[CIR][CIRGen] Implement "if constexpr" code generation (#436)

Author: keryell

clang/lib/CIR/CodeGen/CIRGenStmt.cpp

@@ -401,6 +401,17 @@ mlir::LogicalResult CIRGenFunction::buildIfStmt(const IfStmt &S) {
     bool CondConstant;
     if (ConstantFoldsToSimpleInteger(S.getCond(), CondConstant,
                                      S.isConstexpr())) {
+      if (S.isConstexpr()) {
+        // Handle "if constexpr" explicitly here to avoid generating some
+        // ill-formed code since in CIR the "if" is no longer simplified
+        // in this lambda like in Clang but postponed to other MLIR
+        // passes.
+        if (const Stmt *Executed = CondConstant ? S.getThen() : S.getElse())
+          return buildStmt(Executed, /*useCurrentScope=*/true);
+        // There is nothing to execute at runtime.
+        // TODO(cir): there is still an empty cir.scope generated by the caller.
+        return mlir::success();
+      }
       assert(!UnimplementedFeature::constantFoldsToSimpleInteger());
     }
 

clang/test/CIR/CodeGen/if-constexpr.cpp

@@ -0,0 +1,95 @@
+// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-linux-gnu -fclangir-enable -emit-cir %s -o %t.cir
+// RUN: FileCheck --input-file=%t.cir %s
+
+void if0() {
+  int x = 0;
+  if constexpr (0 == 0) {
+    // Declare a variable with same name to be sure we handle the
+    // scopes correctly
+    int x = 2;
+  } else {
+    int x = 3;
+  }
+  if constexpr (0 == 1) {
+    int x = 4;
+  } else {
+    int x = 5;
+  }
+  if constexpr (int x = 7; 8 == 8) {
+    int y = x;
+  } else {
+    int y = 2*x;
+  }
+  if constexpr (int x = 9; 8 == 10) {
+    int y = x;
+  } else {
+    int y = 3*x;
+  }
+  if constexpr (10 == 10) {
+    int x = 20;
+  }
+  if constexpr (10 == 11) {
+    int x = 30;
+  }
+  if constexpr (int x = 70; 80 == 80) {
+    int y = 10*x;
+  }
+  if constexpr (int x = 90; 80 == 100) {
+    int y = 11*x;
+  }
+}
+
+// CHECK: cir.func @_Z3if0v() {{.*}}
+// CHECK: cir.store %1, %0 : !s32i, cir.ptr <!s32i> loc({{.*}})
+// CHECK-NEXT: cir.scope {
+// CHECK-NEXT:   %2 = cir.alloca !s32i, cir.ptr <!s32i>, ["x", init] {{.*}}
+// CHECK-NEXT:   %3 = cir.const(#cir.int<2> : !s32i) : !s32i loc({{.*}})
+// CHECK-NEXT:   cir.store %3, %2 : !s32i, cir.ptr <!s32i> loc({{.*}})
+// CHECK-NEXT: } loc({{.*}})
+// CHECK-NEXT: cir.scope {
+// CHECK-NEXT:   %2 = cir.alloca !s32i, cir.ptr <!s32i>, ["x", init] {{.*}}
+// CHECK-NEXT:   %3 = cir.const(#cir.int<5> : !s32i) : !s32i loc({{.*}})
+// CHECK-NEXT:   cir.store %3, %2 : !s32i, cir.ptr <!s32i> loc({{.*}})
