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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Message-ID: <20150806065548.GB52262@Alexeis-MBP.westell.com>
Date:	Wed, 5 Aug 2015 23:55:49 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	"Wangnan (F)" <wangnan0@...wei.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 Thu, Aug 06, 2015 at 12:35:30PM +0800, Wangnan (F) wrote:
> 
> 
> 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.

that's expected. We don't have to use default lowering
of typeid_for with getTypeIDFor. bpf backend specific
lowering can be different, though in this case it's odd
that id for __gv_my_str and __gv_my_str_ are different.
__gv_my_str and __gv_my_str2 should be different.

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