Drew T
2.1 KiB
§256 — GOTOS IN THE TARGET'S BLOCK ORDER REPRODUCE SWITCH PLACEMENT WITHOUT SWITCH'S SIDE EFFECTS (P31 S58b)
Three cards found the same escape from opposite directions.
THE PROBLEM. For a small dispatch, switch and if/else-if are not the only two spellings,
and both can perturb things outside the dispatch. func_80181F78 (ov_SC06_000, 244 ins): switch
nested the bodies inside the compare region and rotated the registers of an unrelated upstream copy
loop; if/else-if emitted a bne staircase. The actual source shape is a flat two-target
dispatch written as explicit gotos in the target's own block order:
if (t == 0) goto body0;
if (t == 1) goto body1;
goto end;
body0: …; goto end;
body1: …;
end: ;
which emits beqz v1,.Lbody0 / beq v1,v0,.Lbody1 / j .Lend with both bodies after the tests and an
unconditional j over body0 — the target layout — while leaving the loop's allocation untouched.
THE DISCRIMINATOR vs a switch's inline median tree (func_8017FB98, ov_SC01_009): both carry
slti. What separates them is whether the arm bodies sit inline after their tests (switch's
inline-tree emission) or after the whole dispatch, reached by forward jumps with delay-slot-filled
compare constants (goto-dispatch). Inline-tree emission never produces the delay-slot-filled
compares.
AND THE THIRD CARD. func_801F23D0 (md_SC03_076): a goto-ladder with one shared join is required
for the tail shape — an if/else-if ladder with per-arm returns duplicated the lw/store in every
arm. Zero-byte __asm__("") barriers between the chain tests were what kept gcc from threading the
goto joins back into an else-if ladder; without them every variant collapsed to the 32-ins
duplicated-tail form regardless of goto-vs-return spelling. The t=0/t=1 flag then materialised
through the branch slots on its own.
BOUNDARY. Reach for gotos only when the dispatch has already been proven not to be a switch (no
jtbl, no median tree, or a switch measurably perturbs code outside it). §225's warning stands: gotos
landing mid-flow are for shapes no structured spelling reaches, not a first resort.