One element, loaded and stored, plus a scalar global
Take an indexed read, glue it to an indexed write, and drop a small-data scalar in the middle. That's all tbl[i] = tbl[i] + g is. The base and the scaled index are built once. Then lwzx loads the element, the scalar global comes in on an ordinary @sda21 load, the two get added, and stwx writes the sum back through the same base and index.
The giveaway is that lwzx and stwx share both address operands. The compiler computes &tbl (@ha/@l) and i * 4 (slwi) up front and keeps them in registers, so the load and store hit the same element. The scalar shows up separately as a lone lwz ...@sda21 — a different global, reached the small-data way rather than through the array's address pair.
Here's advanceCell(k). It adds the int global gStep to element k of the int array gCells, in place:
lis r4, gCells@ha # high half of &gCells
slwi r5, r3, 2 # k * 4 (held in r5 for both accesses)
addi r4, r4, gCells@l # r4 = &gCells (held for both accesses)
lwz r3, gStep@sda21(r13) # read the scalar global gStep
lwzx r0, r4, r5 # r0 = gCells[k]
add r0, r3, r0 # gCells[k] + gStep
stwx r0, r4, r5 # gCells[k] = sum (same base r4, same index r5)
blr
r4 (base) and r5 (index) are written once, then both lwzx and stwx read them back; the gStep scalar arrives through its own @sda21 load. Your target does the same in-place update on a different array with a different scalar. Match the reloc names, and check that the load and store hit the same element.
Your task
The globals are already declared: gGrid (int[]) and gBonus (int). Write func_803e93b4 to reproduce the read-modify-write above.