From ec657b923df37503a8e2e6958afa2cf4b9d5dde3 Mon Sep 17 00:00:00 2001
From: Nekotekina <nekotekina@gmail.com>
Date: Sun, 21 Apr 2019 12:32:17 +0300
Subject: [PATCH] X86: avoid vector-scalar shifts if splat amount is directly a
 vector ADD/SUB/AND op.

Prefer vector-vector shifts if available (AVX2+).
Improves code generated for rotate and funnel shifts.
Otherwise it would generate a shuffle + slower vector-scalar shift.
---
 lib/Target/X86/X86ISelLowering.cpp | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 0387cc2fa5d..09a32d1c422 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -27131,6 +27131,16 @@ static SDValue LowerScalarVariableShift(SDValue Op, SelectionDAG &DAG,
 
   if (SDValue BaseShAmt = DAG.getSplatValue(Amt)) {
     if (SupportedVectorShiftWithBaseAmnt(VT, Subtarget, Opcode)) {
+      if (SupportedVectorVarShift(VT, Subtarget, Opcode)) {
+        // Avoid vector-scalar shift in some cases (TODO).
+        unsigned AmtOp = Amt.getOpcode();
+        switch (AmtOp) {
+        case ISD::ADD:
+        case ISD::SUB:
+        case ISD::AND:
+          return SDValue();
+        }
+      }
       MVT EltVT = VT.getVectorElementType();
       assert(EltVT.bitsLE(MVT::i64) && "Unexpected element type!");
       if (EltVT != MVT::i64 && EltVT.bitsGT(MVT::i32))