linux-kernel - Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

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Date:	Thu, 23 Jul 2015 19:54:36 +0800
From:	He Kuang <hekuang@...wei.com>
To:	Alexei Starovoitov <ast@...mgrid.com>,
	"Wangnan (F)" <wangnan0@...wei.com>, pi3orama <pi3orama@....com>,
	Namhyung Kim <namhyung@...nel.org>
CC:	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"masami.hiramatsu.pt@...achi.com" <masami.hiramatsu.pt@...achi.com>,
	"acme@...nel.org" <acme@...nel.org>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"jolsa@...nel.org" <jolsa@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to
 perf event

Hi, Alexi

Thank you for your guidence, and by referencing your last mail
and other llvm backends, I found setting
BPFMCAsmInfo::SupportsDebugInformation = true in BPFMCAsmInfo.h
and fix some unhandeled switch can make llc output debug_info,
but important information is missing in the result:

bpf:
<1><2a>: Abbrev Number: 2 (DW_TAG_subprogram)
    <2b>   DW_AT_low_pc      : 0x0
    <33>   DW_AT_high_pc     : 0x60
    <37>   Unknown AT value: 3fe7: 1
    <37>   DW_AT_frame_base  : 1 byte block: 5a         (DW_OP_reg10 (r10))
    <39>   DW_AT_name        : (indirect string, offset: 0x0): clang
                                version 3.7.0 (http://llvm.org/git/clang.git..
    <3d>   DW_AT_decl_file   : 1
    <3e>   DW_AT_decl_line   : 3
    <3f>   DW_AT_prototyped  : 1
    <3f>   DW_AT_type        : <0x65>
    <43>   DW_AT_external    : 1
    <43>   Unknown AT value: 3fe1: 1
<2><43>: Abbrev Number: 3 (DW_TAG_formal_parameter)
    <44>   DW_AT_name        : (indirect string, offset: 0x0): clang
                                version 3.7.0 (http://llvm.org/git/clang.git..
    <48>   DW_AT_decl_file   : 1
    <49>   DW_AT_decl_line   : 3
    <4a>   DW_AT_type        : <0x65>

Compares to x86 platform result:

<1><26>: Abbrev Number: 2 (DW_TAG_subprogram)
    <27>   DW_AT_low_pc      : 0x0
    <2b>   DW_AT_high_pc     : 0x16
    <2f>   Unknown AT value: 3fe7: 1
    <2f>   DW_AT_frame_base  : 1 byte block: 54         (DW_OP_reg4 (esp))
    <31>   DW_AT_name        : (indirect string, offset: 0xcf): testprog
    <35>   DW_AT_decl_file   : 1
    <36>   DW_AT_decl_line   : 3
    <37>   DW_AT_prototyped  : 1
    <37>   DW_AT_type        : <0x65>
    <3b>   DW_AT_external    : 1
    <3b>   Unknown AT value: 3fe1: 1
<2><3b>: Abbrev Number: 3 (DW_TAG_formal_parameter)
    <3c>   DW_AT_location    : 2 byte block: 91 4       (DW_OP_fbreg: 4)
    <3f>   DW_AT_name        : (indirect string, offset: 0xdc): myvar_a
    <43>   DW_AT_decl_file   : 1
    <44>   DW_AT_decl_line   : 3
    <45>   DW_AT_type        : <0x65>

The bpf result lacks of DW_AT_location, and DW_AT_name gives no
infomation.

Then I used 'llc print-after*' command to check each pass and
wanted to find by which step the debug infomation is dropped,
things looks similar until the passes between 'verify' and
'expand-isel-pseudos':

x86:
   $ llc -march=x86 --print-before-all -print-after-all
     -stop-after=expand-isel-pseudos test.ll

   # *** IR Dump Before Expand ISel Pseudo-instructions ***:
   # Machine code for function testprog: SSA
   Frame Objects:
   fi#-2: size=4, align=4, fixed, at location [SP+8]
   fi#-1: size=4, align=16, fixed, at location [SP+4]

   BB#0: derived from LLVM BB %entry
   DBG_VALUE <fi#-1>, 0, !"myvar_a", <!15>; line no:3
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   DBG_VALUE <fi#-2>, 0, !"myvar_b", <!15>; line no:3
   %vreg0<def> = MOV32rm <fi#-2>, 1, %noreg, 0, %noreg; mem:LD4[FixedStack-2]
                                                      ; GR32:%vreg0

bpf:
   $ llc -march=bpf --print-before-all -print-after-all
     -stop-after=expand-isel-pseudos test.ll

   # *** IR Dump Before Expand ISel Pseudo-instructions ***:
   # Machine code for function testprog: SSA
   Function Live Ins: %R1 in %vreg0, %R2 in %vreg1

   BB#0: derived from LLVM BB %entry
   Live Ins: %R1 %R2
   %vreg1<def> = COPY %R2; GPR:%vreg1
   %vreg0<def> = COPY %R1; GPR:%vreg0
   %vreg2<def> = LD_imm64 2147483648; GPR:%vreg2

I think maybe this missing 'DBG_VALUE' causes the problem, but
I'm stuck here and hope you can give more advice.

Thank you!

On 2015/7/17 12:27, Alexei Starovoitov wrote:
> clang -O2 -emit-llvm -g a.c -S -o a.ll

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