[PATCH] D46129: [SelectionDAG] Improve selection of DBG_VALUE using a PHI node result

[Bjorn Pettersson via Phabricator via llvm-commits]

bjope added inline comments.


================
Comment at: test/DebugInfo/COFF/pieces.ll:46
+; ASM: [[oy_ox_start:\.Ltmp[0-9]+]]:
 ; ASM:        #DEBUG_VALUE: loop_csr:o <- [DW_OP_LLVM_fragment 0 32] $edi
+; ASM:        #DEBUG_VALUE: loop_csr:o <- [DW_OP_LLVM_fragment 32 32] $esi
----------------
aprantl wrote:
> I'm having a hard time interpreting the assembler comment. Does this mean that the range of the DBG_VALUE get's immediately terminated by the assignment to edi, or does that mean "starting with the assignment"?
The original loop in C looks like this (at least according to the comment in the test case, I've not regenerated it from C since it is an old test case):
```
;   for (i = 0; i < n; i++) {
;     o.x = g(o.x);
;     o.y = g(o.y);
;   }
;   return o.x + o.y;
```

Without the patch we get this kind of structure in the loop body:

```
ox_start:
  #DEBUG_VALUE: loop_csr:o <- [DW_OP_LLVM_fragment 0 32] $edi
  o.x = g(o.x);
oy_start:
  #DEBUG_VALUE: loop_csr:o <- [DW_OP_LLVM_fragment 0 32] $edi
  #DEBUG_VALUE: loop_csr:o <- [DW_OP_LLVM_fragment 32 32] $esi
  o.y = g(o.y);
  #DEBUG_VALUE: loop_csr:o <- [DW_OP_LLVM_fragment 32 32] $esi
oy_end:
```

after the patch we get

```
ox_oy_start:
  #DEBUG_VALUE: loop_csr:o <- [DW_OP_LLVM_fragment 0 32] $edi
  #DEBUG_VALUE: loop_csr:o <- [DW_OP_LLVM_fragment 32 32] $esi
  o.x = g(o.x);
ox_start:
  #DEBUG_VALUE: loop_csr:o <- [DW_OP_LLVM_fragment 0 32] $edi
  o.y = g(o.y);
oy_start:
  #DEBUG_VALUE: loop_csr:o <- [DW_OP_LLVM_fragment 32 32] $esi
oy_end:
```
So the difference is basically that we now track "o.y" also for the first part of the loop body. That should be an improvement!

Perhaps I should rename ox_start as ox_restart, and oy_start as oy_restart, to indicate that those labels indicate where a new live range starts for the DEBUG_VALUEs (after the updates of o.x and o.y)? Or we could just call them LBL1, LBL2, LBL3 and LBL4.




Repository:
  rL LLVM

https://reviews.llvm.org/D46129