[llvm-dev] My own codegen is 2.5x slower than llc?

[David Jones via llvm-dev]

My back-end code generator uses LLVM 5.0.1 to optimize and generate code
for x86_64.

If I run it on a given sample of IR, it takes almost 5 minutes to generate
object code.  95%+ of this time is spent in MergeConsecutiveStores().  (One
function has a basic block with 14000 instructions, which is a pathological
case for MergeConsecutiveStores.)

If, instead, I dump out the LLVM IR, and manually run both opt and llc on
it with -O2, the whole affair takes only 2 minutes.

I am using a dynamically linked LLVM library.  I have verified using GDB
that both my code generator and llc are invoking the shared library (i.e.
the exact same code) so I would not expect to see a 2.5x performance
difference.

What could explain this?
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