Add a new target hook "predictableSelectIsExpensive".
This will be used to determine whether it's profitable to turn a select into a
branch when the branch is likely to be predicted.
Currently enabled for everything but Atom on X86 and Cortex-A9 devices on ARM.
I'm not entirely happy with the name of this flag, suggestions welcome ;)
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156233 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/llvm/Target/TargetLowering.h b/include/llvm/Target/TargetLowering.h
index 78ccb12..29abc19 100644
--- a/include/llvm/Target/TargetLowering.h
+++ b/include/llvm/Target/TargetLowering.h
@@ -150,6 +150,12 @@
/// that should be avoided.
bool isJumpExpensive() const { return JumpIsExpensive; }
+ /// isPredictableSelectExpensive - Return true if selects are only cheaper
+ /// than branches if the branch is unlikely to be predicted right.
+ bool isPredictableSelectExpensive() const {
+ return predictableSelectIsExpensive;
+ }
+
/// getSetCCResultType - Return the ValueType of the result of SETCC
/// operations. Also used to obtain the target's preferred type for
/// the condition operand of SELECT and BRCOND nodes. In the case of
@@ -2028,6 +2034,10 @@
/// optimization.
bool benefitFromCodePlacementOpt;
+ /// predictableSelectIsExpensive - Tells the code generator that select is
+ /// more expensive than a branch if the branch is usually predicted right.
+ bool predictableSelectIsExpensive;
+
private:
/// isLegalRC - Return true if the value types that can be represented by the
/// specified register class are all legal.
diff --git a/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 5132f01..5c9dc47 100644
--- a/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -605,6 +605,7 @@
IntDivIsCheap = false;
Pow2DivIsCheap = false;
JumpIsExpensive = false;
+ predictableSelectIsExpensive = false;
StackPointerRegisterToSaveRestore = 0;
ExceptionPointerRegister = 0;
ExceptionSelectorRegister = 0;
diff --git a/lib/Target/ARM/ARMISelLowering.cpp b/lib/Target/ARM/ARMISelLowering.cpp
index 711321c..48cad61 100644
--- a/lib/Target/ARM/ARMISelLowering.cpp
+++ b/lib/Target/ARM/ARMISelLowering.cpp
@@ -824,6 +824,9 @@
benefitFromCodePlacementOpt = true;
+ // Prefer likely predicted branches to selects on out-of-order cores.
+ predictableSelectIsExpensive = Subtarget->isCortexA9();
+
setMinFunctionAlignment(Subtarget->isThumb() ? 1 : 2);
}
diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 7872a49..24d95a9 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -1243,6 +1243,9 @@
setPrefLoopAlignment(4); // 2^4 bytes.
benefitFromCodePlacementOpt = true;
+ // Predictable cmov don't hurt on atom because it's in-order.
+ predictableSelectIsExpensive = !Subtarget->isAtom();
+
setPrefFunctionAlignment(4); // 2^4 bytes.
}