tree 6e1b70c54e6b76c81f979e002e388310cdb23d2b
parent 30458d6f35d365b675fe9549473f12b5d7a7abb2
author YunQiang Su <wzssyqa@gmail.com> 1667871821 -0800
committer Copybara-Service <copybara-worker@google.com> 1667871932 -0800

MIPS/compiler_rt: use synci to flush icache on r6

syscall makes it failed to build on mips64 for mipsel:
```
compiler-rt/lib/builtins/clear_cache.c:97:3: error:
call to undeclared function 'syscall'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
  syscall(__NR_cacheflush, start, (end_int - start_int), BCACHE);
```

In this patch, we use `rdhwr` to get synci_step.
If synci_step is zero, it means that the hardware will maintain the coherence. We need to do nothing.
Then for r6+, `synci` is required to keep icache global.
So we can use `synci` to flush icache.
The ISA documents ask a `sync` and a `jr.hb` after `synci`.

For pre-r6, we can use cacheflush libc function, which is same on Linux and FreeBSD.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D135565

GitOrigin-RevId: 674a17e9bbe82e8c53952fd94dcd862b17cb2d2f