[llvm-dev] Reducing overhead of Verifier on complex data

[David Jones via llvm-dev]

I have an IR file that takes an unacceptably long time to optimize and
generate code for.

I am using LLVM 3.5.1 for now.

If I run

opt -time-passes -std-compile-opts -o opt.bc my.ll

opt takes about 3-4 minutes, only to report that it spent about 25 seconds
optimizing the code.

After running perf, it appears to me that opt is not accounting for a lot
of time spent in Verifier::visitGlobalVariable(). This routine appears to
traverse complex initializer expressions looking for bitcasts between
address spaces. Such bitcasts do not occur in my code - I use only the
default address space.

My IR has lots of complex interlinked data structures, e.g.:

@xxx.Std = constant [24 x i64] [i64 5665, i64 ptrtoint ([13 x i8]* @25342
to i64), i64 72, i64 ptrtoint ([2360 x i64]* @yyy.Std to i64), i64 ptrtoint
([4 x i64]* @25343 to i64), i64 ptrtoint ([11 x i8]* @25344 to i64), i64
24, i64 -1, i64 ptrtoint ([4 x i64]* @25345 to i64), i64 ptrtoint ([6 x
i8]* @25346 to i64), i64 48, i64 -1, i64 ptrtoint ([2 x i64]* @25348 to
i64), i64 ptrtoint ([10 x i8]* @25349 to i64), i64 0, i64 -1, i64 ptrtoint
([2 x i64]* @25351 to i64), i64 ptrtoint ([4 x i8]* @25352 to i64), i64 0,
i64 -1, i64 0, i64 0, i64 0, i64 0]

@yyy.Std likely contains a reference to @xxx.Std.

These data structures document the layout of types and variables in the
program in ways that runtime functions can easily use.

If I hand-edit my IR to remove all function definitions, replacing them
with equivalent declarations, but otherwise keep all type and data
definitions the same, then opt takes almost just as long, but happily
reports that it was able to optimize my code in 0.2 seconds. Not bad,
because there is no code. :-)

My question: what can I do about this?
- Does it make sense to disable the verifier in production?
- Can I recode my data declarations in some way to reduce the impact? I was
planning on changing the format of the data structures; the planned changes
would remove the ptrtoint instructions. Would doing so help?
- Would it help to reduce the amount of other constant data in the IR?
Although LLVM itself collapses duplicate constant data, I could easily do
so myself. Would doing so help?

(Note: the interlinked data structures do not contain duplicate data, so
any attempt at manually reducing the duplicate data would have zero impact
on the linked structures.)
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