[LLVMdev] Valid debug information being deleted by DAGCombiner

Relph, Richard Richard.Relph at amd.com
Fri Apr 15 08:42:29 PDT 2011

    Mem2reg actually modifies debug information in a way that at least suggests it's trying to maintain debugability. Specifically, it changes llvm.dbg.declare() calls (appropriate for variables that permanently reside in a single place) to llvm.dbg.value() calls (specifying that at this instant, this variable is in this register).
    In fact, the .bc after inlining and mem2reg optimizations seems correct. Llvm.dbg.value() refers to a chain of single-input phi nodes left behind by inlining that ultimately resolve to the value in the inlined function that corresponds to the return value. But when code gen does its thing, DAGCombiner thinks some of the llvm.dbg.value() calls are associated with dead assignments and removes them. Only we haven't been able to figure out why DAGCombiner is sometimes confused. For example (from a slightly different test case than Micah posted), here's a block of post-optimized IR for a dbg.value() call that the DAGCombiner ends up deleting...

  %8 = extractelement <4 x i32> %7, i32 0         ; <i32> [#uses=1]
  br label %9

; <label>:9                                       ; preds = %get_local_id.exit
  %10 = phi i32 [ %8, %get_local_id.exit ]        ; <i32> [#uses=1]
  br label %11

; <label>:11                                      ; preds = %9
  %12 = phi i32 [ %10, %9 ]                       ; <i32> [#uses=1]
  br label %get_global_id.exit

get_global_id.exit:                               ; preds = %11
  %13 = phi i32 [ %12, %11 ]                      ; <i32> [#uses=4]
  call void @llvm.dbg.value(metadata !{i32 %13}, i64 0, metadata !27), !dbg !28

    However, a similar sequence of IR 'works', IF that sequence's analog to %13 is used in the SAME basic block as the assignment to %13. It's only if %13 is not used in the same basic block that the dbg.value gets removed. Or at least that's the feeling we have at this point.


From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of John Criswell
Sent: Thursday, April 14, 2011 7:07 PM
To: Villmow, Micah
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Valid debug information being deleted by DAGCombiner

On 4/14/11 8:22 PM, Villmow, Micah wrote:
Found another bitcode file where a debug symbol is being dropped.

In the attached bitcode file, the variable gid is not in the debug output.

Dumb question: Have you looked to see if mem2reg is destroying (or not maintaining) the debug information of interest (or put another way, was the variable gid promoted to an LLVM register and therefore had its debug information destroyed)?

I haven't used LLVM's new debug facilities, so my knowledge is out of date, but I think optimizations are still permitted to remove debug info.

-- John T.

From: llvmdev-bounces at cs.uiuc.edu<mailto:llvmdev-bounces at cs.uiuc.edu> [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Villmow, Micah
Sent: Thursday, April 14, 2011 5:31 PM
To: llvmdev at cs.uiuc.edu<mailto:llvmdev at cs.uiuc.edu>
Subject: [LLVMdev] Valid debug information being deleted by DAGCombiner

I am working on some debug problems that we are seeing with the x86 backend and OpenCL.

The input is linked.bc. There are three debug values, ip, tid and gid.

llc -march=x86 linked.bc -o linked-x86.s <-- assembly file has all three values in the debug info section

However, if I attempt to optimize the bitcode with the following command:
opt -disable-opt -inline-all -mem2reg linked.bc -o optimized.bc
llc -march=x86 optimized.bc -o optimized-x86.s <-- Assembly file only has ip and gid in the debug info section.

Any idea on how to get this to work? I've attached the output, which is missing the 'tid' debug variable.



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