From 27220f8958f4c31a27a4fead41c7556b2a8c3d29 Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Mon, 27 Feb 2006 00:22:28 +0000
Subject: [PATCH] Just like we use the RHS of an AND to simplify the LHS, use
 the LHS to simplify the RHS.  This allows for the elimination of many
 thousands of ands from multisource, and compiles
 CodeGen/PowerPC/and-elim.ll:test2 into this:

_test2:
        srwi r2, r3, 1
        xori r3, r2, 40961
        blr

instead of this:

_test2:
        rlwinm r2, r3, 31, 17, 31
        xori r2, r2, 40961
        rlwinm r3, r2, 0, 16, 31
        blr

llvm-svn: 26388
---
 llvm/lib/Target/TargetLowering.cpp | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/llvm/lib/Target/TargetLowering.cpp b/llvm/lib/Target/TargetLowering.cpp
index 2ab2074bd18f..1dd2f5f3ed94 100644
--- a/llvm/lib/Target/TargetLowering.cpp
+++ b/llvm/lib/Target/TargetLowering.cpp
@@ -223,6 +223,23 @@ bool TargetLowering::SimplifyDemandedBits(SDOperand Op, uint64_t DemandedMask,
     // If the RHS is a constant, see if we can simplify it.
     if (TLO.ShrinkDemandedConstant(Op, DemandedMask & ~KnownZero2))
       return true;
+      
+    // If the RHS is a constant, check to see if the LHS would be zero without
+    // using the bits from the RHS.  Above, we used knowledge about the RHS to
+    // simplify the LHS, here we're using information from the LHS to simplify
+    // the RHS.
+    if (ConstantSDNode *RHSC = dyn_cast<ConstantSDNode>(Op.getOperand(1))) {
+      uint64_t LHSZero, LHSOne;
+      ComputeMaskedBits(Op.getOperand(0), DemandedMask,
+                        LHSZero, LHSOne, Depth+1);
+      // If the LHS already has zeros where RHSC does, this and is dead.
+      if ((LHSZero & DemandedMask) == (~RHSC->getValue() & DemandedMask))
+        return TLO.CombineTo(Op, Op.getOperand(0));
+      // If any of the set bits in the RHS are known zero on the LHS, shrink
+      // the constant.
+      if (TLO.ShrinkDemandedConstant(Op, ~LHSZero & DemandedMask))
+        return true;
+    }
         
     // Output known-1 bits are only known if set in both the LHS & RHS.
     KnownOne &= KnownOne2;