Re: [PATCH 5/9][v5] powerpc: implement is_instr_load_store().

Sukadev Bhattiprolu
2013-10-03T19:03:54Z

Michael Ellerman [michael@ellerman.id.au] wrote:
| On Tue, Oct 01, 2013 at 05:15:06PM -0700, Sukadev Bhattiprolu wrote:
| > Implement is_instr_load_store() to detect whether a given instruction
| > is one of the fixed-point or floating-point load/store instructions.
| > This function will be used in a follow-on patch to save memory hierarchy
| > information of the load/store.
|
| The search over the array is a bit of a pity, especially as the worst
| case penalises you when you haven't hit a load/store.

Agree. Will try this out. This is certainly more efficient.

|
| I think we can do better. If you look at the opcode maps, and in
| particular the extended table for opcode 31, you'll see there's a
| reasonable amount of structure.
|
| The following is only valid for arch 2.06, ie. it will classify reserved
| opcodes as being load/store, but I think that's fine for the moment. If
| we need to use it somewhere in future we can update it. But we should
| add a big comment saying it's only valid in that case.
|
| Anyway, I think the following logic is all we need for opcode 31:
|
| bool is_load_store(int ext_opcode)

how about I call this is_load_store_2_06() and add a comment. Horrible
but minimizes chance of misuse.

| {
| upper = ext_opcode >> 5;
| lower = ext_opcode & 0x1f;
|
| /* Short circuit as many misses as we can */
| if (lower < 3 || lower > 23)
| return false;
|
| if (lower == 3)
| if (upper >= 16)
| return true;
|
| return false;
|
| if (lower == 6)
| if (upper <= 1)
| return true;
| return false;
|
| if (lower == 7 || lower == 12)
| return true;
|
| if (lower >= 20) /* && lower <= 23 (implicit) */
| return true;
|
| return false;
| }
|
|
| Which is not pretty, but I think it's preferable to the full search over the
| array.
|
| cheers