[clang] [CIR] Lowering __builtin_stdc_trailing_zeros (PR #214931)

[Kunal Dubey via cfe-commits]

https://github.com/xakep8 created https://github.com/llvm/llvm-project/pull/214931

Added CIRGen support for __builtin_stdc_trailing_zeros by lowering it to cir.ctz with zero behaviour defined, matching with the C23 stdbit semantics.

Added CodeGen test for the same.

FIxes #214443 

>From 72297a6b7a99238527e926e29392fda314201c4f Mon Sep 17 00:00:00 2001
From: Kunal Dubey <xakep8 at protonmail.com>
Date: Sat, 8 Aug 2026 10:19:23 +0530
Subject: [PATCH] [CIR] Lowering __builtin_stdc_trailing_zeros

Added CIRGen support for __builtin_stdc_trailing_zeros by lowering it to
cir.ctz with zero behaviour defined, matching with the C23 stdbit
semantics.

Added CodeGen test for the same.
---
 clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp           |  7 +++++++
 clang/test/CIR/CodeGenBuiltins/builtin-stdc-bit.c | 10 ++++++++++
 2 files changed, 17 insertions(+)
 create mode 100644 clang/test/CIR/CodeGenBuiltins/builtin-stdc-bit.c

diff --git a/clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp b/clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp
index 1efe2b81d5cae..2eb9d6c3bb661 100644
--- a/clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp
@@ -1247,6 +1247,13 @@ RValue CIRGenFunction::emitBuiltinExpr(const GlobalDecl &gd, unsigned builtinID,
                                            getTarget().isCLZForZeroUndef());
   case Builtin::BI__builtin_ctzg:
     return emitBuiltinBitOpWithFallback<cir::BitCtzOp>(*this, e);
+  case Builtin::BIstdc_trailing_zeros_uc:
+  case Builtin::BIstdc_trailing_zeros_us:
+  case Builtin::BIstdc_trailiing_zeros_ui:
+  case Builtin::BIstdc_trailing_zeros_ul:
+  case Builtin::BIstdc_trailing_zeros_ull:
+  case Builtin::BI__builtin_stdc_trailing_zeros:
+    return emitBuiltinBitOp<cir::BitCtzOp>(*this, e, /*poisonZero=*/false);
 
   case Builtin::BI__builtin_clzs:
   case Builtin::BI__builtin_clz:
diff --git a/clang/test/CIR/CodeGenBuiltins/builtin-stdc-bit.c b/clang/test/CIR/CodeGenBuiltins/builtin-stdc-bit.c
new file mode 100644
index 0000000000000..6d285c259a96f
--- /dev/null
+++ b/clang/test/CIR/CodeGenBuiltins/builtin-stdc-bit.c
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o - | FileCheck %s
+
+void test_stdc_trailing_zeros(unsigned long long x) {
+  int cnt = __builtin_stdc_trailing_zeros(x);
+  (void)cnt;
+}
+
+// CHECK-LABEL: test_stdc_trailing_zeros
+// CHECK: cir.ctz
+// CHECK-NOT: poison_zero