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

clang/lib/CIR/CodeGen/CIRGenStmt.cpp

@@ -383,6 +383,16 @@ mlir::LogicalResult CIRGenFunction::buildIfStmt(const IfStmt &S) {
       if (buildStmt(S.getInit(), /*useCurrentScope=*/true).failed())
         return mlir::failure();
 
+    // Handle "if constexpr" explicitly here to avoid generating some
+    // ill-formed code since in CIR the "if" are no longer simplified
+    // in this lambda like in Clang but postponed to other MLIR
+    // passes.
+    if (S.isConstexpr())
+      if (auto ndc = S.getNondiscardedCase(getContext())) {
+        // Replace the "if constexpr" by its non-discarded branch
+        return buildStmt(ndc.value(), /*useCurrentScope=*/true);
+      }
+
     if (S.getConditionVariable())
       buildDecl(*S.getConditionVariable());
 

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

@@ -0,0 +1,60 @@
+// 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;
+  }
+}
+
+// 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.return loc({{.*}})