+// CHECK-NEXT: } loc({{.*}})
+// CHECK-NEXT: cir.scope {
+// CHECK-NEXT:   %2 = cir.alloca !s32i, cir.ptr <!s32i>, ["x", init] {{.*}}
+// CHECK-NEXT:   %3 = cir.alloca !s32i, cir.ptr <!s32i>, ["y", init] {{.*}}
+// CHECK-NEXT:   %4 = cir.const(#cir.int<7> : !s32i) : !s32i loc({{.*}})
+// CHECK-NEXT:   cir.store %4, %2 : !s32i, cir.ptr <!s32i> loc({{.*}})
+// CHECK-NEXT:   %5 = cir.load %2 : cir.ptr <!s32i>, !s32i loc({{.*}})
+// CHECK-NEXT:   cir.store %5, %3 : !s32i, cir.ptr <!s32i> loc({{.*}})
+// CHECK-NEXT: } loc({{.*}})
+// CHECK-NEXT: cir.scope {
+// CHECK-NEXT:   %2 = cir.alloca !s32i, cir.ptr <!s32i>, ["x", init] {{.*}}
+// CHECK-NEXT:   %3 = cir.alloca !s32i, cir.ptr <!s32i>, ["y", init] {{.*}}
+// CHECK-NEXT:   %4 = cir.const(#cir.int<9> : !s32i) : !s32i loc({{.*}})
+// CHECK-NEXT:   cir.store %4, %2 : !s32i, cir.ptr <!s32i> loc({{.*}})
+// CHECK-NEXT:   %5 = cir.const(#cir.int<3> : !s32i) : !s32i loc({{.*}})
+// CHECK-NEXT:   %6 = cir.load %2 : cir.ptr <!s32i>, !s32i loc({{.*}})
+// CHECK-NEXT:   %7 = cir.binop(mul, %5, %6) : !s32i loc({{.*}})
+// CHECK-NEXT:   cir.store %7, %3 : !s32i, cir.ptr <!s32i> loc({{.*}})
+// CHECK-NEXT: } loc({{.*}})
+// CHECK-NEXT: cir.scope {
+// CHECK-NEXT:   %2 = cir.alloca !s32i, cir.ptr <!s32i>, ["x", init] {{.*}}
+// CHECK-NEXT:   %3 = cir.const(#cir.int<20> : !s32i) : !s32i loc({{.*}})
+// CHECK-NEXT:   cir.store %3, %2 : !s32i, cir.ptr <!s32i> loc({{.*}})
+// CHECK-NEXT: } loc({{.*}})
+// CHECK-NEXT: cir.scope {
+// Note that Clang does not even emit a block in this case
+// CHECK-NEXT: } loc({{.*}})
+// CHECK-NEXT: cir.scope {
+// CHECK-NEXT:   %2 = cir.alloca !s32i, cir.ptr <!s32i>, ["x", init] {{.*}}
+// CHECK-NEXT:   %3 = cir.alloca !s32i, cir.ptr <!s32i>, ["y", init] {{.*}}
+// CHECK-NEXT:   %4 = cir.const(#cir.int<70> : !s32i) : !s32i loc({{.*}})
+// CHECK-NEXT:   cir.store %4, %2 : !s32i, cir.ptr <!s32i> loc({{.*}})
+// CHECK-NEXT:   %5 = cir.const(#cir.int<10> : !s32i) : !s32i loc({{.*}})
+// CHECK-NEXT:   %6 = cir.load %2 : cir.ptr <!s32i>, !s32i loc({{.*}})
+// CHECK-NEXT:   %7 = cir.binop(mul, %5, %6) : !s32i loc({{.*}})
+// CHECK-NEXT:   cir.store %7, %3 : !s32i, cir.ptr <!s32i> loc({{.*}})
+// CHECK-NEXT: } loc({{.*}})
+// CHECK-NEXT: cir.scope {
+// CHECK-NEXT:   %2 = cir.alloca !s32i, cir.ptr <!s32i>, ["x", init] {{.*}}
+// CHECK-NEXT:   %3 = cir.const(#cir.int<90> : !s32i) : !s32i loc({{.*}})
+// CHECK-NEXT:   cir.store %3, %2 : !s32i, cir.ptr <!s32i> loc({{.*}})
+// CHECK-NEXT: } loc({{.*}})
+// CHECK-NEXT: cir.return loc({{.*}})