[PowerPC][AIX] Support toc-data attribute for read-only globals

[yoalione]

Summary

The patch handles the addition of constant global variables to the table of contents.

[sfertile]

LGTM.

llvm/lib/MC/MCSectionXCOFF.cpp

if (getKind().isText()) {
     if (getMappingClass() != XCOFF::XMC_PR)
       report_fatal_error("Unhandled storage-mapping class for .text csect");
 
     printCsectDirective(OS);
     return;
   }
 
   if (getKind().isReadOnly()) {
-    if (getMappingClass() != XCOFF::XMC_RO)
+    if (getMappingClass() != XCOFF::XMC_RO &&
+        getMappingClass() != XCOFF::XMC_TD)
       report_fatal_error("Unhandled storage-mapping class for .rodata csect.");
     printCsectDirective(OS);
     return;
   }
 
   // Initialized TLS data.
   if (getKind().isThreadData()) {
     // We only expect XMC_TL here for initialized TLS data.
if (getKind().isData()) {
     default:
       report_fatal_error(
           "Unhandled storage-mapping class for .data csect.");
     }
     return;
   }
 
   if (isCsect() && getMappingClass() == XCOFF::XMC_TD) {
-    assert((getKind().isBSSExtern() || getKind().isBSSLocal()) &&
+    assert((getKind().isBSSExtern() || getKind().isBSSLocal() ||
+            getKind().isReadOnlyWithRel()) &&
            "Unexepected section kind for toc-data");
     printCsectDirective(OS);
     return;
   }
   // Common csect type (uninitialized storage) does not have to print csect
   // directive for section switching.
   if (isCsect() && getCSectType() == XCOFF::XTY_CM) {
     assert((getMappingClass() == XCOFF::XMC_RW ||

llvm/test/CodeGen/PowerPC/toc-data-const.ll

+; RUN: llc -mtriple powerpc-ibm-aix-xcoff < %s | FileCheck %s --check-prefix CHECK
+; RUN: llc -mtriple powerpc64-ibm-aix-xcoff < %s | FileCheck %s --check-prefix CHECK
+
+@i1 = external constant i32 #0
+@i2 = constant i32* @i1 #0
+
+define i32 @read() {
+  %1  = load i32, i32* @i1, align 4
+  ret i32 %1
+}
+
+define i32** @retptr() {
+  ret i32** @i2
+}
+
+; CHECK:       .read:
+; CHECK:        la 3, i1[TD](2)
+
+; CHECK:       .retptr:
+; CHECK:        la 3, i2[TD](2)
+
+; CHECK-DAG:   .toc
+; CHECK:         .extern i1[TD]
+; CHECK:         .csect i2[TD]
+
+attributes #0 = { "toc-data" }