Re: [tcf-dev] Stack trace evaluation in the UI

[Eugene Tarassov <eugene.tarassov@xxxxxxxxxx>]

Hi Christophe,

The rationale behind it goes like this:
Since it is not a top frame, PC is return address - it points to an instruction after call instruction, and PC-1 is address of last byte of the call instruction. So, to lookup a caller, PC-1 is used. In other words, we want to show a function that owns the call instruction. Same is done with line number info. Actually, it is done mostly because of the line number info - most users want see line that contain the call, not the line which contains return address.

Regards,
Eugene


-----Original Message-----
From: tcf-dev-bounces@xxxxxxxxxxx [mailto:tcf-dev-bounces@xxxxxxxxxxx] On Behalf Of Christophe Augier
Sent: Monday, May 13, 2013 3:01 AM
To: TCF Development
Subject: [tcf-dev] Stack trace evaluation in the UI

Hi,

I've been working on a stack trace problem recently and found a probable
cause in the UI. Here is a log of the UI evaluating a stack trace:

448.180 Out: C 173 StackTrace getContext ["FP0.tracker0.ctx169"]
448.180 Inp: R 173
[{IP:266729288,ID:"FP0.tracker0.ctx169",Level:0,ProcessID:"tracker0.ctx168",ParentID:"tracker0.ctx169"}]
null
448.181 Out: C 174 LineNumbers mapToSource "tracker0.ctx168" 266729287
266729288
448.181 Out: C 175 Symbols findByAddr "tracker0.ctx168" 266729287
448.181 Inp: R 174 null []
448.182 Inp: R 175 null
"@M0.@S3.25.3630810.50A3C8EC.0.0.3.-1.909.0.0.tracker0.ctx168"
448.182 Out: C 176 Symbols getContext
"@M0.@S3.25.3630810.50A3C8EC.0.0.3.-1.909.0.0.tracker0.ctx168"
448.182 Inp: R 176 null
{Name:"__libc_start_main",TypeClass:8,UpdatePolicy:0,Class:3,Address:266729064,ID:"@M0.@S3.25.3630810.50A3C8EC.0.0.3.-1.909.0.0.tracker0.ctx168",OwnerID:"tracker0.ctx168",Size:224}

I'd like to understand why the UI looks for the symbol at pc -1 instead
of pc?

Because of the minus one, the stack trace finds the frame in a specific
function when it should not:

__libc_start_main starts at 266729064 with size 224
pc in __libc_start_main equals to pc < (266729064 + 224)
so while it's false for pc = 266729288, it's true for pc = 266729287

Any reason it's done that way? Thanks for your help.

- Christophe

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