linux-kernel - Re: [llvm-dev] [LLVMdev] Cc llvmdev: Re: llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

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Date:	Thu, 6 Aug 2015 12:35:30 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:	Alexei Starovoitov <ast@...mgrid.com>,
	He Kuang <hekuang@...wei.com>, pi3orama <pi3orama@....com>,
	<llvm-dev@...ts.llvm.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [llvm-dev] [LLVMdev] Cc llvmdev: Re: llvm bpf debug info. Re:
 [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event



On 2015/8/6 11:22, Alexei Starovoitov wrote:
> On Wed, Aug 05, 2015 at 04:28:13PM +0800, Wangnan (F) wrote:
>> It doesn't work for me at first since in my llvm there's only
>> llvm.bpf.load.*.
>>
>> I think llvm.bpf.store.* belone to some patches you haven't posted yet?
> nope. only loads have special instructions ld_abs/ld_ind
> which are represented by these intrinsics.
> stores, so far, are done via single bpf_store_bytes() helper function.
>
>>> the typeid changing ids with order is surprising.
>>> I think the assertion in ExtractTypeInfo() is not hard.
>>> Just there were no such use cases. May be we can do something
>>> similar to what LowerIntrinsicCall() does and lower it differently
>>> in the backend.
>>>
>> But in backend can we still get type information? I thought type is
>> meaningful in frontend only, and backend behaviors is unable to affect
>> DWARF generation, right?
> why do we need to affect type generation? we just need to know dwarf
> type id in the backend, so we can emit it as a constant.
> I still think lowering eh_typeid_for differently may work.
> Like instead of doing
> GV = ExtractTypeInfo(I.getArgOperand(0)) followed by
> getMachineFunction().getMMI().getTypeIDFor(GV)
> we can get dwarf type id from I.getArgOperand(0) if it's
> any pointer to struct type.

I have a bad news to tell:

#include <stdio.h>
struct my_str {
         int x;
         int y;
} __gv_my_str;
struct my_str __gv_my_str_;

struct my_str2 {
         int x;
         int y;
} __gv_my_str2;

int typeid(void *p) asm("llvm.eh.typeid.for");

int main()
{
         printf("%d\n", typeid(&__gv_my_str));
         printf("%d\n", typeid(&__gv_my_str_));
         printf("%d\n", typeid(&__gv_my_str2));
         return 0;
}

Compiled with clang into x86 executable, then:

$ ./a.out
3
2
1

See? I have two types but reported 3 IDs.

And here is the implementation of getTypeIDFor, in 
lib/CodeGen/MachineModuleInfo.cpp:

unsigned MachineModuleInfo::getTypeIDFor(const GlobalValue *TI) {
   for (unsigned i = 0, N = TypeInfos.size(); i != N; ++i)
     if (TypeInfos[i] == TI) return i + 1;

   TypeInfos.push_back(TI);
   return TypeInfos.size();
}

It only checks value in a stupid way.

Now the dwarf side becomes clear (see my other response), but the 
frontend may require
totally reconsidering.

Do you know someone in LLVM-dev who can help us?

Thank you.

> I'm not familiar with dwarf handling part of llvm, but feels possible.
>


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