Add a target hook to allow changing the tail duplication limit based on the
contents of the block to be duplicated. Use this for ARM Cortex A8/9 to
be more aggressive tail duplicating indirect branches, since it makes it
much more likely that they will be predicted in the branch target buffer.
Testcase coming soon.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89187 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/BranchFolding.cpp b/lib/CodeGen/BranchFolding.cpp
index d344af0..94bfb72 100644
--- a/lib/CodeGen/BranchFolding.cpp
+++ b/lib/CodeGen/BranchFolding.cpp
@@ -1033,12 +1033,13 @@
if (TailBB->isSuccessor(TailBB))
return false;
- // Duplicate up to one less than the tail-merge threshold. When optimizing
- // for size, duplicate only one, because one branch instruction can be
- // eliminated to compensate for the duplication.
+ // Set the limit on the number of instructions to duplicate, with a default
+ // of one less than the tail-merge threshold. When optimizing for size,
+ // duplicate only one, because one branch instruction can be eliminated to
+ // compensate for the duplication.
unsigned MaxDuplicateCount =
MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize) ?
- 1 : (TailMergeSize - 1);
+ 1 : TII->TailDuplicationLimit(*TailBB, TailMergeSize - 1);
// Check the instructions in the block to determine whether tail-duplication
// is invalid or unlikely to be profitable.
diff --git a/lib/Target/ARM/ARMBaseInstrInfo.cpp b/lib/Target/ARM/ARMBaseInstrInfo.cpp
index 828041c..b50b609 100644
--- a/lib/Target/ARM/ARMBaseInstrInfo.cpp
+++ b/lib/Target/ARM/ARMBaseInstrInfo.cpp
@@ -1005,6 +1005,16 @@
return TargetInstrInfoImpl::isIdentical(MI0, MI1, MRI);
}
+unsigned ARMBaseInstrInfo::TailDuplicationLimit(const MachineBasicBlock &MBB,
+ unsigned DefaultLimit) const {
+ // If the target processor can predict indirect branches, it is highly
+ // desirable to duplicate them, since it can often make them predictable.
+ if (!MBB.empty() && isIndirectBranchOpcode(MBB.back().getOpcode()) &&
+ getSubtarget().hasBranchTargetBuffer())
+ return DefaultLimit + 2;
+ return DefaultLimit;
+}
+
/// getInstrPredicate - If instruction is predicated, returns its predicate
/// condition, otherwise returns AL. It also returns the condition code
/// register by reference.
diff --git a/lib/Target/ARM/ARMBaseInstrInfo.h b/lib/Target/ARM/ARMBaseInstrInfo.h
index 29855e2..73e854f 100644
--- a/lib/Target/ARM/ARMBaseInstrInfo.h
+++ b/lib/Target/ARM/ARMBaseInstrInfo.h
@@ -272,6 +272,9 @@
virtual bool isIdentical(const MachineInstr *MI, const MachineInstr *Other,
const MachineRegisterInfo *MRI) const;
+
+ virtual unsigned TailDuplicationLimit(const MachineBasicBlock &MBB,
+ unsigned DefaultLimit) const;
};
static inline
diff --git a/lib/Target/ARM/ARMSubtarget.cpp b/lib/Target/ARM/ARMSubtarget.cpp
index 87b78a9..432ed78 100644
--- a/lib/Target/ARM/ARMSubtarget.cpp
+++ b/lib/Target/ARM/ARMSubtarget.cpp
@@ -109,6 +109,8 @@
if (UseNEONFP.getPosition() == 0)
UseNEONForSinglePrecisionFP = true;
}
+ HasBranchTargetBuffer = (CPUString == "cortex-a8" ||
+ CPUString == "cortex-a9");
}
/// GVIsIndirectSymbol - true if the GV will be accessed via an indirect symbol.
diff --git a/lib/Target/ARM/ARMSubtarget.h b/lib/Target/ARM/ARMSubtarget.h
index a95c8a5..3d0e01e 100644
--- a/lib/Target/ARM/ARMSubtarget.h
+++ b/lib/Target/ARM/ARMSubtarget.h
@@ -50,6 +50,9 @@
/// determine if NEON should actually be used.
bool UseNEONForSinglePrecisionFP;
+ /// HasBranchTargetBuffer - True if processor can predict indirect branches.
+ bool HasBranchTargetBuffer;
+
/// IsThumb - True if we are in thumb mode, false if in ARM mode.
bool IsThumb;
@@ -123,6 +126,8 @@
bool isThumb2() const { return IsThumb && (ThumbMode == Thumb2); }
bool hasThumb2() const { return ThumbMode >= Thumb2; }
+ bool hasBranchTargetBuffer() const { return HasBranchTargetBuffer; }
+
bool isR9Reserved() const { return IsR9Reserved; }
const std::string & getCPUString() const { return CPUString; }