[InstCombine] narrow width of rotate patterns, part 2 (PR39624)

The sub-pattern for the shift amount in a rotate can take on
several different forms, and there's apparently no way to
canonicalize those without seeing the entire rotate sequence.

This is the form noted in:
https://bugs.llvm.org/show_bug.cgi?id=39624

https://rise4fun.com/Alive/qnT

  %zx = zext i8 %x to i32
  %maskedShAmt = and i32 %shAmt, 7
  %shl = shl i32 %zx, %maskedShAmt
  %negShAmt = sub i32 0, %shAmt
  %maskedNegShAmt = and i32 %negShAmt, 7
  %shr = lshr i32 %zx, %maskedNegShAmt
  %rot = or i32 %shl, %shr
  %r = trunc i32 %rot to i8
  =>
  %truncShAmt = trunc i32 %shAmt to i8
  %maskedShAmt2 = and i8 %truncShAmt, 7
  %shl2 = shl i8 %x, %maskedShAmt2
  %negShAmt2 = sub i8 0, %truncShAmt
  %maskedNegShAmt2 = and i8 %negShAmt2, 7
  %shr2 = lshr i8 %x, %maskedNegShAmt2
  %r = or i8 %shl2, %shr2

llvm-svn: 346713

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index d976cb11910..8729ecc911b 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -522,6 +522,14 @@ Instruction *InstCombiner::narrowRotate(TruncInst &Trunc) {
     if (match(R, m_OneUse(m_Sub(m_SpecificInt(Width), m_Specific(L)))))
       return L;
 
+    // The shift amount may be masked with negation:
+    // (shl ShVal, (X & (Width - 1))) | (lshr ShVal, ((-X) & (Width - 1)))
+    Value *X;
+    unsigned Mask = Width - 1;
+    if (match(L, m_And(m_Value(X), m_SpecificInt(Mask))) &&
+        match(R, m_And(m_Neg(m_Specific(X)), m_SpecificInt(Mask))))
+      return X;
+
     return nullptr;
   };