D79276 caused the following builder to fail:
Specifically, FileCheck dumped stack in the following tests:
LLVM :: MC/Mips/micromips-jump-pc-region.s LLVM :: MC/Mips/mips-jump-pc-region.s
Those tests contained characters encoded as 160 but that render (at
least for me in vim) like a single space (32). Those characters
appeared between the # and RUN: on several lines, and D79276
caused FileCheck to process those lines differently: RUN: is a
comment directive. As a result, D79276 caused FileCheck to start
calling is isalnum on those characters.
The problem is that FileCheck calls isalnum on type char without
casting to unsigned char first, so it sign-extends 160 beyond what
unsigned char or EOF can represent. C says that has undefined
behavior. This problem is general to FileCheck's prefix parsing and
so exists independently of D79276.