[Bug libgcc/90929] New: libgcc MIPS __clear_cache shouldn't be a no-op

[coypu at sdf dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90929

            Bug ID: 90929
           Summary: libgcc MIPS __clear_cache shouldn't be a no-op
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libgcc
          Assignee: unassigned at gcc dot gnu.org
          Reporter: coypu at sdf dot org
  Target Milestone: ---

MIPS needs cache synchronization.

libgcc's __clear_cache expands to:

000119b0 <__clear_cache>:
   119b0:       03e00008        jr      ra
   119b4:       00000000        nop
        ...

(a no-op)

Running compiler-rt's test[1] on real hardware running netbsd/mips64 (n32)
without optimizations fails.

If -O2 is used it succeeds, presumably because the call is inlined.

# gcc clear_cache_test.c -o clear_cache_test; nm clear_cache_test |grep
clear_cache; ./clear_cache_test; echo $?
         U __clear_cache@@GCC_3.0
1
# gcc -O2 clear_cache_test.c -o clear_cache_test; nm clear_cache_test |grep
clear_cache; ./clear_cache_test; echo $?
0


To confirm this isn't caused by local changes or build oddities, I built with
trunk for "mips64-linux-gnuabi64" on linux, and it still shows the no-op
function.

The correct behaviour is to emit a call to libc (it's a privileged instruction
on older MIPS, so the kernel must do it).

[1]
https://github.com/llvm-mirror/compiler-rt/blob/master/test/builtins/Unit/clear_cache_test.c