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, 27 insertions, 0 deletions

diff --git a/test/CodeGen/ARM/extload-knownzero.ll b/test/CodeGen/ARM/extload-knownzero.ll
new file mode 100644
index 0000000000..348285399a
--- /dev/null
+++ b/test/CodeGen/ARM/extload-knownzero.ll
@@ -0,0 +1,27 @@
+; RUN: llc < %s -march=arm | FileCheck %s
+; rdar://12771555
+
+define void @foo(i16* %ptr, i32 %a) nounwind {
+entry:
+; CHECK: foo:
+  %tmp1 = icmp ult i32 %a, 100
+  br i1 %tmp1, label %bb1, label %bb2
+bb1:
+; CHECK: %bb1
+; CHECK: ldrh
+  %tmp2 = load i16* %ptr, align 2
+  br label %bb2
+bb2:
+; CHECK-NOT: uxth
+; CHECK: cmp
+  %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
+}
+
+declare void @bar ()