[PATCH] D62650: [DebugInfo] DBG_VALUE instruction generated for loop counter is placed incorrectly.

Thu May 30 03:07:32 PDT 2019

avl created this revision.
avl added reviewers: rob.lougher, aprantl, dblaikie.
Herald added subscribers: javed.absar, kosarev, hiraditya, kristof.beyls, qcolombet, MatzeB.
Herald added a project: LLVM.

512B    bb.2.for.body:
520B      MOVaddr 
524B      MOVi32imm

  DBG_VALUE $x19, $noreg, !"i", <<<<<<<<<<<<<<<

Above DBG_VALUE should be placed before MOV* instructions. Test case presented after
the problem description.

DBG_VALUE instruction is placed incorrectly because wrong debug variable range
is calculated at "COMPUTING LIVE DEBUG VARIABLES" stage. Range for debug variable 
instruction (for "i" variable) is computed according to the current state of instructions
inside of loop basic block. But later - Register Allocator creates new instructions 
which are not taken into account when Live Debug Variables are computed. Please check 
real data(full log in the end): after COMPUTING LIVE DEBUG VARIABLES there is following
instruction range for "i" variable - [544B;848r). It is shorter than Live Interval for
corresponding register [512B,848r:1). Register Allocator inserts two instructions 
before 544B. As the result Debug Value record is inserted after these additional 
instructions. That is incorrect. Debug Value for the loop counter should be inserted 
before any loop instruction.

related reviews: D35953 <https://reviews.llvm.org/D35953>, https://bugs.llvm.org/show_bug.cgi?id=33730, D46599 <https://reviews.llvm.org/D46599>, https://bugs.llvm.org/show_bug.cgi?id=37149.

Debug variable range become incorrect when it was trimmed 
at lib/CodeGen/LiveDebugVariables.cpp:

if (I.start() < RStart) {

  // Interval start overlaps range - trim to the scope range.
  I.setStartUnchecked(RStart);
  // Remember that this interval was trimmed.
  trimmedDefs.insert(RStart);

}

The fix is to not trim Debug Variable Range. 
It does not brake live-debug-variables.ll(no new .debug_loc entries).
It does not affect compilation time for test case from https://bugs.llvm.org/show_bug.cgi?id=33730.

cat test_debug_val.cpp:

void func(int, ...);

int array[0x100];

int main( int argc, char **argv )    
{

  int var = 56;

  int a1 = array[1]; int a2 = array[2]; int a3 = array[3]; int a4 = array[4];
  int a5 = array[5]; int a6 = array[6]; int a7 = array[7]; int a8 = array[8];
  int a9 = array[9]; int a10 = array[10];

  for( int i = 0; i < 0x100; i++ ) {    

      array[i] = var;

      func(0, i, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10 );
  }

  return 0;

}

clang -O3 -g -c --target=aarch64-unknown-linux -std=gnu++14 test_debug_val.cpp -S -mllvm -print-after-all -mllvm -debug

test_debug_val.s:
.Ltmp7:

  //DEBUG_VALUE: main:a2 <- $w21

.LBB0_1:                                // %for.body

                                  // =>This Inner Loop Header: Depth=1
  ldr     w2, [sp, #44]           // 4-byte Folded Reload

.Ltmp8:

  .loc    1 16 17 is_stmt 1       // test_debug_val.cpp:16:17
  adrp    x8, array
  add     x8, x8, :lo12:array
  mov     w9, #56

.Ltmp9:

  //DEBUG_VALUE: i <- $x19

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

DEBUG_VALUE is reported not in the very beginning of Loop. 
It is reported after several instructions.

print-after-all data:

- COMPUTING LIVE DEBUG VARIABLES: main **********
- DEBUG VARIABLES **********

!"i,14"  [432B;448B):0 [544B;848r):1 Loc0=0 Loc1=%35

1. *** IR Dump After Debug Variable Analysis ***:
2. Machine code for function main: NoPHIs, TracksLiveness

512B    bb.2.for.body:

  ; predecessors: %bb.0, %bb.2
    successors: %bb.1(0x04000000), %bb.2(0x7c000000); %bb.1(3.12%), %bb.2(96.88%)

544B      STRWroX %28:gpr32, %27:gpr64common, %35:gpr64common, 0, 1, debug-location !54 :: (store 4 into %ir.scevgep, !tbaa !38); test_debug_val.cpp:16:17

- INTERVALS **********

%35 [8r,448B:0)[512B,848r:1)[848r,944B:2)  0 at 8r 1 at 512B-phi 2 at 848r weight:2.289400e-01

1. *** IR Dump After Greedy Register Allocator ***:
2. Machine code for function main: NoPHIs, TracksLiveness

512B    bb.2.for.body:

  ; predecessors: %bb.0, %bb.2
    successors: %bb.1(0x04000000), %bb.2(0x7c000000); %bb.1(3.12%), %bb.2(96.88%)

520B      %37:gpr64common = MOVaddr target-flags(aarch64-page) @array, target-flags(aarch64-pageoff, aarch64-nc) @array, debug-location !54; test_debug_val.cpp:16:17
524B      %39:gpr32 = MOVi32imm 56, debug-location !54; test_debug_val.cpp:16:17
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
544B      STRWroX %39:gpr32, %37:gpr64common, %35:gpr64common, 0, 1, debug-location !54 :: (store 4 into %ir.scevgep, !tbaa !38); test_debug_val.cpp:16:17

1. *** IR Dump After Virtual Register Rewriter ***:
2. Machine code for function main: NoPHIs, TracksLiveness, NoVRegs

512B    bb.2.for.body:

  ; predecessors: %bb.0, %bb.2
    successors: %bb.1(0x04000000), %bb.2(0x7c000000); %bb.1(3.12%), %bb.2(96.88%)
    liveins: $w20, $w21, $w22, $w23, $w24, $w25, $w26, $w27, $w28, $x19

520B      renamable $x8 = MOVaddr target-flags(aarch64-page) @array, target-flags(aarch64-pageoff, aarch64-nc) @array, debug-location !54; test_debug_val.cpp:16:17
524B      renamable $w9 = MOVi32imm 56, debug-location !54; test_debug_val.cpp:16:17
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  DBG_VALUE $x19, $noreg, !"i", !DIExpression(), debug-location !51; test_debug_val.cpp:0 line no:14

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
544B      STRWroX killed renamable $w9, killed renamable $x8, renamable $x19, 0, 1, debug-location !54 :: (store 4 into %ir.scevgep, !tbaa !38); test_debug_val.cpp:16:17

https://reviews.llvm.org/D62650

Files:
  llvm/lib/CodeGen/LiveDebugVariables.cpp
  llvm/test/CodeGen/AArch64/wrong_debug_loc_after_regalloc.ll
  llvm/test/DebugInfo/X86/dbg-addr-dse.ll
  llvm/test/DebugInfo/X86/live-debug-variables.ll

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