[all-commits] [llvm/llvm-project] fc93ba: [GlobalISel][DebugInfo] Remove debug info with zer...

[David Blaikie via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: fc93ba061aa05a428057e7a7753128f27af0dd41
      https://github.com/llvm/llvm-project/commit/fc93ba061aa05a428057e7a7753128f27af0dd41
  Author: Vladislav Dzhidzhoev <vdzhidzhoev at accesssoftek.com>
  Date:   2022-07-25 (Mon, 25 Jul 2022)

  Changed paths:
    M llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
    M llvm/test/CodeGen/AArch64/GlobalISel/combine-shift-of-shifted-dbg-value-fallback.ll
    M llvm/test/CodeGen/AArch64/GlobalISel/constant-dbg-loc.ll

  Log Message:
  -----------
  [GlobalISel][DebugInfo] Remove debug info with zero line from constants inserted at entry block

Emission of constants having DebugLoc with line 0 causes significant increase of debug_line section size for some source files.

To illustrate, we can compare section sizes of several files from llvm test-suite, built with SelectionDAG vs GlobalISel, on Aarch64 (macOS), using -O0 optimization level:

| Source path                                                    | SDAG text sz | GISel text sz | SDAG debug_line sz |  GISel debug_line sz
| -------------------------------------------------------------- | ------------ | ------------- | ------------------ | --------------------
| `SingleSource/Regression/C/gcc-c-torture/execute/strlen-2.c`   | 15320        | 660           | 14872              | 6340
| `SingleSource/Regression/C/gcc-c-torture/execute/20040629-1.c` | 33640        | 26300         | 2812               | 6693
| `SingleSource/Benchmarks/Misc/flops-4.c`                       | 1428         | 1196          | 594                | 1008
| `MultiSource/Benchmarks/MiBench/consumer-typeset/z31.c`        | 2716         | 964           | 809                | 903
| `MultiSource/Benchmarks/Prolangs-C/gnugo/showinst.c`           | 2534         | 2502          | 189                | 573

For instance, here is a fragment of `flops-4.c.o` debug line section dump

```
Address            Line   Column File   ISA Discriminator Flags
------------------ ------ ------ ------ --- ------------- -------------
0x0000000000000000    174      0      1   0             0  is_stmt
0x0000000000000010      0      0      1   0             0
0x0000000000000018    185      4      1   0             0  is_stmt prologue_end
0x000000000000001c      0      0      1   0             0
0x0000000000000024    186      4      1   0             0  is_stmt
0x000000000000002c    189     10      1   0             0  is_stmt
0x0000000000000030      0      0      1   0             0
0x0000000000000038    207     11      1   0             0  is_stmt
0x0000000000000044    208     11      1   0             0  is_stmt
0x0000000000000048      0      0      1   0             0
0x0000000000000058    210     10      1   0             0  is_stmt
0x000000000000005c      0      0      1   0             0
0x0000000000000060    211     10      1   0             0  is_stmt
0x0000000000000064      0      0      1   0             0
0x000000000000006c    212     10      1   0             0  is_stmt
0x0000000000000070      0      0      1   0             0
0x000000000000007c    213     10      1   0             0  is_stmt
0x0000000000000080      0      0      1   0             0
0x0000000000000088    214     10      1   0             0  is_stmt
0x000000000000008c      0      0      1   0             0
0x0000000000000094    215     10      1   0             0  is_stmt
```

Lot of zero lines are produced by constants (global values) having DebugLoc with line 0.
It seems that they're not significant for debugging experience.

With the commit applied, total size of debug_line sections of llvm shared libraries has reduced by 2.5%.
Change of debug line section size of files listed above:

| Source path                                                    | GISel debug_line sz | Patch debug_line sz
| -------------------------------------------------------------- | ------------------- | --------------------
| `SingleSource/Regression/C/gcc-c-torture/execute/strlen-2.c`   | 6340                | 1465
| `SingleSource/Regression/C/gcc-c-torture/execute/20040629-1.c` | 6693                | 3782
| `SingleSource/Benchmarks/Misc/flops-4.c`                       | 1008                | 609
| `MultiSource/Benchmarks/MiBench/consumer-typeset/z31.c`        | 903                 | 841
| `MultiSource/Benchmarks/Prolangs-C/gnugo/showinst.c`           | 573                 | 190

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D127488