[SystemZ] Do not use LOC(G) for volatile loads

It is not safe to use LOAD ON CONDITION to implement access to a memory
location marked "volatile", since the architecture leaves it unspecified
whether or not an access happens if the condition is false.

The current code already appears to care about that:
  def LOC  : CondUnaryRSY<"loc",  0xEBF2, nonvolatile_load, GR32, 4>;

Unfortunately, that "nonvolatile_load" operator is simply ignored
by the CondUnaryRSY class, and there was no test to catch it.

llvm-svn: 285077

llvm/lib/Target/SystemZ/SystemZInstrFormats.td

@@ -1665,7 +1665,7 @@ class CondUnaryRSY<string mnemonic, bits<16> opcode,
             (ins cls:$R1src, mode:$BD2, cond4:$valid, cond4:$R3),
             mnemonic#"$R3\t$R1, $BD2",
             [(set cls:$R1,
-                  (z_select_ccmask (load bdaddr20only:$BD2), cls:$R1src,
+                  (z_select_ccmask (operator bdaddr20only:$BD2), cls:$R1src,
                                    cond4:$valid, cond4:$R3))]>,
     Requires<[FeatureLoadStoreOnCond]> {
   let Constraints = "$R1 = $R1src";

llvm/test/CodeGen/SystemZ/cond-load-01.ll

@@ -128,3 +128,17 @@ exit:
   %res = phi i32 [ %easy, %entry ], [ %other, %load ]
   ret i32 %res
 }
+
+; Test that volatile loads do not use LOC, since if the condition is false,
+; it is unspecified whether or not the load happens.  LOCR is fine though.
+define i32 @f10(i32 %easy, i32 *%ptr, i32 %limit) {
+; CHECK-LABEL: f10:
+; CHECK: l {{%r[0-9]*}}, 0(%r3)
+; CHECK: locr
+; CHECK: br %r14
+  %cond = icmp ult i32 %limit, 42
+  %other = load volatile i32, i32 *%ptr
+  %res = select i1 %cond, i32 %easy, i32 %other
+  ret i32 %res
+}
+

llvm/test/CodeGen/SystemZ/cond-load-02.ll

@@ -128,3 +128,17 @@ exit:
   %res = phi i64 [ %easy, %entry ], [ %other, %load ]
   ret i64 %res
 }
+
+; Test that volatile loads do not use LOCG, since if the condition is false,
+; it is unspecified whether or not the load happens.  LOCGR is fine though.
+define i64 @f10(i64 %easy, i64 *%ptr, i64 %limit) {
+; CHECK-LABEL: f10:
+; CHECK: lg {{%r[0-9]*}}, 0(%r3)
+; CHECK: locgr
+; CHECK: br %r14
+  %cond = icmp ult i64 %limit, 42
+  %other = load volatile i64, i64 *%ptr
+  %res = select i1 %cond, i64 %easy, i64 %other
+  ret i64 %res
+}
+