Add TargetLowering::prepareVolatileOrAtomicLoad

One unusual feature of the z architecture is that the result of a
previous load can be reused indefinitely for subsequent loads, even if
a cache-coherent store to that location is performed by another CPU.
A special serializing instruction must be used if you want to force
a load to be reattempted.

Since volatile loads are not supposed to be omitted in this way,
we should insert a serializing instruction before each such load.
The same goes for atomic loads.

The patch implements this at the IR->DAG boundary, in a similar way
to atomic fences.  It is a no-op for targets other than SystemZ.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196906 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/SystemZ/atomic-load-01.ll

@@ -2,11 +2,10 @@
 ;
 ; RUN: llc < %s -mtriple=s390x-linux-gnu | FileCheck %s
 
-; This is just a placeholder to make sure that loads are handled.
-; The CS-based sequence is probably far too conservative.
 define i8 @f1(i8 *%src) {
 ; CHECK-LABEL: f1:
-; CHECK: cs
+; CHECK: bcr 1{{[45]}}, %r0
+; CHECK: lb %r2, 0(%r2)
 ; CHECK: br %r14
   %val = load atomic i8 *%src seq_cst, align 1
   ret i8 %val