From the S76 func_80011380 agent, which upgraded an empirical closeness-6
plateau to a floor proved from the gcc sources in tools/reference/.
The target needs MULT(MULT(i,2),2) unmerged, but fold-const.c:882 split_tree
decomposes any MULT whose op1 is TREE_CONSTANT — all 20 spellings measured
collapse to one sll 2, and STRIP_NOPS eats NON_LVALUE_EXPR so the usual |0
+0 *1 &~0 ^0 >>0 shields cannot protect it.
Both escapes cost an instruction, each for a named reason: a stmt-expr gives
the exact 5-insn RTL but its BLOCK_END note breaks the adjacency that
stupid.c:497-508 needs for a copy to conflict with its source, so the copy
self-coalesces and final.c deletes it; and (t = i*2)*2 with register s32 t
reaches exact length and shape but expand_decl's zero-byte (use) brackets
make t the longest interval, seizing $v0 and rotating the register ring.
Clinching fact that the target has no variable there: its 4th insn
sll $v1,$a0,1 reads $a0, not insn 2's dest.
Bonus: expand_binop allocates the PLUS dest before force_reg'ing the symbol,
so the symbol pseudo loses stupid_reg_compare's tie-break — that is the
la-on-$a0 colour.
Recorded as the TEMPLATE for a wall claim: name the pass, cite file and line,
measure each escape, and give the byte fact ruling out the alternative. A
wall asserted without that is a belief (§473).
Drew T
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