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SubjectRe: [PATCH] perf probe: Add support for DW_OP_call_frame_cfa vars
Hi Daniel,

On Thu, 2 Apr 2020 20:25:24 +0000
Daniel Shaulov <daniel.shaulov@gmail.com> wrote:

> On Thu, Apr 02, 2020 at 03:49:43PM -0300, Arnaldo Carvalho de Melo wrote:
> > Daniel, next time please consider providing precise steps to show what
> > wasn't working and then how the patch solves the problem, i.e. get some
> > go program, show the command lines used to build, probe, etc.
> >
> > - Arnaldo
>
> Sure thing:
>
> ---- contents of example.go ----
> package main
>
> import "fmt"
>
> func example(param1 int, param2 int) {
> fmt.Println(param1, param2)
> }
>
> func main() {
> example(111, 222)
> }
> ---- end contents of example.go ----
>
> Build with:
> $ go build example.go
>
> Note that the output file is named after the directory.
>
> Then try to probe it with:
> $ perf probe -x ./example 'main.example param1 param2'
>
> The result is:
> > Target program is compiled without optimization. Skipping prologue.
> > Probe on address 0x4820f0 to force probing at the function entry.
> >
> > Sorry, we don't support this variable location yet.
> > Error: Failed to add events.
>
> With my changes the event is added successfuly.
> The reason for the change can be observed in the output of:
> $ readelf --debug-dump=info ./example
>
> Where we have the following for the main.example function (with go >= 1.10):
> <1><63dd2>: Abbrev Number: 2 (DW_TAG_subprogram)
> <63dd3> DW_AT_name : main.example
> <63de0> DW_AT_low_pc : 0x4820f0
> <63de8> DW_AT_high_pc : 0x4821c2
> <63df0> DW_AT_frame_base : 1 byte block: 9c (DW_OP_call_frame_cfa)
> <63df2> DW_AT_decl_file : 0x1
> <63df6> DW_AT_external : 1
> <2><63df7>: Abbrev Number: 14 (DW_TAG_formal_parameter)
> <63df8> DW_AT_name : param1
> <63dff> DW_AT_variable_parameter: 0
> <63e00> DW_AT_decl_line : 5
> <63e01> DW_AT_type : <0x220f>
> <63e05> DW_AT_location : 1 byte block: 9c (DW_OP_call_frame_cfa)
> <2><63e07>: Abbrev Number: 14 (DW_TAG_formal_parameter)
> <63e08> DW_AT_name : param2
> <63e0f> DW_AT_variable_parameter: 0
> <63e10> DW_AT_decl_line : 5
> <63e11> DW_AT_type : <0x220f>
> <63e15> DW_AT_location : 2 byte block: 91 8 (DW_OP_fbreg: 8)
> <2><63e18>: Abbrev Number: 0
>
> The go compiler (after 1.10) uses DW_OP_call_frame_cfa for the first parameter and DW_OP_fbreg for the rest.
> On go versions lower than 1.10, it used DW_OP_call_frame_cfa for all parameters.
> Here is example output of the readelf on main.example, compiled with go1.9.7:
> <1><2634d>: Abbrev Number: 2 (DW_TAG_subprogram)
> <2634e> DW_AT_name : main.example
> <2635b> DW_AT_low_pc : 0x487db0
> <26363> DW_AT_high_pc : 0x487e99
> <2636b> DW_AT_frame_base : 1 byte block: 9c (DW_OP_call_frame_cfa)
> <2636d> DW_AT_external : 1
> <2><2636e>: Abbrev Number: 5 (DW_TAG_formal_parameter)
> <2636f> DW_AT_name : param1
> <26376> DW_AT_location : 1 byte block: 9c (DW_OP_call_frame_cfa)
> <26378> DW_AT_type : <0x27f1c>
> <2><2637c>: Abbrev Number: 5 (DW_TAG_formal_parameter)
> <2637d> DW_AT_name : param2
> <26384> DW_AT_location : 4 byte block: 9c 11 8 22 (DW_OP_call_frame_cfa; DW_OP_consts: 8; DW_OP_plus)
> <26389> DW_AT_type : <0x27f1c>
> <2><2638d>: Abbrev Number: 0
>
> Hope this makes things easier to understand.

Thanks for the explanation. That is a useful info to understand what go compiler does on dwarf.
OK, so that is same as "DW_OP_fbreg: 0".

Thank you,

>
> Daniel.
>


--
Masami Hiramatsu <mhiramat@kernel.org>

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