[clang][ExprConst] Fix second arg of __builtin_{clzg,ctzg} not always being evaluated

[overmighty]

See #86577 (comment).

cc @efriedma-quic @nickdesaulniers

[llvmbot]

@llvm/pr-subscribers-clang

Author: OverMighty (overmighty)

Changes

See #86577 (comment).

cc @efriedma-quic @nickdesaulniers

---

Full diff: https://github.com/llvm/llvm-project/pull/86742.diff

2 Files Affected:

• (modified) clang/lib/AST/ExprConstant.cpp (+20-10)
• (added) clang/test/Sema/constant-builtins-all-args-evaluated.cpp (+39)

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 5a36621dc5cce2..dae8f32fc02951 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -12361,12 +12361,17 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
     if (!EvaluateInteger(E->getArg(0), Val, Info))
       return false;
 
+    std::optional<APSInt> Fallback;
+    if (BuiltinOp == Builtin::BI__builtin_clzg && E->getNumArgs() > 1) {
+      APSInt FallbackTemp;
+      if (!EvaluateInteger(E->getArg(1), FallbackTemp, Info))
+        return false;
+      Fallback = FallbackTemp;
+    }
+
     if (!Val) {
-      if (BuiltinOp == Builtin::BI__builtin_clzg && E->getNumArgs() > 1) {
-        if (!EvaluateInteger(E->getArg(1), Val, Info))
-          return false;
-        return Success(Val, E);
-      }
+      if (Fallback)
+        return Success(*Fallback, E);
 
       // When the argument is 0, the result of GCC builtins is undefined,
       // whereas for Microsoft intrinsics, the result is the bit-width of the
@@ -12425,12 +12430,17 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
     if (!EvaluateInteger(E->getArg(0), Val, Info))
       return false;
 
+    std::optional<APSInt> Fallback;
+    if (BuiltinOp == Builtin::BI__builtin_ctzg && E->getNumArgs() > 1) {
+      APSInt FallbackTemp;
+      if (!EvaluateInteger(E->getArg(1), FallbackTemp, Info))
+        return false;
+      Fallback = FallbackTemp;
+    }
+
     if (!Val) {
-      if (BuiltinOp == Builtin::BI__builtin_ctzg && E->getNumArgs() > 1) {
-        if (!EvaluateInteger(E->getArg(1), Val, Info))
-          return false;
-        return Success(Val, E);
-      }
+      if (Fallback)
+        return Success(*Fallback, E);
 
       return Error(E);
     }
diff --git a/clang/test/Sema/constant-builtins-all-args-evaluated.cpp b/clang/test/Sema/constant-builtins-all-args-evaluated.cpp
new file mode 100644
index 00000000000000..0dee74ddf690cf
--- /dev/null
+++ b/clang/test/Sema/constant-builtins-all-args-evaluated.cpp
@@ -0,0 +1,39 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+// expected-no-diagnostics
+
+constexpr int increment(int& x) {
+  x++;
+  return x;
+}
+
+constexpr int test_clzg_0() {
+  int x = 0;
+  [[maybe_unused]] int unused = __builtin_clzg(0U, increment(x));
+  return x;
+}
+
+static_assert(test_clzg_0() == 1);
+
+constexpr int test_clzg_1() {
+  int x = 0;
+  [[maybe_unused]] int unused = __builtin_clzg(1U, increment(x));
+  return x;
+}
+
+static_assert(test_clzg_1() == 1);
+
+constexpr int test_ctzg_0() {
+  int x = 0;
+  [[maybe_unused]] int unused = __builtin_ctzg(0U, increment(x));
+  return x;
+}
+
+static_assert(test_ctzg_0() == 1);
+
+constexpr int test_ctzg_1() {
+  int x = 0;
+  [[maybe_unused]] int unused = __builtin_ctzg(1U, increment(x));
+  return x;
+}
+
+static_assert(test_ctzg_1() == 1);

[overmighty]

I guess using a bool HasFallback instead of std::optional would be more efficient but defaulting to std::optional feels like better practice.

[efriedma-quic]

LGTM

[overmighty]

Random thought that popped up while in bed at 2 AM: I probably could have just used ++x as second argument.

[overmighty]

Could someone merge this for me if it looks good to everyone?