[CodeGen] Use temporary 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.

It's not possible to reuse createTempSymbol, because on BPF has a
different prefix for globals and basic blocks right now.

Author: aengelke

llvm/include/llvm/MC/MCContext.h

@@ -458,6 +458,12 @@ class MCContext {
   MCSymbol *createNamedTempSymbol();
   MCSymbol *createNamedTempSymbol(const Twine &Name);
 
+  /// Get or create a symbol for a basic block. For non-always-emit symbols,
+  /// this behaves like createTempSymbol, except that it uses the
+  /// PrivateLabelPrefix instead of the PrivateGlobalPrefix. When AlwaysEmit is
+  /// true, behaves like getOrCreateSymbol, prefixed with PrivateLabelPrefix.
+  MCSymbol *createBlockSymbol(const Twine &Name, bool AlwaysEmit = false);
+
   /// Create the definition of a directional local symbol for numbered label
   /// (used for "1:" definitions).
   MCSymbol *createDirectionalLocalSymbol(unsigned LocalLabelVal);

llvm/lib/CodeGen/MachineBasicBlock.cpp

@@ -80,10 +80,11 @@ MCSymbol *MachineBasicBlock::getSymbol() const {
       }
       CachedMCSymbol = Ctx.getOrCreateSymbol(MF->getName() + Suffix);
     } else {
-      const StringRef Prefix = Ctx.getAsmInfo()->getPrivateLabelPrefix();
-      CachedMCSymbol = Ctx.getOrCreateSymbol(Twine(Prefix) + "BB" +
-                                             Twine(MF->getFunctionNumber()) +
-                                             "_" + Twine(getNumber()));
+      // If the block occurs as label in inline assembly, parsing the assembly
+      // needs an actual label name => set AlwaysEmit in these cases.
+      CachedMCSymbol = Ctx.createBlockSymbol(
+          "BB" + Twine(MF->getFunctionNumber()) + "_" + Twine(getNumber()),
+          /*AlwaysEmit=*/hasLabelMustBeEmitted());
     }
   }
   return CachedMCSymbol;
@@ -104,10 +105,9 @@ 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.createBlockSymbol(
+        "BB_END" + Twine(MF->getFunctionNumber()) + "_" + Twine(getNumber()),
+        /*AlwaysEmit=*/false);
   }
   return CachedEndMCSymbol;
 }

llvm/lib/MC/MCContext.cpp

@@ -309,6 +309,18 @@ MCSymbol *MCContext::createNamedTempSymbol(const Twine &Name) {
   return createSymbol(NameSV, true, false);
 }
 
+MCSymbol *MCContext::createBlockSymbol(const Twine &Name, bool AlwaysEmit) {
+  if (AlwaysEmit)
+    return getOrCreateSymbol(MAI->getPrivateLabelPrefix() + Name);
+
+  if (!UseNamesOnTempLabels)
+    return createSymbolImpl(nullptr, /*IsTemporary=*/true);
+
+  SmallString<128> NameSV;
+  raw_svector_ostream(NameSV) << MAI->getPrivateLabelPrefix() << Name;
+  return createSymbol(NameSV, false, /*IsTemporary=*/true);
+}
+
 MCSymbol *MCContext::createLinkerPrivateTempSymbol() {
   return createLinkerPrivateSymbol("tmp");
 }

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:

llvm/test/CodeGen/BPF/objdump_cond_op.ll

@@ -27,7 +27,7 @@ define i32 @test(i32, i32) local_unnamed_addr #0 {
   br label %13
 ; CHECK: r1 <<= 32
 ; CHECK: r1 >>= 32
-; CHECK: if r1 != 2 goto +6 <LBB0_2>
+; CHECK: if r1 != 2 goto +6 <test+0x48>
 
 ; <label>:8:                                      ; preds = %2
   %9 = icmp eq i32 %0, %1
@@ -38,32 +38,29 @@ define i32 @test(i32, i32) local_unnamed_addr #0 {
 ; CHECK: r0 = *(u32 *)(r1 + 0)
 ; CHECK: r0 *= r0
 ; CHECK: r0 <<= 1
-; CHECK: goto +7 <LBB0_4>
+; CHECK: goto +7 <test+0x80>
 
 ; <label>:11:                                     ; preds = %8
   %12 = shl nsw i32 %10, 2
   br label %13
 
-; CHECK-LABEL: <LBB0_2>:
 ; CHECK: r3 = 0 ll
 ; CHECK: r0 = *(u32 *)(r3 + 0)
 ; CHECK: r2 <<= 32
 ; CHECK: r2 >>= 32
-; CHECK: if r1 == r2 goto +4 <LBB0_5>
+; CHECK: if r1 == r2 goto +4 <test+0x98>
 ; CHECK: r0 <<= 2
 
 ; <label>:13:                                     ; preds = %4, %11
   %14 = phi i32 [ %12, %11 ], [ %7, %4 ]
   store i32 %14, ptr @gbl, align 4
   br label %15
-; CHECK-LABEL: <LBB0_4>:
 ; CHECK: r1 = 0 ll
 ; CHECK: *(u32 *)(r1 + 0) = r0
 
 ; <label>:15:                                     ; preds = %8, %13
   %16 = phi i32 [ %14, %13 ], [ %10, %8 ]
   ret i32 %16
-; CHECK-LABEL: <LBB0_5>:
 ; CHECK: exit
 }
 attributes #0 = { norecurse nounwind }

llvm/test/CodeGen/BPF/objdump_cond_op_2.ll

@@ -14,8 +14,7 @@ define i32 @test(i32, i32) local_unnamed_addr #0 {
 
 ; <label>:4:                                      ; preds = %2
   br label %5
-; CHECK: if r4 s>= r3 goto +10 <LBB0_2>
-; CHECK-LABEL: <LBB0_1>:
+; CHECK: if r4 s>= r3 goto +10 <test+0x90>
 
 ; <label>:5:                                      ; preds = %4, %5
   %6 = phi i32 [ %9, %5 ], [ 0, %4 ]
@@ -27,12 +26,11 @@ define i32 @test(i32, i32) local_unnamed_addr #0 {
   %12 = icmp slt i32 %10, %11
   br i1 %12, label %5, label %13
 ; CHECK: r1 = r3
-; CHECK: if r2 s> r3 goto -10 <LBB0_1>
+; CHECK: if r2 s> r3 goto -10 <test+0x40>
 
 ; <label>:13:                                     ; preds = %5, %2
   %14 = phi i32 [ 0, %2 ], [ %9, %5 ]
   ret i32 %14
-; CHECK-LABEL: <LBB0_2>:
 ; CHECK: exit
 }
 attributes #0 = { norecurse nounwind readnone }