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: <20150806032221.GA52057@Alexeis-MBP.westell.com>
Date:	Wed, 5 Aug 2015 20:22:22 -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 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'm not familiar with dwarf handling part of llvm, but feels possible.

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