[clang][Interp] Implement __builtin_ffs (#72988)

Author: tbaederr

clang/lib/AST/Interp/InterpBuiltin.cpp

@@ -602,6 +602,17 @@ static bool interp__builtin_rotate(InterpState &S, CodePtr OpPC,
   return true;
 }
 
+static bool interp__builtin_ffs(InterpState &S, CodePtr OpPC,
+                                const InterpFrame *Frame, const Function *Func,
+                                const CallExpr *Call) {
+  PrimType ArgT = *S.getContext().classify(Call->getArg(0)->getType());
+  APSInt Value = peekToAPSInt(S.Stk, ArgT);
+
+  uint64_t N = Value.countr_zero();
+  pushInt(S, N == Value.getBitWidth() ? 0 : N + 1);
+  return true;
+}
+
 bool InterpretBuiltin(InterpState &S, CodePtr OpPC, const Function *F,
                       const CallExpr *Call) {
   InterpFrame *Frame = S.Current;
@@ -803,6 +814,13 @@ bool InterpretBuiltin(InterpState &S, CodePtr OpPC, const Function *F,
       return false;
     break;
 
+  case Builtin::BI__builtin_ffs:
+  case Builtin::BI__builtin_ffsl:
+  case Builtin::BI__builtin_ffsll:
+    if (!interp__builtin_ffs(S, OpPC, Frame, F, Call))
+      return false;
+    break;
+
   default:
     return false;
   }

clang/test/AST/Interp/builtin-functions.cpp

@@ -353,3 +353,13 @@ namespace rotateright {
   char rotateright3[__builtin_rotateright32(0x76543210, 22) == 0x50C841D9 ? 1 : -1];
   char rotateright4[__builtin_rotateright64(0xFEDCBA9876543210ULL, 55) == 0xB97530ECA86421FDULL ? 1 : -1];
 }
+
+namespace ffs {
+  char ffs1[__builtin_ffs(0) == 0 ? 1 : -1];
+  char ffs2[__builtin_ffs(1) == 1 ? 1 : -1];
+  char ffs3[__builtin_ffs(0xfbe71) == 1 ? 1 : -1];
+  char ffs4[__builtin_ffs(0xfbe70) == 5 ? 1 : -1];
+  char ffs5[__builtin_ffs(1U << (BITSIZE(int) - 1)) == BITSIZE(int) ? 1 : -1];
+  char ffs6[__builtin_ffsl(0x10L) == 5 ? 1 : -1];
+  char ffs7[__builtin_ffsll(0x100LL) == 9 ? 1 : -1];
+}