[llvm-dev] X86 Backend - How to push and pop eflags?

[David Majnemer via llvm-dev]

On Mon, Feb 29, 2016 at 2:06 PM, Fabio Pagani via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello llvm-dev list,
>
> i am implementing an X86 Machine Pass that at some point needs to push/pop
> eflags on the stack. This pass is hooked at preRegAlloc and LLVM is 3.7.0.
> I got two big problems:
> 1) I didn't found a way to emit a pushfq instruction in a clean way, i.e.
> with   BuildMI(*MBB, MI, DL, TII.get(X86::PUSHF64)). Even if both EFLAGS
> and RSP are added to the MBB liveins, the Machine Verifier complains saying:
> *** Bad machine code: Using an undefined physical register ***
>
> - function:    main
>
> - basic block: BB#238 for.inc.121.4 (0x43133b0)
> - instruction: PUSHF64- operand 2:   %EFLAGS<imp-use,kill>
>

We had a similar issue with the machine verifier for our blessed intrinsic.
The fixe was to mark the EFLAGS MachineOperand as Undef:
https://github.com/llvm-mirror/llvm/commit/60cefca131b1e4847f7206f99f334baac95772e9


>
> Anyway right now i'm able to push it via some "dirty" INLINE_ASM.
>
> 2) INLINE_ASM works pretty well, except in one randomly generated test
> case where the register allocator spills a register in between the
> pushfq/popfq, resulting in a crash of the compiled application.
>
> So the question is: is there a recommended way to save and restore the
> value of eflags?
>
> Any help is really appreciated!
>
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>
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