llvm/test/Analysis/ValueTracking/knownnonzero-shift.ll
James Molloy bf7b3fed5c [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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251294 91177308-0d34-0410-b5e6-96231b3b80d8
2015-10-26 14:10:46 +00:00

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LLVM

; 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 }