Do not emit intermediate register for zero FP immediate

This updates check for double precision zero floating point constant to allow
use of instruction with immediate value rather than temporary register.
Currently "a == 0.0", where "a" is of "double" type generates:

vmov.i32        d16, #0x0
vcmpe.f64       d0, d16

With this change it becomes:

vcmpe.f64        d0, #0

Patch by Sergey Dmitrouk.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220486 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/ARM/ARMISelLowering.cpp

@@ -3245,6 +3245,18 @@ static bool isFloatingPointZero(SDValue Op) {
         if (const ConstantFP *CFP = dyn_cast<ConstantFP>(CP->getConstVal()))
           return CFP->getValueAPF().isPosZero();
     }
+  } else if (Op->getOpcode() == ISD::BITCAST &&
+             Op->getValueType(0) == MVT::f64) {
+    // Handle (ISD::BITCAST (ARMISD::VMOVIMM (ISD::TargetConstant 0)) MVT::f64)
+    // created by LowerConstantFP().
+    SDValue BitcastOp = Op->getOperand(0);
+    if (BitcastOp->getOpcode() == ARMISD::VMOVIMM) {
+      SDValue MoveOp = BitcastOp->getOperand(0);
+      if (MoveOp->getOpcode() == ISD::TargetConstant &&
+          cast<ConstantSDNode>(MoveOp)->getZExtValue() == 0) {
+        return true;
+      }
+    }
   }
   return false;
 }

test/CodeGen/ARM/fpcmp-f64-neon-opt.ll

@@ -0,0 +1,12 @@
+; RUN: llc -mtriple=linux-arm-gnueabihf -mattr=+neon %s -o - | FileCheck %s
+
+; Check that no intermediate integer register is used.
+define i32 @no-intermediate-register-for-zero-imm(double %x) #0 {
+entry:
+; CHECK-LABEL: no-intermediate-register-for-zero-imm
+; CHECK-NOT: vmov
+; CHECK: vcmp
+  %cmp = fcmp une double %x, 0.000000e+00
+  %conv = zext i1 %cmp to i32
+  ret i32 %conv
+}