[ValueTracking] Extend r251146 to catch a fairly common case

Even though we may not know the value of the shifter operand, it's possible we know the shifter operand is non-zero. This can allow us to infer more known bits - for example:

  %1 = load %p !range {1, 5}
  %2 = shl %q, %1

We don't know %1, but we do know that it is nonzero so %2[0] is known zero, and importantly %2 is known non-zero.

Calling isKnownNonZero is nontrivially expensive so use an Optional to run it lazily and cache its result.

llvm-svn: 251294

1 files changed, 13 insertions, 0 deletions

diff --git a/llvm/test/Analysis/ValueTracking/knownnonzero-shift.ll b/llvm/test/Analysis/ValueTracking/knownnonzero-shift.ll
new file mode 100644
index 00000000000..e59d19cc2e2
--- /dev/null
+++ b/llvm/test/Analysis/ValueTracking/knownnonzero-shift.ll
@@ -0,0 +1,13 @@
+; 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 = shl i8 %p, %q              ; because %q is nonzero, %1[0] is known to be zero.
+  %2 = and i8 %1, 1
+  %x = icmp eq i8 %2, 0
+  ; CHECK: ret i1 true
+  ret i1 %x
+}
+
+!0 = !{ i8 1, i8 5 }