Saving anything except `a` trashes `a`.
Chris Pressey
7 years ago
| 776 | 776 |
self.analyze_block(instr.block, context)
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| 777 | 777 |
# TODO assert no goto was encountered
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| 778 | 778 |
context.re_introduce(baton)
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|
779 |
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|
780 |
if location == REG_A:
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|
781 |
pass
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782 |
else:
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|
783 |
context.set_touched(REG_A)
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|
784 |
context.set_unmeaningful(REG_A)
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| 1954 | 1954 |
| }
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| 1955 | 1955 |
= ok
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| 1956 | 1956 |
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1957 |
Saving any location (other than `a`) will trash `a`.
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1958 |
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1959 |
| routine main
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|
1960 |
| inputs a, x
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1961 |
| outputs a, x
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1962 |
| trashes z, n
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1963 |
| {
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1964 |
| ld a, 1
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1965 |
| save x {
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1966 |
| ld a, 2
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1967 |
| }
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1968 |
| }
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1969 |
? UnmeaningfulOutputError
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1970 |
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1971 |
Saving `a` does not trash anything.
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1972 |
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1973 |
| routine main
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1974 |
| inputs a, x
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1975 |
| outputs a, x
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1976 |
| trashes z, n
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1977 |
| {
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1978 |
| ld x, 1
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1979 |
| save a {
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1980 |
| ld x, 2
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|
1981 |
| }
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1982 |
| ld x, 3
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1983 |
| }
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1984 |
= ok
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1985 |
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| 1957 | 1986 |
A defined value that has been saved can be trashed inside the block.
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| 1958 | 1987 |
It will continue to be defined outside the block.
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| 1959 | 1988 |
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| 1960 | 1989 |
| routine main
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| 1961 | |
| inputs a
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| 1962 | |
| outputs a, x
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| 1963 | |
| trashes z, n
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|
1990 |
| outputs x, y
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1991 |
| trashes a, z, n
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| 1964 | 1992 |
| {
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| 1965 | 1993 |
| ld x, 0
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| 1966 | 1994 |
| save x {
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| 1967 | |
| ld a, 0
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1995 |
| ld y, 0
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| 1968 | 1996 |
| trash x
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| 1969 | 1997 |
| }
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| 1970 | 1998 |
| }
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| 1991 | 2019 |
routine; and can appear in a `save` exactly because a `save` preserves it.
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| 1992 | 2020 |
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| 1993 | 2021 |
| routine main
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2022 |
| outputs y
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|
2023 |
| trashes a, z, n
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|
2024 |
| {
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2025 |
| save x {
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2026 |
| ld y, 0
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2027 |
| ld x, 1
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|
2028 |
| }
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2029 |
| }
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|
2030 |
= ok
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2031 |
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2032 |
Because saving anything except `a` trashes `a`, a common idiom is to save `a`
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2033 |
first in a nested series of `save`s.
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2034 |
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2035 |
| routine main
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| 1994 | 2036 |
| inputs a
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| 1995 | 2037 |
| outputs a
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| 1996 | 2038 |
| trashes z, n
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| 1997 | 2039 |
| {
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| 1998 | |
| save x {
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| 1999 | |
| ld a, 0
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| 2000 | |
| ld x, 1
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2040 |
| save a {
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|
2041 |
| save x {
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2042 |
| ld a, 0
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|
2043 |
| ld x, 1
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|
2044 |
| }
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| 2001 | 2045 |
| }
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| 2002 | 2046 |
| }
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| 2003 | 2047 |
= ok
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