[CIR][CIRGen] Implement "if constexpr" code generation

Author: keryell

clang/lib/CIR/CodeGen/CIRGenStmt.cpp

@@ -374,6 +374,14 @@ mlir::LogicalResult CIRGenFunction::buildIfStmt(const IfStmt &S) {
   if (S.isConsteval()) {
     llvm_unreachable("consteval nyi");
   }
+  // Handle "if constexpr" explicitly here to avoid generating some
+  // ill-formed code since in CIR the "if" are no longer simplified in
+  // this function like in Clang but postponed to other MLIR passes.
+  if (auto ndc = S.getNondiscardedCase(getContext())) {
+    // Replace the "if constexpr" by its non-discarded branch in a new
+    // scope
+    return buildStmt(ndc.value(), /*useCurrentScope=*/false);
+  }
   mlir::LogicalResult res = mlir::success();
 
   // C99 6.8.4.1: The first substatement is executed if the expression

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

@@ -0,0 +1,31 @@
+// 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 scope
+    int x = 2;
+  } else {
+    int x = 3;
+  }
+  if constexpr (0 == 1) {
+    int x = 4;
+  } else {
+    int x = 5;
+  }
+}
+
+// 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] {alignment = 4 : i64} loc({{.*}})
+// 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] {alignment = 4 : i64} loc({{.*}})
+// 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.return loc({{.*}})