Let targets provide hooks that compute known zero and ones for any_extend

and extload's. If they are implemented as zero-extend, or implicitly
zero-extend, then this can enable more demanded bits optimizations. e.g.

define void @foo(i16* %ptr, i32 %a) nounwind {
entry:
  %tmp1 = icmp ult i32 %a, 100
  br i1 %tmp1, label %bb1, label %bb2
bb1:
  %tmp2 = load i16* %ptr, align 2
  br label %bb2
bb2:
  %tmp3 = phi i16 [ 0, %entry ], [ %tmp2, %bb1 ]
  %cmp = icmp ult i16 %tmp3, 24
  br i1 %cmp, label %bb3, label %exit
bb3:
  call void @bar() nounwind
  br label %exit
exit:
  ret void
}

This compiles to the followings before:
        push    {lr}
        mov     r2, #0
        cmp     r1, #99
        bhi     LBB0_2
@ BB#1:                                 @ %bb1
        ldrh    r2, [r0]
LBB0_2:                                 @ %bb2
        uxth    r0, r2
        cmp     r0, #23
        bhi     LBB0_4
@ BB#3:                                 @ %bb3
        bl      _bar
LBB0_4:                                 @ %exit
        pop     {lr}
        bx      lr

The uxth is not needed since ldrh implicitly zero-extend the high bits. With
this change it's eliminated.

rdar://12771555


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169459 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 10 insertions, 0 deletions

diff --git a/include/llvm/Target/TargetLowering.h b/include/llvm/Target/TargetLowering.h
index 687109a1b0..2cdc05096b 100644
--- a/include/llvm/Target/TargetLowering.h
+++ b/include/llvm/Target/TargetLowering.h
@@ -935,6 +935,16 @@ public:
                                               const SelectionDAG &DAG,
                                               unsigned Depth = 0) const;
 
+  /// computeMaskedBitsForAnyExtend - Since each target implement ANY_EXTEND
+  /// and ExtLoad nodes specifically, let the target determine which of the bits
+  /// specified in Mask are known to be either zero or one and return them in
+  /// the KnownZero/KnownOne bitsets.
+  virtual void computeMaskedBitsForAnyExtend(const SDValue Op,
+                                             APInt &KnownZero,
+                                             APInt &KnownOne,
+                                             const SelectionDAG &DAG,
+                                             unsigned Depth = 0) const;
+
   /// ComputeNumSignBitsForTargetNode - This method can be implemented by
   /// targets that want to expose additional information about sign bits to the
   /// DAG Combiner.