[llvm-commits] [llvm] r114599 - in /llvm/trunk: lib/CodeGen/SelectionDAG/DAGCombiner.cpp test/CodeGen/X86/bit-test-shift.ll

[Owen Anderson]

Author: resistor
Date: Wed Sep 22 17:58:22 2010
New Revision: 114599

URL: http://llvm.org/viewvc/llvm-project?rev=114599&view=rev
Log:
A select between a constant and zero, when fed by a bit test, can be efficiently
lowered using a series of shifts.
Fixes <rdar://problem/8285015>.

Added:
    llvm/trunk/test/CodeGen/X86/bit-test-shift.ll
Modified:
    llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

Modified: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp?rev=114599&r1=114598&r2=114599&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Wed Sep 22 17:58:22 2010
@@ -6808,6 +6808,35 @@
     }
   }
 
+  // fold (select_cc seteq (and x, y), 0, 0, A) -> (and (shr (shl x)) A)
+  // where y is has a single bit set.
+  // A plaintext description would be, we can turn the SELECT_CC into an AND
+  // when the condition can be materialized as an all-ones register.  Any
+  // single bit-test can be materialized as an all-ones register with
+  // shift-left and shift-right-arith.
+  if (CC == ISD::SETEQ && N0->getOpcode() == ISD::AND &&
+      N0->getValueType(0) == VT &&
+      N1C && N1C->isNullValue() && 
+      N2C && N2C->isNullValue()) {
+    SDValue AndLHS = N0->getOperand(0);
+    ConstantSDNode *ConstAndRHS = dyn_cast<ConstantSDNode>(N0->getOperand(1));
+    if (ConstAndRHS && ConstAndRHS->getAPIntValue().countPopulation() == 1) {
+      // Shift the tested bit over the sign bit.
+      APInt AndMask = ConstAndRHS->getAPIntValue();
+      SDValue ShlAmt =
+        DAG.getConstant(AndMask.countLeadingZeros(), getShiftAmountTy());
+      SDValue Shl = DAG.getNode(ISD::SHL, N0.getDebugLoc(), VT, AndLHS, ShlAmt);
+      
+      // Now arithmetic right shift it all the way over, so the result is either
+      // all-ones, or zero.
+      SDValue ShrAmt =
+        DAG.getConstant(AndMask.getBitWidth()-1, getShiftAmountTy());
+      SDValue Shr = DAG.getNode(ISD::SRA, N0.getDebugLoc(), VT, Shl, ShrAmt);
+      
+      return DAG.getNode(ISD::AND, DL, VT, Shr, N3);
+    }
+  }
+
   // fold select C, 16, 0 -> shl C, 4
   if (N2C && N3C && N3C->isNullValue() && N2C->getAPIntValue().isPowerOf2() &&
       TLI.getBooleanContents() == TargetLowering::ZeroOrOneBooleanContent) {

Added: llvm/trunk/test/CodeGen/X86/bit-test-shift.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/bit-test-shift.ll?rev=114599&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/X86/bit-test-shift.ll (added)
+++ llvm/trunk/test/CodeGen/X86/bit-test-shift.ll Wed Sep 22 17:58:22 2010
@@ -0,0 +1,13 @@
+; RUN: llc < %s -march=x86 | FileCheck %s
+; <rdar://problem/8285015>
+
+define i32 @x(i32 %t) nounwind readnone ssp {
+entry:
+; CHECK: shll	$23, %eax
+; CHECK: sarl	$31, %eax
+; CHECK: andl	$-26, %eax
+  %and = and i32 %t, 256
+  %tobool = icmp eq i32 %and, 0
+  %retval.0 = select i1 %tobool, i32 0, i32 -26
+  ret i32 %retval.0
+}