[ARM] Fix handling of mapping symbols when changing sections

[john.brawn]

Summary

ChangeSection incorrectly registers LastEMSInfo as belonging to the previous section, not the current section. This happens to work when changing sections using .section, as the previous section is set to the current section before the call to ChangeSection, but not when using .popsection.

[rengolin]

Good catch, thanks! LGTM.

llvm/trunk/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp

public:
   void emitHandlerData();
   void emitSetFP(unsigned NewFpReg, unsigned NewSpReg, int64_t Offset = 0);
   void emitMovSP(unsigned Reg, int64_t Offset = 0);
   void emitPad(int64_t Offset);
   void emitRegSave(const SmallVectorImpl<unsigned> &RegList, bool isVector);
   void emitUnwindRaw(int64_t Offset, const SmallVectorImpl<uint8_t> &Opcodes);
 
   void ChangeSection(MCSection *Section, const MCExpr *Subsection) override {
-    LastMappingSymbols[getPreviousSection().first] = std::move(LastEMSInfo);
+    LastMappingSymbols[getCurrentSection().first] = std::move(LastEMSInfo);
     MCELFStreamer::ChangeSection(Section, Subsection);
     auto LastMappingSymbol = LastMappingSymbols.find(Section);
     if (LastMappingSymbol != LastMappingSymbols.end()) {
       LastEMSInfo = std::move(LastMappingSymbol->second);
       return;
     }
     LastEMSInfo.reset(new ElfMappingSymbolInfo(SMLoc(), nullptr, 0));
   }

llvm/trunk/test/MC/ARM/multi-section-mapping.s

@ A setion should be able to start with a $d
         .section .starts_data
         .word 42
 
 @ Changing back to .text should not emit a redundant $a
         .text
         .arm
         add r0, r0, r0
 
+@ Similarly no $t if we change back .starts_thumb using .pushsection
+        .pushsection .starts_thumb
+        .thumb
+        adds r0, r0, r0
+
+@ When we change back to .text using .popsection .thumb is still active, so we
+@ should emit a $t
+        .popsection
+        add r0, r0, r0
+
+@ .ident does a push then pop of the .comment section, so the .word should
+@ cause $d to appear in the .text section
+        .ident "ident"
+        .word 0
+
 @ With all those constraints, we want:
-@   + .text to have $a at 0 and no others
+@   + .text to have $a at 0, $t at 8, $d at 12
 @   + .wibble to have $a at 0
 @   + .starts_thumb to have $t at 0
 @   + .starts_data to have $d at 0
 
 @ CHECK: 00000000 .text 00000000 $a
 @ CHECK-NEXT: 00000000 .wibble 00000000 $a
+@ CHECK-NEXT: 0000000a .text 00000000 $d
 @ CHECK-NEXT: 00000000 .starts_thumb 00000000 $t
+@ CHECK-NEXT: 00000008 .text 00000000 $t
 @ CHECK-NOT: ${{[adt]}}