Simplify raw_eq to Transmute+Eq for sizes with primitives

scottmcm · scottmcm · commit c468ee338679 · 2026-05-10T19:28:57.000-07:00
diff --git a/compiler/rustc_mir_transform/src/instsimplify.rs b/compiler/rustc_mir_transform/src/instsimplify.rs
@@ -1,11 +1,12 @@
 //! Performs various peephole optimizations.
 
-use rustc_abi::ExternAbi;
+use rustc_abi::{ExternAbi, Integer};
 use rustc_hir::{LangItem, find_attr};
+use rustc_index::IndexVec;
 use rustc_middle::bug;
 use rustc_middle::mir::visit::MutVisitor;
 use rustc_middle::mir::*;
-use rustc_middle::ty::layout::ValidityRequirement;
+use rustc_middle::ty::layout::{IntegerExt, ValidityRequirement};
 use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt, layout};
 use rustc_span::{Symbol, sym};
 
@@ -33,10 +34,10 @@ impl<'tcx> crate::MirPass<'tcx> for InstSimplify {
         if !preserve_ub_checks {
             SimplifyUbCheck { tcx }.visit_body(body);
         }
-        let ctx = InstSimplifyContext {
+        let mut ctx = InstSimplifyContext {
             tcx,
-            local_decls: &body.local_decls,
             typing_env: body.typing_env(tcx),
+            local_decls: &mut body.local_decls,
         };
         for block in body.basic_blocks.as_mut() {
             for statement in block.statements.iter_mut() {
@@ -55,6 +56,7 @@ impl<'tcx> crate::MirPass<'tcx> for InstSimplify {
             let terminator = block.terminator.as_mut().unwrap();
             ctx.simplify_primitive_clone(terminator, &mut block.statements);
             ctx.simplify_size_or_align_of_val(terminator, &mut block.statements);
+            ctx.simplify_raw_eq(terminator, &mut block.statements);
             ctx.simplify_intrinsic_assert(terminator);
             ctx.simplify_nounwind_call(terminator);
             simplify_duplicate_switch_targets(terminator);
@@ -68,7 +70,7 @@ impl<'tcx> crate::MirPass<'tcx> for InstSimplify {
 
 struct InstSimplifyContext<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
-    local_decls: &'a LocalDecls<'tcx>,
+    local_decls: &'a mut IndexVec<Local, LocalDecl<'tcx>>,
     typing_env: ty::TypingEnv<'tcx>,
 }
 
@@ -318,6 +320,63 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
         }
     }
 
