[CIR] Generate SelectOp instead of TernaryOp for if cheap enough to evaluate unconditionally (llvm#1642)

This came up during the review of
llvm/llvm-project#138156

During codegen we check whether the LHS and RHS of the conditional
operator are cheap enough to evaluate uncondionally. Unlike classic
codegen we still emit `TernaryOp` instead of `SelectOp` and defer that
optimization to cir-simplify.

This patch changes codegen to directly emit `SelectOp` for `cond ?
constant : constant` expressions.

Author: mmha

clang/lib/CIR/CodeGen/CIRGenExprScalar.cpp

@@ -2516,30 +2516,22 @@ mlir::Value ScalarExprEmitter::VisitAbstractConditionalOperator(
   if (isCheapEnoughToEvaluateUnconditionally(lhsExpr, CGF) &&
       isCheapEnoughToEvaluateUnconditionally(rhsExpr, CGF)) {
     bool lhsIsVoid = false;
-    auto condV = CGF.evaluateExprAsBool(condExpr);
+    mlir::Value condV = CGF.evaluateExprAsBool(condExpr);
     assert(!cir::MissingFeatures::incrementProfileCounter());
 
-    return builder
-        .create<cir::TernaryOp>(
-            loc, condV, /*thenBuilder=*/
-            [&](mlir::OpBuilder &b, mlir::Location loc) {
-              auto lhs = Visit(lhsExpr);
-              if (!lhs) {
-                lhs = builder.getNullValue(CGF.VoidTy, loc);
-                lhsIsVoid = true;
-              }
-              builder.create<cir::YieldOp>(loc, lhs);
-            },
-            /*elseBuilder=*/
-            [&](mlir::OpBuilder &b, mlir::Location loc) {
-              auto rhs = Visit(rhsExpr);
-              if (lhsIsVoid) {
-                assert(!rhs && "lhs and rhs types must match");
-                rhs = builder.getNullValue(CGF.VoidTy, loc);
-              }
-              builder.create<cir::YieldOp>(loc, rhs);
-            })
-        .getResult();
+    mlir::Value lhs = Visit(lhsExpr);
+    if (!lhs) {
+      lhs = builder.getNullValue(CGF.VoidTy, loc);
+      lhsIsVoid = true;
+    }
+
+    mlir::Value rhs = Visit(rhsExpr);
+    if (lhsIsVoid) {
+      assert(!rhs && "lhs and rhs types must match");
+      rhs = builder.getNullValue(CGF.VoidTy, loc);
+    }
+
+    return builder.createSelect(loc, condV, lhs, rhs);
   }
 
   mlir::Value condV = CGF.emitOpOnBoolExpr(loc, condExpr);

clang/test/CIR/CodeGen/ternary.c

@@ -13,3 +13,10 @@ double f1(int cond, int n, ...) {
 
 // Fine enough to check it passes the verifying.
 // CIR: cir.ternary
+
+int unconditional_evaluation(_Bool cond) {
+  return cond ? 123 : 456;
+  // CIR: %[[TRUE_CONST:.+]] = cir.const #cir.int<123>
+  // CIR: %[[FALSE_CONST:.+]] = cir.const #cir.int<456>
+  // CIR: cir.select if {{.+}} then %[[TRUE_CONST]] else %[[FALSE_CONST]] : (!cir.bool, !s32i, !s32i) -> !s32i
+}