[LLVMdev] what is the Line number of Phi Node with addr2line

[John Criswell]

On 4/22/15 2:53 AM, Eric Lu wrote:
> Hey, Daniel
>
>   Is it better to instrument in the CodeGen phase ? I am now doing it 
> in the opt phases, and it seems a little complex when instrument phi 
> nodes, because I have to decide where the load operation should be if 
> there are no phi nodes.
>
> %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
>
> It may load 0 or %indvars.iv.next, I have to add checkLoad(addr, size) 
> in different BBs for different incoming value.

Eric, I think you might be confusing people with your use of the term 
"load."  In LLVM parlance, "load" means an LLVM load instruction.  When 
an instruction (such as a phi node) uses an SSA register as an operand, 
we don't say that it "loads" the value from the SSA register; we say 
that it "reads" the value of the SSA register.  Only the LLVM load 
instruction "loads" values.  The reasoning for this is that SSA 
registers may be assigned to stack spill slots or to registers, 
depending upon what the code generator decides.

Getting back to your question, it is not clear what you want to do or 
why you want to do it.  If you want to add a call to checkLoad() prior 
to every memory access that does a read, then all you need to do is to 
look for all the instructions that read memory (the LLVM load 
instruction, the atomic instructions, and a few intrinsics like 
llvm.memcpy()) and insert your call before those instructions.  That 
will allow you to instrument all memory accesses that read memory that 
are visible at the LLVM IR level.

Now, if you want to instrument memory accesses that are *not* visible at 
the LLVM IR level (e.g., reads to stack spill slots, pushing parameters 
on the stack for function calls, etc.), then you need to instrument at 
the MachineInstr level.  These memory access instructions are not 
visible in the LLVM IR.

Hope this helps,

John Criswell

>
> I guess in CodeGen Phase, the code have been changed back to code with 
> load/store already.
>
> Are there examples to show me how to instrument in the CodeGen Phase?
>
> On Wed, Apr 22, 2015 at 1:23 PM, Daniel Berlin <dberlin at dberlin.org 
> <mailto:dberlin at dberlin.org>> wrote:
>
>     reg2mem does not eliminate phi nodes the way codegen does, it just
>     converts ssa values to non-ssa values.
>
>     It's not the same thing.
>
>
>     On Tue, Apr 21, 2015 at 10:14 PM, Eric Lu <eirc.lew at gmail.com
>     <mailto:eirc.lew at gmail.com>> wrote:
>     > By the way. When I remove these phi nodes with -reg2mem, some
>     new load
>     > operations will be inserted, but when I try cache load
>     operations with:
>     > visitFunction
>     > visitLoadInst
>     > It seems we can't see these new load operations.
>     >
>     > On Wed, Apr 22, 2015 at 1:10 PM, Eric Lu <eirc.lew at gmail.com
>     <mailto:eirc.lew at gmail.com>> wrote:
>     >>
>     >> Hi, Daniel
>     >>
>     >> I want to profile load/store operations, in order to reduce the
>     overhead
>     >> of profiling, I try to instrument the optimized llvm ir, which
>     has phi
>     >> nodes.
>     >>
>     >> BTW, when the value of some load/store operations may have
>     multi-source,
>     >> then the load will be translated into phi nodes, and all phi
>     nodes are
>     >> placed in the front of BB. Sometimes, the position is not where
>     the load
>     >> happens, is there any way to figure out where the load should
>     be placed?( If
>     >> the phi node translated back to load operations)
>     >>
>     >> Best Regards!
>     >> Eric Lew
>     >>
>     >> On 周三, 4月 22, 2015 at 12:24 下午, Daniel Berlin
>     <dberlin at dberlin.org <mailto:dberlin at dberlin.org>> wrote:
>     >>
>     >> Hey Eric,
>     >> phi nodes don't exist for real, so you can't.
>     >> The are removed by PHI elimination as part of codegen.
>     >>
>     >> What are you trying to achieve?
>     >>
>     >>
>     >>
>     >>
>     >> On Tue, Apr 21, 2015 at 9:12 PM, Eric Lu <eirc.lew at gmail.com
>     <mailto:eirc.lew at gmail.com>> wrote:
>     >> > Hi, all
>     >> >
>     >> >    When compiling a program with -g -O0, and if we have a PC,
>     then with
>     >> > addr2line, we can get the line number of the instruction.
>     >> >
>     >> > My quesions are:  what is the result of Phi node instruction,
>     can we get
>     >> > the
>     >> > similar results ?
>     >> >
>     >> > --
>     >> > Best Regards!
>     >> > Eric Lew
>     >> >
>     >> > _______________________________________________
>     >> > LLVM Developers mailing list
>     >> > LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu>
>     http://llvm.cs.uiuc.edu
>     >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>     >> >
>     >
>     >
>     >
>     >
>     > --
>     > Best Regards!
>     > Eric Lew
>
>
>
>
> -- 
> Best Regards!
> Eric Lew
>
>
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-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

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