[Bug c/97329] New: POWER9 default cache and line sizes appear to be wrong

[kip at thevertigo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97329

            Bug ID: 97329
           Summary: POWER9 default cache and line sizes appear to be wrong
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kip at thevertigo dot com
  Target Milestone: ---

While investigating the memory hierarchy on my Romulus POWER9 (CPU revision
2.2) I discovered GCC's default L1 cache and line sizes on POWER9 are not
correct. 

I think whoever specified the default cache size of 64KB may not have realized
the L1 cache is banked, and not unified. On POWER9 that 64KB is split between
separate instructions and data spaces. Only 32KB is actually available for
data.

GCC's documentation specifies that the l1-cache-size parameter is suppose to
refer to data only, but not instructions.

Further, the default l1-cache-line-size is wrong. It's currently set at 32
bytes. The correct value is actually four times that at 128 bytes.

As things are right now, the resulting generated code may not be properly
optimized because the optimizer plans around the wrong parameters.

When this happens the generated program may have a higher than necessary cache
miss rate. This could be a big deal since the CPU retrieving data from the L1
may only take one or two cycles, but a cache miss could mean several hundred
while the block is transferred.