[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.

llvm-svn: 346711

1 files changed, 17 insertions, 12 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 9fa27d89911..d976cb11910 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -514,22 +514,27 @@ Instruction *InstCombiner::narrowRotate(TruncInst &Trunc) {
   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'.