[all-commits] [llvm/llvm-project] fa9cef: Only guard loop metadata that has non-debug info i...

[Dominik Steenken via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: fa9cef50b1afc203b6b653396f9e775862c26e68
      https://github.com/llvm/llvm-project/commit/fa9cef50b1afc203b6b653396f9e775862c26e68
  Author: Dominik Steenken <dost at de.ibm.com>
  Date:   2024-12-20 (Fri, 20 Dec 2024)

  Changed paths:
    M llvm/include/llvm/IR/Instruction.h
    M llvm/lib/IR/Instruction.cpp
    M llvm/lib/Transforms/Utils/Local.cpp
    M llvm/test/Transforms/SimplifyCFG/preserve-llvm-loop-metadata.ll

  Log Message:
  -----------
  Only guard loop metadata that has non-debug info in it (#118825)

This PR is motivated by a mismatch we discovered between compilation
results with vs. without `-g3`. We noticed this when compiling SPEC2017
testcases. The specific instance we saw is fixed in this PR by modifying
a guard (see below), but it is likely similar instances exist elsewhere
in the codebase.

The specific case fixed in this PR manifests itself in the `SimplifyCFG`
pass doing different things depending on whether DebugInfo is generated
or not. At the end of this comment, there is reduced example code that
shows the behavior in question.

The differing behavior has two root causes:
1. Commit https://github.com/llvm/llvm-project/commit/c07e19b adds loop
metadata including debug locations to loops that otherwise would not
have loop metadata
2. Commit https://github.com/llvm/llvm-project/commit/ac28efa6c100 adds
a guard to a simplification action in `SImplifyCFG` that prevents it
from simplifying away loop metadata

So, the change in 2. does not consider that when compiling with debug
symbols, loops that otherwise would not have metadata that needs
preserving, now have debug locations in their loop metadata. Thus, with
`-g3`, `SimplifyCFG` behaves differently than without it.

The larger issue is that while debug info is not supposed to influence
the final compilation result, commits like 1. blur the line between what
is and is not debug info, and not all optimization passes account for
this.

This PR does not address that and rather just modifies this particular
guard in order to restore equivalent behavior between debug and
non-debug builds in this one instance.

---

Here is a reduced version of a file from `f526.blender_r` that showcases
the behavior in question:
```C
struct LinkNode;
typedef struct LinkNode {
 struct LinkNode *next;
 void *link;
} LinkNode;

void do_projectpaint_thread_ph_v_state() {
  int *ps = do_projectpaint_thread_ph_v_state;
  LinkNode *node;
  while (do_projectpaint_thread_ph_v_state)
    for (node = ps; node; node = node->next)
      ;
}
```
Compiling this with and without DebugInfo, and then disassembling the
results, leads to different outcomes (tested on SystemZ and X86). The
reason for this is that the `SimplifyCFG` pass does different things in
either case.



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