[llvm] r346711 - [InstCombine] refactor code for matching shift amount of a rotate; NFC

[Sanjay Patel via llvm-commits]

Author: spatel
Date: Mon Nov 12 14:00:00 2018
New Revision: 346711

URL: http://llvm.org/viewvc/llvm-project?rev=346711&view=rev
Log:
[InstCombine] refactor code for matching shift amount of a rotate; NFC

As shown in existing test cases and with:
https://bugs.llvm.org/show_bug.cgi?id=39624
...we're missing at least 2 more patterns for rotate narrowing.

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp?rev=346711&r1=346710&r2=346711&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp Mon Nov 12 14:00:00 2018
@@ -514,22 +514,27 @@ Instruction *InstCombiner::narrowRotate(
   if (ShiftOpcode0 == ShiftOpcode1)
     return nullptr;
 
-  // The shift amounts must add up to the narrow bit width.
-  Value *ShAmt;
-  bool SubIsOnLHS;
+  // Match the shift amount operands for a rotate pattern. This always matches
+  // a subtraction on the R operand.
+  auto matchShiftAmount = [](Value *L, Value *R, unsigned Width) -> Value * {
+    // The shift amounts may add up to the narrow bit width:
+    // (shl ShVal, L) | (lshr ShVal, Width - L)
+    if (match(R, m_OneUse(m_Sub(m_SpecificInt(Width), m_Specific(L)))))
+      return L;
+
+    return nullptr;
+  };
+
   Type *DestTy = Trunc.getType();
   unsigned NarrowWidth = DestTy->getScalarSizeInBits();
-  if (match(ShAmt0,
-            m_OneUse(m_Sub(m_SpecificInt(NarrowWidth), m_Specific(ShAmt1))))) {
-    ShAmt = ShAmt1;
+  Value *ShAmt = matchShiftAmount(ShAmt0, ShAmt1, NarrowWidth);
+  bool SubIsOnLHS = false;
+  if (!ShAmt) {
+    ShAmt = matchShiftAmount(ShAmt1, ShAmt0, NarrowWidth);
     SubIsOnLHS = true;
-  } else if (match(ShAmt1, m_OneUse(m_Sub(m_SpecificInt(NarrowWidth),
-                                          m_Specific(ShAmt0))))) {
-    ShAmt = ShAmt0;
-    SubIsOnLHS = false;
-  } else {
-    return nullptr;
   }
+  if (!ShAmt)
+    return nullptr;
 
   // The shifted value must have high zeros in the wide type. Typically, this
   // will be a zext, but it could also be the result of an 'and' or 'shift'.