[llvm] [BPF] Remove 'may_goto 0' instructions (PR #123482)

[via llvm-commits]

================
@@ -682,6 +685,56 @@ bool BPFMIPreEmitPeephole::insertMissingCallerSavedSpills() {
   return Changed;
 }
 
+bool BPFMIPreEmitPeephole::removeMayGotoZero() {
+  bool Changed = false;
+  MachineBasicBlock *Prev_MBB, *Curr_MBB = nullptr;
+
+  for (MachineBasicBlock &MBB : make_early_inc_range(reverse(*MF))) {
+    Prev_MBB = Curr_MBB;
+    Curr_MBB = &MBB;
+    if (Prev_MBB == nullptr)
+      continue;
+
+    MachineInstr &MI = MBB.back();
+    if (MI.getOpcode() != TargetOpcode::INLINEASM_BR)
+      continue;
+
+    const char *AsmStr = MI.getOperand(0).getSymbolName();
+    SmallVector<StringRef, 4> AsmPieces;
+    SplitString(AsmStr, AsmPieces, ";\n");
+
+    // Do not support multiple insns in one inline asm.
+    if (AsmPieces.size() != 1)
+      continue;
+
+    // The asm insn must be a may_goto insn.
+    SmallVector<StringRef, 4> AsmOpPieces;
+    SplitString(AsmPieces[0], AsmOpPieces, " ");
+    if (AsmOpPieces[0] != "may_goto")
+      continue;
+
+    // Get the may_goto branch target.
+    MachineOperand &MO = MI.getOperand(InlineAsm::MIOp_FirstOperand + 1);
+    if (MO.getMBB() != Prev_MBB)
----------------
eddyz87 wrote:

I think that `MO.isMBB()` is needed, otherwise this would abort on (bogus) input as below:

```
$ cat test.c
void bar(int i) {
  asm volatile goto ("may_goto %l[lbl];"::"r"(i)::lbl);
 lbl:
  asm volatile("r0 = 42;");
  return;
}
$ clang -O2 --target=bpf -S test.c -o - 2>&1 | head -n 20
clang: /home/eddy/work/llvm-project/llvm/include/llvm/CodeGen/MachineOperand.h:572: MachineBasicBlock *llvm::MachineOperand::getMBB() const: Assertion `isMBB() && "Wrong MachineOperand accessor"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.	Program arguments: clang -O2 --target=bpf -S test.c -o -
1.	<eof> parser at end of file
2.	Code generation
3.	Running pass 'Function Pass Manager' on module 'test.c'.
4.	Running pass 'BPF PreEmit Peephole Optimization' on function '@bar'
 #0 0x00007f8711bfe388 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /home/eddy/work/llvm-project/llvm/lib/Support/Unix/Signals.inc:794:13
 #1 0x00007f8711bfc260 llvm::sys::RunSignalHandlers() /home/eddy/work/llvm-project/llvm/lib/Support/Signals.cpp:106:18
 #2 0x00007f8711b3e7d6 (anonymous namespace)::CrashRecoveryContextImpl::HandleCrash(int, unsigned long) /home/eddy/work/llvm-project/llvm/lib/Support/CrashRecoveryContext.cpp:73:5
 #3 0x00007f8711b3e7d6 CrashRecoverySignalHandler(int) /home/eddy/work/llvm-project/llvm/lib/Support/CrashRecoveryContext.cpp:390:51
 #4 0x00007f8711426090 __restore_rt (/lib64/libc.so.6+0x1a090)
 #5 0x00007f871147f0f4 __pthread_kill_implementation /usr/src/debug/glibc-2.40-17.fc41.x86_64/nptl/pthread_kill.c:44:76
 #6 0x00007f8711425fde gsignal /usr/src/debug/glibc-2.40-17.fc41.x86_64/signal/../sysdeps/posix/raise.c:27:6
 #7 0x00007f871140d942 abort /usr/src/debug/glibc-2.40-17.fc41.x86_64/stdlib/abort.c:81:7
 #8 0x00007f871140d85e _nl_load_domain.cold /usr/src/debug/glibc-2.40-17.fc41.x86_64/intl/loadmsgcat.c:1177:9
 #9 0x00007f871141e107 (/lib64/libc.so.6+0x12107)
#10 0x00007f8718304657 llvm::MachineOperand::getSymbolName() const /home/eddy/work/llvm-project/llvm/include/llvm/CodeGen/MachineOperand.h:638:5
#11 0x00007f8718304657 (anonymous namespace)::BPFMIPreEmitPeephole::removeMayGotoZero() /home/eddy/work/llvm-project/llvm/lib/Target/BPF/BPFMIPeephole.cpp:702:43
```

https://github.com/llvm/llvm-project/pull/123482