[ValueTracking] Teach isKnownNonZero a new trick

If the shifter operand is a constant, and all of the bits shifted out
are known to be zero, then if X is known non-zero at least one
non-zero bit must remain.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248508 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
index 10776a03cae36ff09fff3994ea75551d5b7a6da1..417122be0d853d96540afbac9b474c8cdfea7ee4 100644 (file)
--- a/lib/Analysis/ValueTracking.cpp
+++ b/lib/Analysis/ValueTracking.cpp
@@ -1827,6 +1827,23 @@ bool isKnownNonZero(Value *V, const DataLayout &DL, unsigned Depth,
     ComputeSignBit(X, XKnownNonNegative, XKnownNegative, DL, Depth, Q);
     if (XKnownNegative)
       return true;
+
+    // If the shifter operand is a constant, and all of the bits shifted
+    // out are known to be zero, and X is known non-zero then at least one
+    // non-zero bit must remain.
+    if (ConstantInt *Shift = dyn_cast<ConstantInt>(Y)) {
+      APInt KnownZero(BitWidth, 0);
+      APInt KnownOne(BitWidth, 0);
+      computeKnownBits(X, KnownZero, KnownOne, DL, Depth, Q);
+      
+      auto ShiftVal = Shift->getLimitedValue(BitWidth - 1);
+      // Is there a known one in the portion not shifted out?
+      if (KnownOne.countLeadingZeros() < BitWidth - ShiftVal)
+        return true;
+      // Are all the bits to be shifted out known zero?
+      if (KnownZero.countTrailingOnes() >= ShiftVal)
+        return isKnownNonZero(X, DL, Depth, Q);
+    }
   }
   // div exact can only produce a zero if the dividend is zero.
   else if (match(V, m_Exact(m_IDiv(m_Value(X), m_Value())))) {

diff --git a/test/Analysis/ValueTracking/knownzero-shift.ll b/test/Analysis/ValueTracking/knownzero-shift.ll
new file mode 100644 (file)
index 0000000..835d87a
--- /dev/null
+++ b/test/Analysis/ValueTracking/knownzero-shift.ll
@@ -0,0 +1,14 @@
+; RUN: opt -instsimplify -S < %s | FileCheck %s
+
+; CHECK-LABEL: @test
+define i1 @test(i8 %p, i8* %pq) {
+  %q = load i8, i8* %pq, !range !0 ; %q is known nonzero; no known bits
+  %1 = or i8 %p, 2                 ; %1[1] = 1
+  %2 = and i8 %1, 254              ; %2[0] = 0, %2[1] = 1
+  %A = lshr i8 %2, 1               ; We should know that %A is nonzero.
+  %x = icmp eq i8 %A, 0
+  ; CHECK: ret i1 false
+  ret i1 %x
+}
+
+!0 = !{ i8 1, i8 5 }