[Builtins] Implement __builtin_clrsb to be compatible with gcc

gcc defines an intrinsic called __builtin_clrsb which counts the number of extra sign bits on a number. This is equivalent to counting the number of leading zeros on a positive number or the number of leading ones on a negative number and subtracting one from the result. Since we can't count leading ones we need to invert negative numbers to count zeros.

This patch will cause the builtin to be expanded inline while gcc uses a call to a function like clrsbdi2 that is implemented in libgcc. But this is similar to what we already do for popcnt. And I don't think compiler-rt supports clrsbdi2.

Differential Revision: https://reviews.llvm.org/D50168

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@339282 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/include/clang/Basic/Builtins.def b/include/clang/Basic/Builtins.def
index c4d11cd827c1344b2a466920d237c724433c1de1..2d69bbf0c67e633093a86032ecf128186c3bb962 100644 (file)
--- a/include/clang/Basic/Builtins.def
+++ b/include/clang/Basic/Builtins.def
@@ -413,6 +413,9 @@ BUILTIN(__builtin_parityll, "iULLi", "nc")
 BUILTIN(__builtin_popcount  , "iUi"  , "nc")
 BUILTIN(__builtin_popcountl , "iULi" , "nc")
 BUILTIN(__builtin_popcountll, "iULLi", "nc")
+BUILTIN(__builtin_clrsb  , "ii"  , "nc")
+BUILTIN(__builtin_clrsbl , "iLi" , "nc")
+BUILTIN(__builtin_clrsbll, "iLLi", "nc")
 
 // FIXME: These type signatures are not correct for targets with int != 32-bits
 // or with ULL != 64-bits.

diff --git a/lib/CodeGen/CGBuiltin.cpp b/lib/CodeGen/CGBuiltin.cpp
index 2060630c3863fab8f4d101638f22d55f1dbb31a9..4c59286291b1ac3594ebe6a72c0f73704d64f39c 100644 (file)
--- a/lib/CodeGen/CGBuiltin.cpp
+++ b/lib/CodeGen/CGBuiltin.cpp
@@ -1537,6 +1537,26 @@ RValue CodeGenFunction::EmitBuiltinExpr(const FunctionDecl *FD,
     return RValue::get(ComplexVal.second);
   }
 
+  case Builtin::BI__builtin_clrsb:
+  case Builtin::BI__builtin_clrsbl:
+  case Builtin::BI__builtin_clrsbll: {
+    // clrsb(x) -> clz(x < 0 ? ~x : x) - 1 or
+    Value *ArgValue = EmitScalarExpr(E->getArg(0));
+
+    llvm::Type *ArgType = ArgValue->getType();
+    Value *F = CGM.getIntrinsic(Intrinsic::ctlz, ArgType);
+
+    llvm::Type *ResultType = ConvertType(E->getType());
+    Value *Zero = llvm::Constant::getNullValue(ArgType);
+    Value *IsNeg = Builder.CreateICmpSLT(ArgValue, Zero, "isneg");
+    Value *Inverse = Builder.CreateNot(ArgValue, "not");
+    Value *Tmp = Builder.CreateSelect(IsNeg, Inverse, ArgValue);
+    Value *Ctlz = Builder.CreateCall(F, {Tmp, Builder.getFalse()});
+    Value *Result = Builder.CreateSub(Ctlz, llvm::ConstantInt::get(ArgType, 1));
+    Result = Builder.CreateIntCast(Result, ResultType, /*isSigned*/true,
+                                   "cast");
+    return RValue::get(Result);
+  }
   case Builtin::BI__builtin_ctzs:
   case Builtin::BI__builtin_ctz:
   case Builtin::BI__builtin_ctzl:

diff --git a/test/CodeGen/builtin_clrsb.c b/test/CodeGen/builtin_clrsb.c
new file mode 100644 (file)
index 0000000..c51777e
--- /dev/null
+++ b/test/CodeGen/builtin_clrsb.c
@@ -0,0 +1,22 @@
+// RUN: %clang_cc1 %s -emit-llvm -o - | FileCheck %s
+
+int test__builtin_clrsb(int x) {
+// CHECK-LABEL: test__builtin_clrsb
+// CHECK: [[C:%.*]] = icmp slt i32 [[X:%.*]], 0
+// CHECK-NEXT: [[INV:%.*]] = xor i32 [[X]], -1
+// CHECK-NEXT: [[SEL:%.*]] = select i1 [[C]], i32 [[INV]], i32 [[X]]
+// CHECK-NEXT: [[CTLZ:%.*]] = call i32 @llvm.ctlz.i32(i32 [[SEL]], i1 false)
+// CHECK-NEXT: [[SUB:%.*]] = sub i32 [[CTLZ]], 1
+  return __builtin_clrsb(x);
+}
+
+int test__builtin_clrsbll(long long x) {
+// CHECK-LABEL: test__builtin_clrsbll
+// CHECK: [[C:%.*]] = icmp slt i64 [[X:%.*]], 0
+// CHECK-NEXT: [[INV:%.*]] = xor i64 [[X]], -1
+// CHECK-NEXT: [[SEL:%.*]] = select i1 [[C]], i64 [[INV]], i64 [[X]]
+// CHECK-NEXT: [[CTLZ:%.*]] = call i64 @llvm.ctlz.i64(i64 [[SEL]], i1 false)
+// CHECK-NEXT: [[SUB:%.*]] = sub i64 [[CTLZ]], 1
+// CHECK-NEXT: trunc i64 [[SUB]] to i32
+  return __builtin_clrsbll(x);
+}