[PATCH] D64630: [DebugInfo] Address performance regression with r364515

[Jeremy Morse via Phabricator via llvm-commits]

jmorse added a comment.

Bjorn wrote inline:

> Not sure if the old code cared much about it either, but I always wonder if we forget to consider sub registers when I see code that only looks at getReg() but not getSubReg()

In this circumstance I think it's legitimate to ignore the subregisters -- we're not considering the precise location of a value with a reg/vreg, just whether the merging vregs are live or not. The riskiest circumstance would be a non-live DBG_VALUE-of-subreg being undef'd when a live value was merged into a different subregister within the same virtual register. This is conservative at the least; and I suspect that kind of merging would require an undef anyway, although I'm not overly familiar with subregisters.

Vedant wrote:

> [A proposal for a different way of doing this]

I think that'd work, trading some memory for some performance. The approach in this patch seems to be Good Enough (TM) though, I don't observe any performance differences on a clang-3.4 RelWithDebInfo build, and only some fractional increases on the (pathalogical) ASan build. My preference is shipping this, and making DBG_VALUE_PACKs happen sometime soon, as that'll eliminate similar problems elsewhere (PR43855 recently bit me, for example).


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