Add __builtin_assume and __builtin_assume_aligned using @llvm.assume.

This makes use of the recently-added @llvm.assume intrinsic to implement a
__builtin_assume(bool) intrinsic (to provide additional information to the
optimizer). This hooks up __assume in MS-compatibility mode to mirror
__builtin_assume (the semantics have been intentionally kept compatible), and
implements GCC's __builtin_assume_aligned as assume((p - o) & mask == 0). LLVM
now contains special logic to deal with assumptions of this form.

llvm-svn: 217349

1 files changed, 21 insertions, 3 deletions

diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index 401764a8ddf..69ad762d18e 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -374,6 +374,27 @@ RValue CodeGenFunction::EmitBuiltinExpr(const FunctionDecl *FD,
                                         "expval");
     return RValue::get(Result);
   }
+  case Builtin::BI__builtin_assume_aligned: {
+    Value *PtrValue = EmitScalarExpr(E->getArg(0));
+    Value *OffsetValue =
+      (E->getNumArgs() > 2) ? EmitScalarExpr(E->getArg(2)) : nullptr;
+
+    Value *AlignmentValue = EmitScalarExpr(E->getArg(1));
+    ConstantInt *AlignmentCI = cast<ConstantInt>(AlignmentValue);
+    unsigned Alignment = (unsigned) AlignmentCI->getZExtValue();
+
+    EmitAlignmentAssumption(PtrValue, Alignment, OffsetValue);
+    return RValue::get(PtrValue);
+  }
+  case Builtin::BI__assume:
+  case Builtin::BI__builtin_assume: {
+    if (E->getArg(0)->HasSideEffects(getContext()))
+      return RValue::get(nullptr);
+
+    Value *ArgValue = EmitScalarExpr(E->getArg(0));
+    Value *FnAssume = CGM.getIntrinsic(Intrinsic::assume);
+    return RValue::get(Builder.CreateCall(FnAssume, ArgValue));
+  }
   case Builtin::BI__builtin_bswap16:
   case Builtin::BI__builtin_bswap32:
   case Builtin::BI__builtin_bswap64: {
@@ -1510,9 +1531,6 @@ RValue CodeGenFunction::EmitBuiltinExpr(const FunctionDecl *FD,
   case Builtin::BI__noop:
     // __noop always evaluates to an integer literal zero.
     return RValue::get(ConstantInt::get(IntTy, 0));
-  case Builtin::BI__assume:
-    // Until LLVM supports assumptions at the IR level, this becomes nothing.
-    return RValue::get(nullptr);
   case Builtin::BI_InterlockedExchange:
   case Builtin::BI_InterlockedExchangePointer:
     return EmitBinaryAtomic(*this, llvm::AtomicRMWInst::Xchg, E);