Handle MFENCE instructions appropriately

[zbrid]

At the time of this report, SpecFuzz crashes when instrumenting code that uses MFENCEs.

Reproduction instructions
mfence.ll

define void @mfence() {
  call void @llvm.x86.sse2.mfence()
  ret void
}

declare void @llvm.x86.sse2.mfence()

./bin/llc -x86-specfuzz mfence.ll

Output:

SpecFuzzPass.cpp:604: bool {anonymous}::X86SpecFuzzPass::visitWrite(llvm::MachineInstr&, llvm::MachineBasicBlock&): Assertion `MemRefBegin >= 0 && "Not a write instruction"' failed.
Stack dump:
0.	Program arguments: bin/llc -x86-specfuzz -disable-tail-calls -O3 utils.sf.bc -o utils.sf.s 
1.	Running pass 'Function Pass Manager' on module 'utils.sf.bc'.
2.	Running pass 'SpecFuzz' on function '@_Z18FlushFromDataCachePKvS0_'
#0 0x00007f8348758cae llvm::sys::PrintStackTrace(llvm::raw_ostream&) src/lib/Support/Unix/Signals.inc:490:22
#1 0x00007f8348758d41 PrintStackTraceSignalHandler(void*) src/lib/Support/Unix/Signals.inc:554:1
#2 0x00007f8348756d73 llvm::sys::RunSignalHandlers() src/lib/Support/Signals.cpp:67:20
#3 0x00007f8348758773 SignalHandler(int) src/lib/Support/Unix/Signals.inc:353:1
#4 0x00007f8348108e30 (/lib/x86_64-linux-gnu/libc.so.6+0x3be30)
#5 0x00007f8348108db1 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x3bdb1)
#6 0x00007f83480f2537 abort (/lib/x86_64-linux-gnu/libc.so.6+0x25537)
#7 0x00007f83480f240f __tls_get_addr (/lib/x86_64-linux-gnu/libc.so.6+0x2540f)
#8 0x00007f83481015b2 (/lib/x86_64-linux-gnu/libc.so.6+0x345b2)
#9 0x00007f834cd5e240 (anonymous namespace)::X86SpecFuzzPass::visitWrite(llvm::MachineInstr&, llvm::MachineBasicBlock&) src/lib/Target/X86/SpecFuzzPass.cpp:605:41
#10 0x00007f834cd5cae3 (anonymous namespace)::X86SpecFuzzPass::visitFunction(llvm::MachineFunction&) src/lib/Target/X86/SpecFuzzPass.cpp:283:26
#11 0x00007f834cd5c5f3 (anonymous namespace)::X86SpecFuzzPass::runOnMachineFunction(llvm::MachineFunction&) src/lib/Target/X86/SpecFuzzPass.cpp:223:25

Workarounds:

• Skip instrumenting MFENCE. I'll put up a pull request for this shortly.
• Remove MFENCE from the code I want to compile.

Longer term solution

• Not sure. Would it be useful for SpecFuzz to take into account the effects of MFENCEs on the state it stores during speculative execution simulation?

[OleksiiOleksenko]

Closed by 46141ef

[canertol]

I got the same error.
I don't see the error after editing SpecFuzzPass.cpp line 282 from this:

SpecFuzz/src/SpecFuzzPass.cpp

Line 282 in 46141ef

if (MI->mayStore()) {

to this:
if (MI->mayStore() && MI->getOpcode()!=2546 && MI->getOpcode()!=1) {

I couldn't figure out what opcode 2546 and 1 are but they cause the same problem.