,
+    param_env: ParamEnv<'tcx>,
 }
 
 impl<'tcx> InstCombineContext<'tcx, '_> {
@@ -200,4 +210,76 @@ impl<'tcx> InstCombineContext<'tcx, '_> {
         });
         terminator.kind = TerminatorKind::Goto { target: destination_block };
     }
+
+    fn combine_intrinsic_assert(
+        &self,
+        terminator: &mut Terminator<'tcx>,
+        _statements: &mut Vec<Statement<'tcx>>,
+    ) {
+        let TerminatorKind::Call { func, target, .. } = &mut terminator.kind  else { return; };
+        let Some(target_block) = target else { return; };
+        let func_ty = func.ty(self.local_decls, self.tcx);
+        let Some((intrinsic_name, substs)) = resolve_rust_intrinsic(self.tcx, func_ty) else {
+            return;
+        };
+        // The intrinsics we are interested in have one generic parameter
+        if substs.is_empty() {
+            return;
+        }
+        let ty = substs.type_at(0);
+
+        // Check this is a foldable intrinsic before we query the layout of our generic parameter
+        let Some(assert_panics) = intrinsic_assert_panics(intrinsic_name) else { return; };
+        let Ok(layout) = self.tcx.layout_of(self.param_env.and(ty)) else { return; };
+        if assert_panics(self.tcx, self.param_env.and(layout)) {
+            // If we know the assert panics, indicate to later opts that the call diverges
+            *target = None;
+        } else {
+            // If we know the assert does not panic, turn the call into a Goto
+            terminator.kind = TerminatorKind::Goto { target: *target_block };
+        }
+    }
+}
+
+fn intrinsic_assert_panics<'tcx>(
+    intrinsic_name: Symbol,
+) -> Option<fn(TyCtxt<'tcx>, ParamEnvAnd<'tcx, TyAndLayout<'tcx>>) -> bool> {
+    fn inhabited_predicate<'tcx>(
+        _tcx: TyCtxt<'tcx>,
+        param_env_and_layout: ParamEnvAnd<'tcx, TyAndLayout<'tcx>>,
+    ) -> bool {
+        let (_param_env, layout) = param_env_and_layout.into_parts();
+        layout.abi.is_uninhabited()
+    }
+    fn zero_valid_predicate<'tcx>(
+        tcx: TyCtxt<'tcx>,
+        param_env_and_layout: ParamEnvAnd<'tcx, TyAndLayout<'tcx>>,
+    ) -> bool {
+        !tcx.permits_zero_init(param_env_and_layout)
+    }
+    fn mem_uninitialized_valid_predicate<'tcx>(
+        tcx: TyCtxt<'tcx>,
+        param_env_and_layout: ParamEnvAnd<'tcx, TyAndLayout<'tcx>>,
+    ) -> bool {
+        !tcx.permits_uninit_init(param_env_and_layout)
+    }
+
+    match intrinsic_name {
+        sym::assert_inhabited => Some(inhabited_predicate),
+        sym::assert_zero_valid => Some(zero_valid_predicate),
+        sym::assert_mem_uninitialized_valid => Some(mem_uninitialized_valid_predicate),
+        _ => None,
+    }
+}
+
+fn resolve_rust_intrinsic<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    func_ty: Ty<'tcx>,
+) -> Option<(Symbol, SubstsRef<'tcx>)> {
+    if let ty::FnDef(def_id, substs) = *func_ty.kind() {
+        if tcx.is_intrinsic(def_id) {
+            return Some((tcx.item_name(def_id), substs));
+        }
+    }
+    None
 }