[PATCH] D109124: [ARM] Implement target hook function to decide folding (mul (add x, c1), c2)

[Ben Shi via Phabricator via llvm-commits]

benshi001 updated this revision to Diff 370142.
Herald added a subscriber: hiraditya.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D109124/new/

https://reviews.llvm.org/D109124

Files:
  llvm/lib/Target/ARM/ARMISelLowering.cpp
  llvm/lib/Target/ARM/ARMISelLowering.h
  llvm/test/CodeGen/ARM/addimm-mulimm.ll


Index: llvm/test/CodeGen/ARM/addimm-mulimm.ll
===================================================================
--- llvm/test/CodeGen/ARM/addimm-mulimm.ll
+++ llvm/test/CodeGen/ARM/addimm-mulimm.ll
@@ -137,10 +137,9 @@
 ;
 ; CHECK-7-LABEL: fold_addimm_mulimm_i:
 ; CHECK-7:       @ %bb.0:
-; CHECK-7-NEXT:    movw r1, #23067
-; CHECK-7-NEXT:    movw r2, #353
-; CHECK-7-NEXT:    movt r1, #1
-; CHECK-7-NEXT:    mla r0, r0, r2, r1
+; CHECK-7-NEXT:    add r0, r0, #251
+; CHECK-7-NEXT:    movw r1, #353
+; CHECK-7-NEXT:    mul r0, r0, r1
 ; CHECK-7-NEXT:    bx lr
   %b = add i32 %a, 251
   %c = mul i32 %b, 353
@@ -162,10 +161,9 @@
 ;
 ; CHECK-7-LABEL: fold_addimm_mulimm_j:
 ; CHECK-7:       @ %bb.0:
-; CHECK-7-NEXT:    movw r1, #42469
-; CHECK-7-NEXT:    movw r2, #353
-; CHECK-7-NEXT:    movt r1, #65534
-; CHECK-7-NEXT:    mla r0, r0, r2, r1
+; CHECK-7-NEXT:    sub r0, r0, #251
+; CHECK-7-NEXT:    movw r1, #353
+; CHECK-7-NEXT:    mul r0, r0, r1
 ; CHECK-7-NEXT:    bx lr
   %b = add i32 %a, -251
   %c = mul i32 %b, 353
Index: llvm/lib/Target/ARM/ARMISelLowering.h
===================================================================
--- llvm/lib/Target/ARM/ARMISelLowering.h
+++ llvm/lib/Target/ARM/ARMISelLowering.h
@@ -514,6 +514,9 @@
                                        const SelectionDAG &DAG,
                                        unsigned Depth) const override;
 
+    bool isMulAddWithConstProfitable(const SDValue &AddNode,
+                                     const SDValue &ConstNode) const override;
+
     bool targetShrinkDemandedConstant(SDValue Op, const APInt &DemandedBits,
                                       const APInt &DemandedElts,
                                       TargetLoweringOpt &TLO) const override;
Index: llvm/lib/Target/ARM/ARMISelLowering.cpp
===================================================================
--- llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -18738,6 +18738,39 @@
   return AbsImm >= 0 && AbsImm <= 255;
 }
 
+// Return false to prevent folding
+// (mul (add r, c0), c1) -> (add (mul r, c1), c0*c1) in DAGCombine,
+// if the folding leads to worse code.
+bool ARMTargetLowering::isMulAddWithConstProfitable(
+    const SDValue &AddNode, const SDValue &ConstNode) const {
+  // TODO: Implement this hook for thumb.
+  if (Subtarget->isThumb())
+    return true;
+
+  // Let the DAGCombiner decide for vector types and large types.
+  const EVT VT = AddNode.getValueType();
+  if (VT.isVector() || VT.getScalarSizeInBits() > 32)
+    return true;
+
+  // It is worse if c0 is legal add immediate, while c1*c0 is not,
+  // and has to be composed by at least two instructions. That means c0*c1
+  // can not be obtained via a single MOV/MVN, and can not be
+  // obtained via a single MOVW (on ARMv6t2 and above).
+  const ConstantSDNode *C0Node = cast<ConstantSDNode>(AddNode.getOperand(1));
+  const ConstantSDNode *C1Node = cast<ConstantSDNode>(ConstNode);
+  const int64_t C0 = C0Node->getSExtValue();
+  const int64_t C1 = C1Node->getSExtValue();
+  if (isLegalAddImmediate(C0) && !isLegalAddImmediate(C0 * C1) &&
+      !isLegalAddImmediate(~std::abs(C0 * C1))) {
+    // TODO: Check for subtargets without movw/movt.
+    if (Subtarget->useMovt() && std::abs(C0 * C1) > 0xffff)
+      return false;
+  }
+
+  // Default to true and let the DAGCombiner decide.
+  return true;
+}
+
 static bool getARMIndexedAddressParts(SDNode *Ptr, EVT VT,
                                       bool isSEXTLoad, SDValue &Base,
                                       SDValue &Offset, bool &isInc,


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