+    /// Simplify `raw_eq` intrinsic calls to `Eq` when the type has the size of a primitive.
+    ///
+    /// For example, replace `raw_eq::<[u8; 4]>(a, b)` with `Eq(Transmute(a), Transmute(b))`.
+    fn simplify_raw_eq(
+        &mut self,
+        terminator: &mut Terminator<'tcx>,
+        statements: &mut Vec<Statement<'tcx>>,
+    ) {
+        let tcx = self.tcx;
+        let source_info = terminator.source_info;
+        let span = source_info.span;
+        if let TerminatorKind::Call {
+            func, args, destination, target: Some(destination_block), ..
+        } = &terminator.kind
+            && args.len() == 2
+            && let Some((fn_def_id, generics)) = func.const_fn_def()
+            && tcx.is_intrinsic(fn_def_id, sym::raw_eq)
+            && let generic_ty = generics.type_at(0)
+            && let Ok(layout) = tcx.layout_of(self.typing_env.as_query_input(generic_ty))
+            && let Ok(integer) = Integer::from_size(layout.size)
+        {
+            let ref_ty = Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, generic_ty);
+            let uint_ty = integer.to_ty(tcx, false);
+
+            let mut transmute_operand = |op: &Operand<'tcx>| -> Operand<'tcx> {
+                let ref_local = self.local_decls.push(LocalDecl::new(ref_ty, span));
+                statements.push(Statement::new(
+                    source_info,
+                    StatementKind::Assign(Box::new((
+                        Place::from(ref_local),
+                        Rvalue::Use(op.clone(), WithRetag::Yes),
+                    ))),
+                ));
+                let place = Place::from(ref_local).project_deeper(&[ProjectionElem::Deref], tcx);
+                let int_local = self.local_decls.push(LocalDecl::new(uint_ty, span));
+                statements.push(Statement::new(
+                    source_info,
+                    StatementKind::Assign(Box::new((
+                        Place::from(int_local),
+                        Rvalue::Cast(CastKind::Transmute, Operand::Copy(place), uint_ty),
+                    ))),
+                ));
+                Operand::Move(Place::from(int_local))
+            };
+            let lhs_op = transmute_operand(&args[0].node);
+            let rhs_op = transmute_operand(&args[1].node);
+            statements.push(Statement::new(
+                source_info,
+                StatementKind::Assign(Box::new((
+                    *destination,
+                    Rvalue::BinaryOp(BinOp::Eq, Box::new((lhs_op, rhs_op))),
+                ))),
+            ));
+            terminator.kind = TerminatorKind::Goto { target: *destination_block };
+        }
+    }
+
     fn simplify_nounwind_call(&self, terminator: &mut Terminator<'tcx>) {
         let TerminatorKind::Call { ref func, ref mut unwind, .. } = terminator.kind else {
             return;
diff --git a/tests/mir-opt/instsimplify/raw_eq.inner_array.InstSimplify-after-simplifycfg.diff b/tests/mir-opt/instsimplify/raw_eq.inner_array.InstSimplify-after-simplifycfg.diff
@@ -0,0 +1,25 @@
+- // MIR for `inner_array` before InstSimplify-after-simplifycfg
++ // MIR for `inner_array` after InstSimplify-after-simplifycfg
+  
+  fn inner_array(_1: &&[i32; 2], _2: &&[i32; 2]) -> bool {
+      let mut _0: bool;
++     let mut _3: &[i32; 2];
++     let mut _4: u64;
++     let mut _5: &[i32; 2];
++     let mut _6: u64;
+  
+      bb0: {
+-         _0 = raw_eq::<[i32; 2]>(copy (*_1), copy (*_2)) -> [return: bb1, unwind unreachable];
++         _3 = copy (*_1);
++         _4 = copy (*_3) as u64 (Transmute);
++         _5 = copy (*_2);
++         _6 = copy (*_5) as u64 (Transmute);
++         _0 = Eq(move _4, move _6);
++         goto -> bb1;
+      }
+  
+      bb1: {
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/instsimplify/raw_eq.rs b/tests/mir-opt/instsimplify/raw_eq.rs
@@ -0,0 +1,27 @@
+//@ test-mir-pass: InstSimplify-after-simplifycfg
+#![crate_type = "lib"]
+#![feature(core_intrinsics)]
+#![feature(custom_mir)]
+
+// Custom MIR so we can get an argument that's not just a local directly
+use std::intrinsics::mir::*;
+use std::intrinsics::raw_eq;
+
+// EMIT_MIR raw_eq.inner_array.InstSimplify-after-simplifycfg.diff
+#[custom_mir(dialect = "runtime")]
+pub fn inner_array(a: &&[i32; 2], b: &&[i32; 2]) -> bool {
+    // CHECK-LABEL: fn inner_array(_1: &&[i32; 2], _2: &&[i32; 2]) -> bool
+    // CHECK: [[AREF:_.+]] = copy (*_1);
+    // CHECK: [[AINT:_.+]] = copy (*[[AREF]]) as u64 (Transmute);
+    // CHECK: [[BREF:_.+]] = copy (*_2);
+    // CHECK: [[BINT:_.+]] = copy (*[[BREF]]) as u64 (Transmute);
+    // CHECK: _0 = Eq(move [[AINT]], move [[BINT]]);
+    mir! {
+        {
+            Call(RET = raw_eq(*a, *b), ReturnTo(ret), UnwindUnreachable())
+        }
+        ret = {
+            Return()
+        }
+    }
+}
diff --git a/tests/mir-opt/pre-codegen/array_eq.eq_ipv4.PreCodegen.after.panic-abort.mir b/tests/mir-opt/pre-codegen/array_eq.eq_ipv4.PreCodegen.after.panic-abort.mir
@@ -4,6 +4,8 @@ fn eq_ipv4(_1: &[u8; 4], _2: &[u8; 4]) -> bool {
     debug a => _1;
     debug b => _2;
     let mut _0: bool;
+    let mut _3: u32;
+    let mut _4: u32;
     scope 1 (inlined std::cmp::impls::<impl PartialEq for &[u8; 4]>::eq) {
         scope 2 (inlined array::equality::<impl PartialEq for [u8; 4]>::eq) {
             scope 3 (inlined <u8 as array::equality::SpecArrayEq<u8, 4>>::spec_eq) {
@@ -12,10 +14,9 @@ fn eq_ipv4(_1: &[u8; 4], _2: &[u8; 4]) -> bool {
     }
 
     bb0: {
-        _0 = raw_eq::<[u8; 4]>(move _1, move _2) -> [return: bb1, unwind unreachable];
-    }
-
-    bb1: {
+        _3 = copy (*_1) as u32 (Transmute);
+        _4 = copy (*_2) as u32 (Transmute);
+        _0 = Eq(move _3, move _4);
         return;
     }
 }
diff --git a/tests/mir-opt/pre-codegen/array_eq.eq_ipv4.PreCodegen.after.panic-unwind.mir b/tests/mir-opt/pre-codegen/array_eq.eq_ipv4.PreCodegen.after.panic-unwind.mir
@@ -4,6 +4,8 @@ fn eq_ipv4(_1: &[u8; 4], _2: &[u8; 4]) -> bool {
     debug a => _1;
     debug b => _2;
     let mut _0: bool;
+    let mut _3: u32;
+    let mut _4: u32;
     scope 1 (inlined std::cmp::impls::<impl PartialEq for &[u8; 4]>::eq) {
         scope 2 (inlined array::equality::<impl PartialEq for [u8; 4]>::eq) {
             scope 3 (inlined <u8 as array::equality::SpecArrayEq<u8, 4>>::spec_eq) {
@@ -12,10 +14,9 @@ fn eq_ipv4(_1: &[u8; 4], _2: &[u8; 4]) -> bool {
     }
 
