[PATCH] D67410: [DWARF] Emit call site parameter info when tuning for lldb

Wed Sep 11 13:35:26 PDT 2019

vsk added a comment.

In D67410#1666505 <https://reviews.llvm.org/D67410#1666505>, @dblaikie wrote:

> In D67410#1666370 <https://reviews.llvm.org/D67410#1666370>, @aprantl wrote:
>
> > I'm very curious what the effect of this patch on our green dragon statistics bot will be (http://green.lab.llvm.org/green/view/LLDB/job/clang-3.4-debuginfo-statistics/).
>
>
> What sort of statistics do we track that would be impacted by this change?

None for now, because debug entry values are gated behind a CC1 flag the bot does not use. I'm not sure what the plan is here exactly, but what I'd like to do is turn this on by default for DWARF5, and leave the CC1 flag for anyone who wants call site information in DWARF4 mode.

> (to me it doesn't seem like this feature would improve coverage, etc - partly mechanically (this adds new tags we might not have any stats for)

(assuming we're compiling for DWARF5 or are using the CC1 flag) -- Variable location coverage should go up, as llvm should be able to describe arguments that are never mutated. @djtodoro has also added some additional stats related to call site info in D66525 <https://reviews.llvm.org/D66525>.

> and partly practically (knowing the value of the parameter in the caller doesn't necessarily mean you know the value of the parameter at various points in the callee - parameters are usually mutable, so the callee might've modified the parameter & the callee's value won't reflect that))

Yes, good point, parameters are generally mutable but clang doesn't request call site parameter info for mutable arguments right now. Doing so would require some extra debug info support (a bit that says "hey, not sure this value is up-to-date!"), and this just hasn't been implemented yet.

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https://reviews.llvm.org/D67410