[ARM]: Assign cost of scaling used in addressing mode for ARM cores

This patch assigns cost of the scaling used in addressing.
On many ARM cores, a negated register offset takes longer than a
non-negated register offset, in a register-offset addressing mode.

For instance:

LDR R0, [R1, R2 LSL #2]
LDR R0, [R1, -R2 LSL #2]

Above, (1) takes less cycles than (2).

By assigning appropriate scaling factor cost, we enable the LLVM
to make the right trade-offs in the optimization and code-selection phase.

Differential Revision: http://reviews.llvm.org/D24857

Reviewers: jmolloy, rengolin
llvm-svn: 284127

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.h b/llvm/lib/Target/ARM/ARMISelLowering.h
index 191235d491d..75da3584337 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.h
+++ b/llvm/lib/Target/ARM/ARMISelLowering.h
@@ -291,6 +291,14 @@ namespace llvm {
     /// by AM is legal for this target, for a load/store of the specified type.
     bool isLegalAddressingMode(const DataLayout &DL, const AddrMode &AM,
                                Type *Ty, unsigned AS) const override;
+
+    /// getScalingFactorCost - Return the cost of the scaling used in
+    /// addressing mode represented by AM.
+    /// If the AM is supported, the return value must be >= 0.
+    /// If the AM is not supported, the return value must be negative.
+    int getScalingFactorCost(const DataLayout &DL, const AddrMode &AM, Type *Ty,
+                             unsigned AS) const override;
+
     bool isLegalT2ScaledAddressingMode(const AddrMode &AM, EVT VT) const;
 
     /// isLegalICmpImmediate - Return true if the specified immediate is legal