Meinersbur added a comment.
In D104601#2831746 <https://reviews.llvm.org/D104601#2831746>, @dblaikie wrote:
> This is probably more @aaron.ballman 's wheelhouse, but for my money this
> seems pretty problematic - will make quoted text in compiler diagnostics
> weird/difficult to read, etc.
It is not meant for human readability but automatic processing. The linked
ccache patch either re-runs the compiler with on the original source file if
there is any diagnostic output (run_second_cpp=false) or(inclusive) uses the
preprocessor output only to determine whether there was a significant change in
the first place (run_second_cpp=true, default).
To discourage use by humans, we could make the flag available only behind
`-Xclang`, but I'd expect people to only use the flag if they have a need for
it anyway. Compiler diagnostics invoked on `-E -P` output is already not very
useful; `-fnormalize-whitespace` would potentially cause the entire source be
on a single line. If this is a show-stopper, I could try making the
diagnostic's quoted text print only an excerpt of a line if it becomes too long.
> Do you have a particular use case that has exceptionally frequent
> whitespace-only changes?
Every edit of a (multi-line) comment and running of clang-format. The former
makes me hesitant to improve comments in central header files because I know it
means that I have to rebuild the entire code base; it should not be a reason to
not improve documentation. The latter is best-practice before any commit.
Again, if it includes a central header file, it means rebuilding significant
portions.
ccache uses preprocessor mode by default while it also supports strict binary
comparison ("direct mode") and I assume there is a reason for this. However,
every non-intra-line change causes the preprocessor output to change due to the
line number in the #line directive changing, making the preprocessor mode much
less effective than it could be. Very few NF changes take place within a single
line only.
Unlike gcc, clang's `-E -P` output still includes empty lines (unless there are
more than 8 consecutive empty lines). That is, there is hardly a benefit to use
`-P` output in ccache with clang.
> Or is this something you think would be generally applicable? How applicable,
> do you think? Do you have some data showing a % of rebuilds that would be
> avoided in practice/based on real code bases, etc?
If it supports my case, I can work with builds using ccache for a while: one
configured using -fnormalize-whitespace, the other without, and compare ccache
statistics. I have used it already, but never comparatively over the same time
frame. However, I think the above argument is already pretty good.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D104601/new/
https://reviews.llvm.org/D104601
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