[llvm-dev] Fwd: Why MachineBasicBlcok doesn't have transferPredecessors() ?

[Reid Kleckner via llvm-dev]

If you are working at the MachineInstr level, try using setPostInstrSymbol
to place your own label after the call in question. That is the reliable
way of getting the address of a specific instruction.

On Fri, Mar 13, 2020 at 8:47 PM PenYiWang via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
>
> ---------- Forwarded message ---------
> 寄件者： PenYiWang <s89162504 at gmail.com>
> Date: 2020年3月14日 週六 11:24
> Subject: Re: [llvm-dev] Why MachineBasicBlcok doesn't have
> transferPredecessors() ?
> To: Nicolai Hähnle <nhaehnle at gmail.com>
>
>
> for example
>
> in llvm ir level, I use BlockAddress to get the address of a BasicBlock,
> which is started with a Call Inst ir.
>
> /*
> I want to get the address of  exactly the call instruction in asm level.
> */
>
> but in llvm backend, llvm will insert some instruction before the call,
> like “sub rsp” or “mov rdi” , so the BlockAddress would get the address in
> front of the call instruction.
>
> that' why i want to spilt the MachineBasicBlock
>
> and move out the ”sub rsp“ or ”mov rdi“ to the new mbb before the original
> mbb,
>
> so that i can keep the call instruction as the first instruction of the
> MachineBasicBlock.
>
> so i need to maintain the predecessor of the new mbb,
> (because i insert a new mbb “before” the original mbb)
>
> Nicolai Hähnle <nhaehnle at gmail.com> 於 2020年3月14日 週六 09:18 寫道：
>
>> On Fri, Mar 13, 2020 at 5:09 PM PenYiWang <s89162504 at gmail.com> wrote:
>> > for example
>> >
>> > I want to insert a new machine bb “before” a specific machine bb.
>> >
>> > or split a mbb and keep the later one as the original one.
>> > (to keep the label/Blackadder's correct
>> > t)
>>
>> Can you explain why that would make a difference? Basic block labels
>> ought to be irrelevant for codegen.
>>
>>
>> > (or keep other property of mbb)
>>
>> What property?
>>
>> Cheers,
>> Nicolai
>>
>>
>> >
>> > so I need to transfer the original mbb's predecessor to the new mbb.
>> >
>> >
>> >
>> >
>> >
>> > Nicolai Hähnle <nhaehnle at gmail.com> 於 2020年3月13日 週五 23:57 寫道：
>> >>
>> >> On Fri, Mar 13, 2020 at 12:22 PM PenYiWang via llvm-dev
>> >> <llvm-dev at lists.llvm.org> wrote:
>> >> > I found that there is transferSuccessors() in MachineBasicBlcok
>> >> >
>> >> > So that when manipulating MachineBasicBlock,
>> >> > we can use transferSuccessors to update the CFG easily.
>> >> >
>> >> > Why there is not transferPredecessors in  MachineBasicBlcok ?
>> >>
>> >> What would you want to use it for? If it's for splitting a block: the
>> >> common pattern for splitting seems to be to create a new block that
>> >> will be the successor of the old one rather than the other way around.
>> >> One good reason for this is that doing it this way only requires
>> >> fixing up (target-independent) phi instructions, while doing it the
>> >> other way around would require fixing up the (target-specific!) branch
>> >> instructions.
>> >>
>> >> Cheers,
>> >> Nicolai
>> >>
>> >>
>> >> --
>> >> Lerne, wie die Welt wirklich ist,
>> >> aber vergiss niemals, wie sie sein sollte.
>>
>>
>>
>> --
>> Lerne, wie die Welt wirklich ist,
>> aber vergiss niemals, wie sie sein sollte.
>>
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