[CodeGen] Use temp symbol for MBBs

Internal label names never occur in the symbol table, so when using an
object streamer, there's no point in constructing these names and then
adding them to hash tables -- they are never visible in the output.

This changes label names to use the PrivateGlobalPrefix instead of the
PrivateLabelPrefix. These are the same for all targets, however, so this
seems acceptable.

Author: aengelke

llvm/lib/CodeGen/MachineBasicBlock.cpp

@@ -79,11 +79,18 @@ MCSymbol *MachineBasicBlock::getSymbol() const {
         Suffix = (Suffix + Twine(".__part.") + Twine(SectionID.Number)).str();
       }
       CachedMCSymbol = Ctx.getOrCreateSymbol(MF->getName() + Suffix);
-    } else {
+    } else if (hasLabelMustBeEmitted()) {
+      // If the block occurs as label in inline assembly, parsing the assembly
+      // needs an actual label name.
       const StringRef Prefix = Ctx.getAsmInfo()->getPrivateLabelPrefix();
       CachedMCSymbol = Ctx.getOrCreateSymbol(Twine(Prefix) + "BB" +
                                              Twine(MF->getFunctionNumber()) +
                                              "_" + Twine(getNumber()));
+    } else {
+      CachedMCSymbol = Ctx.createTempSymbol("BB" +
+                                            Twine(MF->getFunctionNumber()) +
+                                            "_" + Twine(getNumber()),
+                                            /*AlwaysAddSuffix=*/false);
     }
   }
   return CachedMCSymbol;
@@ -104,10 +111,10 @@ MCSymbol *MachineBasicBlock::getEndSymbol() const {
   if (!CachedEndMCSymbol) {
     const MachineFunction *MF = getParent();
     MCContext &Ctx = MF->getContext();
-    auto Prefix = Ctx.getAsmInfo()->getPrivateLabelPrefix();
-    CachedEndMCSymbol = Ctx.getOrCreateSymbol(Twine(Prefix) + "BB_END" +
-                                              Twine(MF->getFunctionNumber()) +
-                                              "_" + Twine(getNumber()));
+    CachedEndMCSymbol = Ctx.createTempSymbol("BB_END" +
+                                             Twine(MF->getFunctionNumber()) +
+                                             "_" + Twine(getNumber()),
+                                             /*AlwaysAddSuffix=*/false);
   }
   return CachedEndMCSymbol;
 }

llvm/test/CodeGen/AArch64/branch-relax-cross-section.mir

@@ -473,8 +473,8 @@ body:             |
   ; INDIRECT-NEXT:    successors: %bb.1
   ; INDIRECT-NEXT:    liveins: $x16
   ; INDIRECT-NEXT:    {{ $}}
-  ; INDIRECT-NEXT:    $[[SCAVENGED_REGISTER:x[0-9]+]] = ADRP target-flags(aarch64-page) <mcsymbol .LBB5_1>
-  ; INDIRECT-NEXT:    $[[SCAVENGED_REGISTER]] = ADDXri $[[SCAVENGED_REGISTER]], target-flags(aarch64-pageoff, aarch64-nc) <mcsymbol .LBB5_1>, 0
+  ; INDIRECT-NEXT:    $[[SCAVENGED_REGISTER:x[0-9]+]] = ADRP target-flags(aarch64-page) <mcsymbol >
+  ; INDIRECT-NEXT:    $[[SCAVENGED_REGISTER]] = ADDXri $[[SCAVENGED_REGISTER]], target-flags(aarch64-pageoff, aarch64-nc) <mcsymbol >, 0
   ; INDIRECT-NEXT:    BR $[[SCAVENGED_REGISTER]]
 
   bb.0.entry: