[LLVMdev] MachineRegisterInfo use_iterator/reg_iterator?

[cheng-chih yang]

Thanks Quentin. I'm trying to examine from the operands of the return
instruction, and then to get the last assignment of those. I thought
use_iterator/reg_iterator may suit better than just loop through the
machine basicblock in the reverse order.

Cheng-Chih

On Thu, Sep 25, 2014 at 1:51 PM, Quentin Colombet <qcolombet at apple.com>
wrote:

> Hi Cheng-Chih,
>
> On Sep 25, 2014, at 9:14 AM, Yang, Cheng-Chih <Cheng-Chih.Yang at amd.com>
> wrote:
>
> Hi folks,
>
> I would like to find out the machine instructions that use some given
> registers in the reverse order, and I came across these iterators
> (use_iterator/reg_iterator). However, there are two things I noticed:
>
> 1) These iterators seem to traverse the machine function a bit differently
> from what I get from the machine function dump. In other words, the
> use_iterator list is not constructed in the same order as the machine
> function?
>
>
> Yes, the use_iterator list is not constructed in the same order as the
> machine function.
>
> Maybe from the order that DAG is constructed?
>
>
> No, this is not the case either.
> The list is constructed via calls to MachineInstr::setReg (which calls
> MachineRegisterInfo::addRegOperandToUseList if you are interested by the
> details), which could basically occur anywhere in the backend.
> That said, the main users of this interface is the VirtRegMap pass, which
> walks through the function and set the register along the way. Therefore,
> most of the use_iterator should be in the order you want, but not all and
> more importantly, we do not ensure that this ordering is fulfilled.
>
>
> 2) Is there a way to go backward with these two iterators? For example,
> from use_end() to use_begin() with some decrement operator?
>
>
> AFAIK, no, because we did not need so far to traverse them in a specific
> order.
>
> What are you trying to achieve?
>
> Cheers,
> -Quentin
>
>
> I was wondering if 1) and 2) are true or just there’s something I missed.
>
> Thanks!
> Cheng-Chih
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