[clang] [clang][Diagnostics] Highlight code snippets (PR #66514)

[Timm Baeder via cfe-commits]

Timm =?utf-8?q?Bäder?= <tbaeder at redhat.com>,
Timm =?utf-8?q?Bäder?= <tbaeder at redhat.com>,
Timm =?utf-8?q?Bäder?= <tbaeder at redhat.com>,
Timm =?utf-8?q?Bäder?= <tbaeder at redhat.com>,
Timm =?utf-8?q?Bäder?= <tbaeder at redhat.com>
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In-Reply-To: <llvm/llvm-project/pull/66514/clang at github.com>


tbaederr wrote:

> > > I have concerns about Lexing twice every single file for which we display a warning.
> > 
> > 
> > Sure. Because of performance, memory usage or code complexity?
> 
> Performance, mostly

In a debug build with sanitizers enabled, I get this when compiling `SemaExpr.cpp` from clang with a `static_assert(false)` added at the end:

```
/home/tbaeder/code/llvm-project/clang/lib/Sema/SemaExpr.cpp:21870:15: error: static assertion failed
Lexed 21870 lines and 174105 Tokens
That took 660402 microseconds
That took 660 milliseconds
That took 0 seconds
```
That doesn't seem too bad to me? I can try again with an optimized build if you want.

> > So, my general procedure for diagnostics is that they shouldn't complicate the non-diagnostic code. And when we emit a diagnostic, performance is basically out the window anyway (printing to stdout is _slow_ and the way we emit code snippets is also slow, etc.)
> 
> That is generally true, but how far do want to push that logic? Lexing is going to be fairly expensive on most source files. I'd expect it to be much more (cpu) expensive than a call to print.

Of course it's more expensive, I was just trying to make a point :) I expect this to be only visible in situations where very few errors are ever shown, i.e. when an actual human compiles a project. Build servers redirect the output to a file anyway, which disables any sort of colored output.



https://github.com/llvm/llvm-project/pull/66514