[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.

llvm-svn: 248508

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/test/Analysis/ValueTracking/knownzero-shift.ll b/llvm/test/Analysis/ValueTracking/knownzero-shift.ll
new file mode 100644
index 00000000000..835d87a9d9c
--- /dev/null
+++ b/llvm/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 }