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| Left: Code generated by ZILF 0.7 (11 instructions). Right: Code after new optimizations (1 instruction). |
How ZILF turns the nasty code on the left into a single instruction, using three new optimizations:
==1==
?L14: BAND STACK,2048 >STACK
BAND STACK,32767 >STACK
ZERO? STACK /?L12
PUSH 1
JUMP ?L13
?L12: PUSH 0
?L13: ZERO? STACK \?L6
PUSH 1
JUMP ?L7
?L6: PUSH 0
?L7: ZERO? STACK \FALSE
Combine sequential bitwise operations (this is new). BAND STACK,32767 goes away, because 2048 & 32767 == 2048.
==2==
?L14: BAND STACK,2048 >STACK
ZERO? STACK /?L12
PUSH 1
JUMP ?L13
?L12: PUSH 0
?L13: ZERO? STACK \?L6
PUSH 1
JUMP ?L7
?L6: PUSH 0
?L7: ZERO? STACK \FALSE
Convert single-bit BAND+ZERO? test to BTST (this is new). BAND STACK,2048 >STACK followed by ZERO? STACK /?L12 becomes BTST STACK,2048 \?L12, because 2048 is a power of two.
==3==
?L14: BTST STACK,2048 \?L12
PUSH 1
JUMP ?L13
?L12: PUSH 0
?L13: ZERO? STACK \?L6
PUSH 1
JUMP ?L7
?L6: PUSH 0
?L7: ZERO? STACK \FALSE
Bypass controlled conditional branch (this is new). PUSH 1 followed by a JUMP to ZERO? STACK \?L6 becomes JUMP ?L6, because we know the negative ZERO? branch will be taken.
==4==
?L14: BTST STACK,2048 \?L12
JUMP ?L6
JUMP ?L13
?L12: PUSH 0
?L13: ZERO? STACK \?L6
PUSH 1
JUMP ?L7
?L6: PUSH 0
?L7: ZERO? STACK \FALSE
Bypass controlled conditional branch. PUSH 0 followed by ZERO? STACK \?L6 becomes a JUMP to the new ?L15 (the instruction after ZERO?), because we know the negative branch won't be taken.
==5==
?L14: BTST STACK,2048 \?L12
JUMP ?L6
JUMP ?L13
?L12: JUMP ?L15
?L13: ZERO? STACK \?L6
?L15: PUSH 1
JUMP ?L7
?L6: PUSH 0
?L7: ZERO? STACK \FALSE
Bypass controlled conditional branch. PUSH 1 followed by a JUMP to ZERO? STACK \FALSE becomes RFALSE, because we know the negative branch will be taken.
==6==
?L14: BTST STACK,2048 \?L12
JUMP ?L6
JUMP ?L13
?L12: JUMP ?L15
?L13: ZERO? STACK \?L6
?L15: RFALSE
JUMP ?L7
?L6: PUSH 0
?L7: ZERO? STACK \FALSE
Bypass controlled conditional branch. PUSH 0 followed by ZERO? STACK \FALSE becomes a JUMP to the new ?L16, because we know the negative branch won't be taken.
==7==
?L14: BTST STACK,2048 \?L12
JUMP ?L6
?L12: JUMP ?L15
?L15: RFALSE
?L6: JUMP ?L16
?L16: ...
Eliminate dead code and unused labels.
==8==
?L14: BTST STACK,2048 \?L12
JUMP ?L6
?L12: JUMP ?L15
?L6: JUMP ?L16
ZERO? STACK \FALSE
?L16: ...
Eliminate branch to next instruction. JUMP ?L15 goes away; ?L15 merges with ?L12.
==9==
?L14: BTST STACK,2048 \?L12
JUMP ?L6
?L12: RFALSE
?L6: JUMP ?L16
ZERO? STACK \FALSE
?L16: ...
Disintermediate branch. BTST STACK,2048 \?L12 becomes BTST STACK,2048 \FALSE, because the instruction at ?L12 is RFALSE.
==10==
?L14: BTST STACK,2048 \FALSE
JUMP ?L6
?L12: RFALSE
?L6: JUMP ?L16
ZERO? STACK \FALSE
?L16: ...
Disintermediate branch. JUMP ?L6 becomes JUMP ?L16, because the instruction at ?L6 is JUMP ?L16.
==11==
?L14: BTST STACK,2048 \FALSE
JUMP ?L16
?L16: ...
Eliminate dead code and unused labels.
==12==
?L14: BTST STACK,2048 \FALSE
Eliminate branch to next instruction. JUMP ?L16 goes away.
==13==
?L14: BTST STACK,2048 \FALSE
