[lldb-dev] Patch to fix REPL for ARMv7 & ARMv6 on linux

[William Dillon via lldb-dev]

Hi Omair,

In a very real sense, no information is lost here.  The addition of the ‘l’ only indicates that the system is little endian.  When the triple is created, the flag setting little endian is set (and defaults to little anyway).  There is no other valid ARM sub architecture with ARMv6 or ARMv7 that ends with an ‘l’.

I based this change somewhat on the existing model for massaging LLVM triples such as in HostInfoLinux.cpp:

void
HostInfoLinux::ComputeHostArchitectureSupport(ArchSpec &arch_32, ArchSpec &arch_64)
{
    HostInfoPosix::ComputeHostArchitectureSupport(arch_32, arch_64);

    const char *distribution_id = GetDistributionId().data();

    // On Linux, "unknown" in the vendor slot isn't what we want for the default
    // triple.  It's probably an artifact of config.guess.
    if (arch_32.IsValid())
    {
        arch_32.SetDistributionId(distribution_id);
        if (arch_32.GetTriple().getVendor() == llvm::Triple::UnknownVendor)
            arch_32.GetTriple().setVendorName(llvm::StringRef());
    }
    if (arch_64.IsValid())
    {
        arch_64.SetDistributionId(distribution_id);
        if (arch_64.GetTriple().getVendor() == llvm::Triple::UnknownVendor)
            arch_64.GetTriple().setVendorName(llvm::StringRef());
    }
}

Would it make you more comfortable is this patch was rewritten within HostInfoLinux::ComputeHostArchitectureSupport alongside the massaging of the vendor name for linux?

- Will

> On Jan 27, 2016, at 1:28 AM, Omair Javaid <omair.javaid at linaro.org> wrote:
> 
> Hi Will,
> 
> I dont understand REPL and thus the benefits it will have by making
> change to architecture name. I would not recommend to drop any
> information that we get from the host operating system.
> 
> LLDB maintains core information alongwith triple in ArchSpec, may be
> you can parse triple to reflect correct core and use core instead of
> architecture name where needed.
> 
> Kindly elaborate in a bit detail what are we getting out of this
> change for more accurate comments.
> 
> Thanks!
> 
> On 26 January 2016 at 14:47, Pavel Labath <labath at google.com> wrote:
>> + Omair
>> 
>> I don't really understand arm (sub)-architectures or REPL. The patch
>> seems mostly harmless, but it also feels like a hack to me. A couple
>> of questions:
>> - why does this only pose a problem for REPL?
>> - If I understand correctly, the problem is that someone is looking at
>> the architecture string contained in the Triple, and not finding what
>> it expects. Is that so? Could you point me to (some of) the places
>> that do that.
>> 
>> Omair, any thoughts on this?
>> 
>> cheers,
>> pl
>> 
>> 
>> On 25 January 2016 at 18:55, Hans Wennborg <hans at chromium.org> wrote:
>>> This patch looks reasonable to me, but I don't know enough about LLDB
>>> to actually review it.
>>> 
>>> +Renato or Pavel maybe?
>>> 
>>> On Thu, Jan 14, 2016 at 11:32 AM, William Dillon via lldb-dev
>>> <lldb-dev at lists.llvm.org> wrote:
>>>> Hi again, everyone
>>>> 
>>>> I’d like to ping on this patch now that the 3.8 branch is fairly new, and merging it over is fairly straight-forward.
>>>> 
>>>> Thanks in advance for your comments!
>>>> - Will
>>>> 
>>>>> There is a small change that enables correct calculation of arm sub architectures while using the REPL on arm-linux.  As you may of may or may not know, linux appends ‘l’ to arm architecture versions to denote little endian.  This sometimes interferes with the determination of the architecture in the triple.  I experimented with adding sub architecture entries for these within lldb, but I discovered a simpler (and less invasive) method.  Because LLVM already knows how to handle some of these cases (I have a patch submitted for review that enables v6l; v7l already works), I am relying on llvm to clean it up.  The gist of it is that the llvm constructor (when given a triple string) retains the provided string unless an accessor mutates it.  Meanwhile, the accessors for the components go through the aliasing and parsing logic.  This code detects whether the sub-architecture that armv6l or armv7l aliases to is detected, and re-sets the architecture in the triple.  This overwrites the architecture that comes from linux, thus sanitizing it.
>>>>> 
>>>>> Some kind of solution is required for the REPL to work on arm-linux.  Without it, the REPL crashes.