[OPT] Tail Loop Optimization

[briannwu]

details:

1. Separate tailLoopOpt for A / B: tailLoopOptA / tailLoopOptB.
2. Not supported: DTV, SparseGemm.
3. Reorder load instructions with more vgprs.

Compare:

globalReadMode = 3 -> use more vgpr to reorder GR, waitcnt, v_or_b32 instructions

Before:
/* g2l=0, load component 0 /
buffer_load_ubyte_d16 v[vgprG2LA+0+0], ..., 0 offen offset:0 // load one buffer value
/ g2l=0, load component 1 /
buffer_load_ubyte_d16 v0, ..., 0 offen offset:1 // load one buffer value
s_waitcnt vmcnt(0)
v_lshlrev_b32 v0, 0x8, v0 // shift left to higher 8 bits
v_or_b32 v[vgprG2LA+0+0], v[vgprG2LA+0+0], v0 // pack a sub 8-bit with dest
/ g2l=0, load component 0 /
buffer_load_ubyte_d16 v[vgprG2LA+0+4], ... offen offset:0 // load one buffer value
/ g2l=0, load component 1 */
buffer_load_ubyte_d16 v0, ... offen offset:1 // load one buffer value
s_waitcnt vmcnt(0)
v_lshlrev_b32 v0, 0x8, v0 // shift left to higher 8 bits
v_or_b32 v[vgprG2LA+0+4], v[vgprG2LA+0+4], v0 // pack a sub 8-bit with dest
...

After:
buffer_load_ubyte_d16 v[vgprG2LA+0+0], ... offen offset:0 // load one buffer value
buffer_load_ubyte_d16 v0, ..., 0 offen offset:1 // load one buffer value
buffer_load_ubyte_d16 v[vgprG2LA+0+4], ... offen offset:0 // load one buffer value
buffer_load_ubyte_d16 v1, ... offen offset:1 // load one buffer value
buffer_load_ubyte_d16 v[vgprG2LA+1+0], offen offset:0 // load one buffer value
...
s_waitcnt vmcnt(10)
v_lshlrev_b32 v0, 0x8, v0 // shift left to higher 8 bits
v_or_b32 v[vgprG2LA+0+0], v[vgprG2LA+0+0], v0 // pack a sub 8-bit with dest
s_waitcnt vmcnt(8)
v_lshlrev_b32 v1, 0x8, v1 // shift left to higher 8 bits
v_or_b32 v[vgprG2LA+0+4], v[vgprG2LA+0+4], v1 // pack a sub 8-bit with dest
...

//////////////////////////////////////////////////////////////////////////////////////////////////////////////////
globalReadMode = 2 -> use wider global load instructions
Before:
/* g2l=0, load component 0 /
buffer_load_ubyte_d16 v[vgprG2LB+0+0], ..., 0 offen offset:0 // load one buffer value
/ g2l=0, load component 1 /
buffer_load_ubyte_d16 v51, ..., 0 offen offset:1 // load one buffer value
/ g2l=0, load component 2 /
buffer_load_ubyte_d16_hi v52, ..., 0 offen offset:2 // load one buffer value
/ g2l=0, load component 3 */
buffer_load_ubyte_d16_hi v53, ..., 0 offen offset:3 // load one buffer value
...
s_waitcnt vmcnt(14)
v_lshlrev_b32 v51, 0x8, v51 // shift left to higher 8 bits
v_or_b32 v[vgprG2LB+0+0], v[vgprG2LB+0+0], v51 // pack a sub 8-bit with dest
s_waitcnt vmcnt(13)
v_or_b32 v[vgprG2LB+0+0], v[vgprG2LB+0+0], v52 // pack a sub 8-bit with dest
s_waitcnt vmcnt(12)
v_lshlrev_b32 v53, 0x8, v53 // shift left to higher 8 bits
v_or_b32 v[vgprG2LB+0+0], v[vgprG2LB+0+0], v53 // pack a sub 8-bit with dest
...

After:
buffer_load_dwordx4 v[vgprG2LB+0:vgprG2LB+0+3], v[vgprGlobalReadOffsetB+0], s[sgprSrdB:sgprSrdB+3], 0 offen offset:0 // G -> Reg 0_0_0_0
... (calculate some data to determine how to load the last data)
label_LoadB:
... (jump to specified load tile)
label_LOAD_B0:
label_LOAD_B0_K1:
s_cmp_ge_u32 s11, 1
s_cbranch_scc0 label_MergeB
/* g2l=0, load component 0 */
buffer_load_ubyte_d16 v54, ... 0 offen offset:0 // load one buffer value
label_LOAD_B0_K2
...
label_LOAD_B0_K15:
... (load code)
s_branch label_MergeB
label_MergeB:
... (jump to specified load tile)
label_MERGE_B0:
label_MERGE_B0_K1:
s_cmp_ge_u32 s11, 1
s_cbranch_scc0 label_CheckB_OOB
s_waitcnt vmcnt(0)
v_or_b32 v[vgprG2LB+0+0], v[vgprG2LB+0+0], v54 // pack a sub 8-bit with dest
label_MERGE_B0_K2:
...
label_MERGE_B0_K15:
... (pack code)
s_branch label_CheckB_OOB
label_CheckB_OOB:
...
label_CheckLoopBeginB:
... (calculate size to be loaded and size can be loaded)
label_B0:
... (check if there's other tile should be loaded again due to OOB)
s_cbranch_scc1 label_LoadB // Reload
s_branch label_CheckLoopBeginB // Re check
label_TailGlobalLoadEnd:
s_waitcnt vmcnt(0)

[hcman2]

Any brief before/after comparison of the tail loop asm code?

[briannwu]

[hcman2]

Good Opt. If you can share the performance gain for sensitive sizes, it will be much better.

[aazz44ss]

LGTM

[aazz44ss]

Can you put more comments about what are globalReadMode1st and globalReadMode2nd here.