[clang] [clang][ExprConst] Fix second arg of __builtin_{clzg,ctzg} not always being evaluated (PR #86742)

[via cfe-commits]

https://github.com/overmighty created https://github.com/llvm/llvm-project/pull/86742

See https://github.com/llvm/llvm-project/pull/86577#discussion_r1540069558.

cc @efriedma-quic @nickdesaulniers

>From c615f656153fcb1d4158548660f87d1d69960864 Mon Sep 17 00:00:00 2001
From: OverMighty <its.overmighty at gmail.com>
Date: Tue, 26 Mar 2024 21:25:59 +0000
Subject: [PATCH] [clang][ExprConst] Fix second arg of __builtin_{clzg,ctzg}
 not always being evaluated

---
 clang/lib/AST/ExprConstant.cpp                | 30 +++++++++-----
 .../constant-builtins-all-args-evaluated.cpp  | 39 +++++++++++++++++++
 2 files changed, 59 insertions(+), 10 deletions(-)
 create mode 100644 clang/test/Sema/constant-builtins-all-args-evaluated.cpp

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);