Fix PR 17368: disable vector mul distribution for square of add/sub for ARM

Generally, it is desirable to distribute (a + b) * c to a*c + b*c for
ARM with VMLx forwarding, where a, b and c are vectors.
However, for (a + b)*(a + b), distribution will result in one extra
instruction.
With distribution:
  x = a + b (add)
  y = a * x (mul)
  z = y + b * y (mla)

Without distribution:
  x = a + b (add)
  z = x * x (mul)

This patch checks if a mul is a square of add/sub. If yes, skip
distribution.

llvm-svn: 191410

llvm/lib/Target/ARM/ARMISelLowering.cpp

@@ -8342,6 +8342,13 @@ static SDValue PerformSUBCombine(SDNode *N,
 /// is faster than
 ///   vadd d3, d0, d1
 ///   vmul d3, d3, d2
+//  However, for (A + B) * (A + B),
+//    vadd d2, d0, d1
+//    vmul d3, d0, d2
+//    vmla d3, d1, d2
+//  is slower than
+//    vadd d2, d0, d1
+//    vmul d3, d2, d2
 static SDValue PerformVMULCombine(SDNode *N,
                                   TargetLowering::DAGCombinerInfo &DCI,
                                   const ARMSubtarget *Subtarget) {
@@ -8361,6 +8368,9 @@ static SDValue PerformVMULCombine(SDNode *N,
     std::swap(N0, N1);
   }
 
+  if (N0 == N1)
+    return SDValue();
+
   EVT VT = N->getValueType(0);
   SDLoc DL(N);
   SDValue N00 = N0->getOperand(0);

llvm/test/CodeGen/ARM/vmul.ll

@@ -515,6 +515,17 @@ entry:
   ret void
 }
 
+define <8 x i8> @no_distribute(<8 x i8> %a, <8 x i8> %b) nounwind {
+entry:
+; CHECK: no_distribute
+; CHECK: vadd.i8
+; CHECK: vmul.i8
+; CHECK-NOT: vmla.i8
+  %0 = add <8 x i8> %a, %b
+  %1 = mul <8x i8> %0, %0
+  ret <8 x i8> %1
+}
+
 ; If one operand has a zero-extend and the other a sign-extend, vmull
 ; cannot be used.
 define i16 @vmullWithInconsistentExtensions(<8 x i8> %vec) {