[compiler-rt] [tsan] Shrink RiscV64 48-bit LowApp region slightly to speed up TSan RestoreAddr (PR #72316)

[Alex Fan via llvm-commits]

alexfanqi wrote:

Tested on qemu system emulation. All tests passed, except again mmap_lots.cpp like before, which is not relevant.

https://github.com/llvm/llvm-project/pull/72316