[BPF] Remove unused weak symbol __bpf_trap (#166003)

Nikita Popov reported an issue ([1]) where a dangling weak symbol
__bpf_trap is in the final binary and this caused libbpf failing like
below:

  $ veristat -v ./t.o
  Processing 't.o'...
  libbpf: elf: skipping unrecognized data section(4) .eh_frame
  libbpf: elf: skipping relo section(5) .rel.eh_frame for section(4) .eh_frame
  libbpf: failed to find BTF for extern '__bpf_trap': -3
  Failed to open './t.o': -3

In llvm, the dag selection phase generates __bpf_trap in code. Later the
UnreachableBlockElim pass removed __bpf_trap from the code, but
__bpf_trap symbol survives in the symbol table.

Having a dangling __bpf_trap weak symbol is not good for old kernels as
seen in the above veristat failure. Although users could use compiler
flag `-mllvm -bpf-disable-trap-unreachable` to workaround the issue,
this patch fixed the issue by removing the dangling __bpf_trap.

  [1] #165696

Author: yonghong-song

llvm/lib/Target/BPF/BPFAsmPrinter.cpp

@@ -88,6 +88,16 @@ bool BPFAsmPrinter::doFinalization(Module &M) {
     }
   }
 
+  for (GlobalObject &GO : M.global_objects()) {
+    if (!GO.hasExternalWeakLinkage())
+      continue;
+
+    if (!SawTrapCall && GO.getName() == BPF_TRAP) {
+      GO.eraseFromParent();
+      break;
+    }
+  }
+
   return AsmPrinter::doFinalization(M);
 }
 
@@ -160,6 +170,20 @@ bool BPFAsmPrinter::PrintAsmMemoryOperand(const MachineInstr *MI,
 }
 
 void BPFAsmPrinter::emitInstruction(const MachineInstr *MI) {
+  if (MI->isCall()) {
+    for (const MachineOperand &Op : MI->operands()) {
+      if (Op.isGlobal()) {
+        if (const GlobalValue *GV = Op.getGlobal())
+          if (GV->getName() == BPF_TRAP)
+            SawTrapCall = true;
+      } else if (Op.isSymbol()) {
+        if (const MCSymbol *Sym = Op.getMCSymbol())
+          if (Sym->getName() == BPF_TRAP)
+            SawTrapCall = true;
+      }
+    }
+  }
+
   BPF_MC::verifyInstructionPredicates(MI->getOpcode(),
                                       getSubtargetInfo().getFeatureBits());
 

llvm/lib/Target/BPF/BPFAsmPrinter.h

@@ -39,6 +39,7 @@ class BPFAsmPrinter : public AsmPrinter {
 private:
   BTFDebug *BTF;
   TargetMachine &TM;
+  bool SawTrapCall = false;
 
   const BPFTargetMachine &getBTM() const;
 };

llvm/test/CodeGen/BPF/bpf_trap.ll

@@ -0,0 +1,32 @@
+; RUN: llc < %s | FileCheck %s
+;
+target triple = "bpf"
+
+define i32 @test(i8 %x) {
+entry:
+  %0 = and i8 %x, 3
+  switch i8 %0, label %default.unreachable4 [
+    i8 0, label %return
+    i8 1, label %sw.bb1
+    i8 2, label %sw.bb2
+    i8 3, label %sw.bb3
+  ]
+
+sw.bb1:                                           ; preds = %entry
+  br label %return
+
+sw.bb2:                                           ; preds = %entry
+  br label %return
+
+sw.bb3:                                           ; preds = %entry
+  br label %return
+
+default.unreachable4:                             ; preds = %entry
+  unreachable
+
+return:                                           ; preds = %entry, %sw.bb3, %sw.bb2, %sw.bb1
+  %retval.0 = phi i32 [ 12, %sw.bb1 ], [ 43, %sw.bb2 ], [ 54, %sw.bb3 ], [ 32, %entry ]
+  ret i32 %retval.0
+}
+
+; CHECK-NOT: __bpf_trap