     bb0: {
-        _0 = raw_eq::<[u8; 4]>(move _1, move _2) -> [return: bb1, unwind unreachable];
-    }
-
-    bb1: {
+        _3 = copy (*_1) as u32 (Transmute);
+        _4 = copy (*_2) as u32 (Transmute);
+        _0 = Eq(move _3, move _4);
         return;
     }
 }
diff --git a/tests/mir-opt/pre-codegen/array_eq.eq_ipv6.PreCodegen.after.panic-abort.mir b/tests/mir-opt/pre-codegen/array_eq.eq_ipv6.PreCodegen.after.panic-abort.mir
@@ -4,6 +4,8 @@ fn eq_ipv6(_1: &[u16; 8], _2: &[u16; 8]) -> bool {
     debug a => _1;
     debug b => _2;
     let mut _0: bool;
+    let mut _3: u128;
+    let mut _4: u128;
     scope 1 (inlined std::cmp::impls::<impl PartialEq for &[u16; 8]>::eq) {
         scope 2 (inlined array::equality::<impl PartialEq for [u16; 8]>::eq) {
             scope 3 (inlined <u16 as array::equality::SpecArrayEq<u16, 8>>::spec_eq) {
@@ -12,10 +14,9 @@ fn eq_ipv6(_1: &[u16; 8], _2: &[u16; 8]) -> bool {
     }
 
     bb0: {
-        _0 = raw_eq::<[u16; 8]>(move _1, move _2) -> [return: bb1, unwind unreachable];
-    }
-
-    bb1: {
+        _3 = copy (*_1) as u128 (Transmute);
+        _4 = copy (*_2) as u128 (Transmute);
+        _0 = Eq(move _3, move _4);
         return;
     }
 }
diff --git a/tests/mir-opt/pre-codegen/array_eq.eq_ipv6.PreCodegen.after.panic-unwind.mir b/tests/mir-opt/pre-codegen/array_eq.eq_ipv6.PreCodegen.after.panic-unwind.mir
@@ -4,6 +4,8 @@ fn eq_ipv6(_1: &[u16; 8], _2: &[u16; 8]) -> bool {
     debug a => _1;
     debug b => _2;
     let mut _0: bool;
+    let mut _3: u128;
+    let mut _4: u128;
     scope 1 (inlined std::cmp::impls::<impl PartialEq for &[u16; 8]>::eq) {
         scope 2 (inlined array::equality::<impl PartialEq for [u16; 8]>::eq) {
             scope 3 (inlined <u16 as array::equality::SpecArrayEq<u16, 8>>::spec_eq) {
@@ -12,10 +14,9 @@ fn eq_ipv6(_1: &[u16; 8], _2: &[u16; 8]) -> bool {
     }
 
     bb0: {
-        _0 = raw_eq::<[u16; 8]>(move _1, move _2) -> [return: bb1, unwind unreachable];
-    }
-
-    bb1: {
+        _3 = copy (*_1) as u128 (Transmute);
+        _4 = copy (*_2) as u128 (Transmute);
+        _0 = Eq(move _3, move _4);
         return;
     }
 }
diff --git a/tests/mir-opt/pre-codegen/array_eq.rs b/tests/mir-opt/pre-codegen/array_eq.rs
@@ -13,13 +13,17 @@ pub unsafe fn eq_odd_length<T: Copy>(a: &[u8; 3], b: &[u8; 3]) -> bool {
 // EMIT_MIR array_eq.eq_ipv4.PreCodegen.after.mir
 pub unsafe fn eq_ipv4<T: Copy>(a: &[u8; 4], b: &[u8; 4]) -> bool {
     // CHECK-LABEL: fn eq_ipv4(_1: &[u8; 4], _2: &[u8; 4]) -> bool
-    // CHECK: _0 = raw_eq::<[u8; 4]>(move _1, move _2)
+    // CHECK: [[A:_.+]] = copy (*_1) as u32 (Transmute);
+    // CHECK: [[B:_.+]] = copy (*_2) as u32 (Transmute);
+    // CHECK: _0 = Eq(move [[A]], move [[B]]);
     a == b
 }
 
 // EMIT_MIR array_eq.eq_ipv6.PreCodegen.after.mir
 pub unsafe fn eq_ipv6<T: Copy>(a: &[u16; 8], b: &[u16; 8]) -> bool {
     // CHECK-LABEL: fn eq_ipv6(_1: &[u16; 8], _2: &[u16; 8]) -> bool
-    // CHECK: _0 = raw_eq::<[u16; 8]>(move _1, move _2)
+    // CHECK: [[A:_.+]] = copy (*_1) as u128 (Transmute);
+    // CHECK: [[B:_.+]] = copy (*_2) as u128 (Transmute);
+    // CHECK: _0 = Eq(move [[A]], move [[B]]);
     a == b
 }
