[llvm] [MC] Use StringTable to reduce dynamic relocations (PR #144202)

[Jay Foad via llvm-commits]

jayfoad wrote:

I can confirm this gives a nice speedup for `check-llvm` in my Release+Asserts build:

no PIE: 104 seconds
PIE: 171 seconds
PIE + this patch: 119 seconds

All times were repeatable within a second or two.

https://github.com/llvm/llvm-project/pull/144202