Simple preopt: fold instructions using simple algebraic identities;

cranelift/codegen/src/simple_preopt.rs

@@ -577,13 +577,17 @@ fn simplify(pos: &mut FuncCursor, inst: Inst) {
             _ => {}
         },
 
-        InstructionData::BinaryImm { opcode, arg, imm } => match opcode {
+        InstructionData::BinaryImm { opcode, arg, imm } => {
+            let ty = pos.func.dfg.ctrl_typevar(inst);
+
+            let mut imm = imm;
+            match opcode {
                 Opcode::IaddImm
                 | Opcode::ImulImm
                 | Opcode::BorImm
                 | Opcode::BandImm
                 | Opcode::BxorImm => {
-                // Fold binary_op(C2, binary_op(C1, x)) into binary_op(binary_op(C1 | C2), x)
+                    // Fold binary_op(C2, binary_op(C1, x)) into binary_op(binary_op(C1, C2), x)
                     if let ValueDef::Result(arg_inst, _) = pos.func.dfg.value_def(arg) {
                         if let InstructionData::BinaryImm {
                             opcode: prev_opcode,
@@ -592,7 +596,6 @@ fn simplify(pos: &mut FuncCursor, inst: Inst) {
                         } = &pos.func.dfg[arg_inst]
                         {
                             if opcode == *prev_opcode {
-                            let ty = pos.func.dfg.ctrl_typevar(inst);
                                 if ty == pos.func.dfg.ctrl_typevar(arg_inst) {
                                     let lhs: i64 = imm.into();
                                     let rhs: i64 = (*prev_imm).into();
@@ -610,6 +613,7 @@ fn simplify(pos: &mut FuncCursor, inst: Inst) {
                                         .dfg
                                         .replace(inst)
                                         .BinaryImm(opcode, ty, new_imm, arg);
+                                    imm = new_imm;
                                 }
                             }
                         }
@@ -621,8 +625,47 @@ fn simplify(pos: &mut FuncCursor, inst: Inst) {
                         return;
                     }
                 }
+
                 _ => {}
-        },
+            };
+
+            // Replace operations that are no-ops.
+            match (opcode, imm.into()) {
+                (Opcode::IaddImm, 0)
+                | (Opcode::ImulImm, 1)
+                | (Opcode::SdivImm, 1)
+                | (Opcode::UdivImm, 1)
+                | (Opcode::BorImm, 0)
+                | (Opcode::BandImm, -1)
+                | (Opcode::BxorImm, 0)
+                | (Opcode::RotlImm, 0)
+                | (Opcode::RotrImm, 0)
+                | (Opcode::IshlImm, 0)
+                | (Opcode::UshrImm, 0)
+                | (Opcode::SshrImm, 0) => {
+                    // Alias the result value with the original argument.
+                    let results = pos.func.dfg.detach_results(inst);
+                    debug_assert!(results.len(&pos.func.dfg.value_lists) == 1);
+                    let first_result = results.get(0, &pos.func.dfg.value_lists).unwrap();
+                    pos.func.dfg.change_to_alias(first_result, arg);
+
+                    // Replace instruction by a nop.
+                    pos.func.dfg.replace(inst).nop();
+                    return;
+                }
+                (Opcode::ImulImm, 0) | (Opcode::BandImm, 0) => {
+                    // Replace by zero.
+                    pos.func.dfg.replace(inst).iconst(ty, 0);
+                    return;
+                }
+                (Opcode::BorImm, -1) => {
+                    // Replace by minus one.
+                    pos.func.dfg.replace(inst).iconst(ty, -1);
+                    return;
+                }
+                _ => {}
+            }
+        }
 
         InstructionData::IntCompare { opcode, cond, args } => {
             debug_assert_eq!(opcode, Opcode::Icmp);

cranelift/filetests/filetests/simple_preopt/div_by_const_power_of_2.clif

@@ -11,11 +11,11 @@ ebb0(v0: i32):
     return v1
 }
 
-; converted to a copy
+; converted to a nop
 function %t_udiv32_p1(i32) -> i32 {
 ebb0(v0: i32):
     v1 = udiv_imm v0, 1
-    ; check: copy v0
+    ; check: nop
     return v1
 }
 
@@ -46,11 +46,11 @@ ebb0(v0: i64):
     return v1
 }
 
-; converted to a copy
+; converted to a nop
 function %t_udiv64_p1(i64) -> i64 {
 ebb0(v0: i64):
     v1 = udiv_imm v0, 1
-    ; check: copy v0
+    ; check: nop
     return v1
 }
 
@@ -81,11 +81,11 @@ ebb0(v0: i32):
     return v1
 }
 
-; converted to a copy
+; converted to a nop
 function %t_sdiv32_p1(i32) -> i32 {
 ebb0(v0: i32):
     v1 = sdiv_imm v0, 1
-    ; check: copy v0
+    ; check: nop
     return v1
 }
 
@@ -192,11 +192,11 @@ ebb0(v0: i64):
     return v1
 }
 
-; converted to a copy
+; converted to a nop
 function %t_sdiv64_p1(i64) -> i64 {
 ebb0(v0: i64):
     v1 = sdiv_imm v0, 1
-    ; check: copy v0
+    ; check: nop
     return v1
 }