[CIR][CIRGen] Implement "if consteval" code generation (#446)

Emit the false-branch of the consteval if statement, if any.

clang/lib/CIR/CodeGen/CIRGenStmt.cpp

@@ -371,16 +371,23 @@ static void terminateBody(CIRGenBuilderTy &builder, mlir::Region &r,
 }
 
 mlir::LogicalResult CIRGenFunction::buildIfStmt(const IfStmt &S) {
+  mlir::LogicalResult res = mlir::success();
   // The else branch of a consteval if statement is always the only branch
   // that can be runtime evaluated.
+  const Stmt *ConstevalExecuted;
   if (S.isConsteval()) {
-    llvm_unreachable("consteval nyi");
+    ConstevalExecuted = S.isNegatedConsteval() ? S.getThen() : S.getElse();
+    if (!ConstevalExecuted)
+      // No runtime code execution required
+      return res;
   }
-  mlir::LogicalResult res = mlir::success();
 
   // C99 6.8.4.1: The first substatement is executed if the expression
   // compares unequal to 0.  The condition must be a scalar type.
   auto ifStmtBuilder = [&]() -> mlir::LogicalResult {
+    if (S.isConsteval())
+      return buildStmt(ConstevalExecuted, /*useCurrentScope=*/true);
+
     if (S.getInit())
       if (buildStmt(S.getInit(), /*useCurrentScope=*/true).failed())
         return mlir::failure();

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

@@ -0,0 +1,33 @@
+// RUN: %clang_cc1 -std=c++23 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o %t.cir
+// RUN: FileCheck --input-file=%t.cir %s
+
+void should_be_used_1();
+void should_be_used_2();
+void should_be_used_3();
+constexpr void should_not_be_used() {}
+
+constexpr void f() {
+  if consteval {
+    should_not_be_used(); // CHECK-NOT: call {{.*}}should_not_be_used
+  } else {
+    should_be_used_1(); // CHECK: call {{.*}}should_be_used_1
+  }
+
+  if !consteval {
+    should_be_used_2(); // CHECK: call {{.*}}should_be_used_2
+  } else {
+    should_not_be_used(); // CHECK-NOT: call {{.*}}should_not_be_used
+  }
+
+  if consteval {
+    should_not_be_used(); // CHECK-NOT: call {{.*}}should_not_be_used
+  }
+
+  if !consteval {
+    should_be_used_3(); // CHECK: call {{.*}}should_be_used_3
+  }
+}
+
+void g() {
+  f();
+}