[cfe-dev] Diagnostic source location accuracy with and without remarks enabled

[Paul Kirth via cfe-dev]

I'm trying to get some insight into why source locations in a warning
resolve differently when remarks are enabled in a cc1 invocation.

In a diagnostic handler I have two diagnostics being emitted(one warning
and one remark), but the accuracy of the source location changes
dramatically if I request remarks to be emitted through '-R<remark>'. Note
that I don't mean the specific remark I'm emitting, I mean *any* remark.

More concretely, when I enable the warning and request no remarks, I get a
source location that is the beginning of the function. However, if I enable
any remarks(such as for the inliner pass), the source location becomes
fully accurate for the warning(i.e. pointing to a branch location in the
function body).

The invocations only differ in a small way:

clang -cc1 source.ll -Wmisexpect <... rest of invocation>

vs.

clang -cc1 source.ll -Wmisexpect -R<anything> <... rest of invocation>

I ran into this when working on some tests in
https://reviews.llvm.org/D66324, and I'm having trouble understanding why
this is happening. The same source location is passed to both diagnostics.
This behavior also doesn't seem to appear if I don't use cc1 directly, but
I would still like to understand this behavior.

Can someone explain why enabling remarks here has an effect on the accuracy
of the source locations? Do they cause a delay in the source location
resolution, or is there an important piece of infrastructure that the
remarks framework enables that I've missed?

For reference my backend diagnostic handler looks something like the
following snippet where the source location is passed to both the warning
and remark:

```
void BackendConsumer::MisExpectDiagHandler(
    const llvm::DiagnosticInfoMisExpect &D) {
  StringRef Filename;
  unsigned Line, Column;
  bool BadDebugInfo = false;
  FullSourceLoc Loc =
      getBestLocationFromDebugLoc(D, BadDebugInfo, Filename, Line, Column);

  Diags.Report(Loc, diag::warn_misexpect) << D.getMsg().str();
  Diags.Report(Loc, diag::remark_misexpect) << D.getMsg().str();

  // ...
}
```


-- 
Paul Kirth
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