[llvm-dev] LLVM X86 MachineBasicBlock inserting push and pop instructions causes segmentation fault

[Craig Topper via llvm-dev]

Best guess is that you're clobbering something in the red zone.
https://en.wikipedia.org/wiki/Red_zone_(computing).  It's not guarantee
that the area of the stack above the stack pointer is unused. I think you
can check if the redzone is used by checking getUsesRedZone() in
X86MachineFunctionInfo. If the red zone is used, its not safe to insert a
push/pop. I think you can turn off the red zone by passing -mno-red-zone to
clang.



~Craig


On Thu, Nov 12, 2020 at 10:18 AM vignesh babu via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello,
>
> I am working on a project where I need to insert some logic before each
> machine basic block.
> In particular, it involves setting some global variables and calling a
> function. I'm able to add the instructions and verify they get added, but
> when the compiled program runs, it stops with a segfault.
>
> For brevity, I'm not sharing the whole code here but basically I have a
> X86 MachineFunctionPass added to addPreEmitPass2 stage which simply inserts
> a push rcx immediately followed by pop rcx before each basic block (only
> the relevant logic portions are included):
>
> /* Inserts push rcx followed by pop rcx before each MachineBasicBlock */
> void VirtualTimeManager::__insertVtlLogic(MachineFunction &MF,
>     MachineBasicBlock* origMBB) {
>     const llvm::TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
>     auto MMI = &MF.getMMI();
>     llvm::Module &M = const_cast<Module &>(*MMI->getModule());
>     if (origMBB->empty() || !origMBB->isLegalToHoistInto())
>          return;
>     llvm::BuildMI(*origMBB, origMBB->begin(), DebugLoc(),
>         TII.get(X86::POP64r)).addReg(X86::RCX);
>     /* SOME ADDITIONAL LOGIC COMMENTED OUT */
>     llvm::BuildMI(*origMBB, origMBB->begin(), DebugLoc(),
>         TII.get(X86::PUSH64r)).addReg(X86::RCX);
> }
> bool VirtualTimeManager::runOnMachineFunction(MachineFunction &MF) {
>          for (auto &MBB : MF) {
>                MachineBasicBlock* origMBB = &MBB;
>                 __insertVtlLogic(MF, origMBB);
>         }
>        return true;
> }
>
> When I compile and run a program (e.g from SPEC-2006 benchmark (sjeng)),
> using this pass, the program segfaults. I don't understand how simply
> adding a push, followed by pop is affecting the program execution. Could
> anyone please offer some insights. It would be greatly appreciated. I'm new
> to LLVM and struggling to understand what is causing this issue.
>
> Thanks,
> Vignesh
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