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Optimize -a == -b, !(a - b), !(a + b)

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-a == -b  ->  a == b

If both a and b either are constants or have a minus sign, negate both.

!(a - b) can be optimized to a == b.

!(a + b) can be optimized to -a == b, relying on the first optimization to remove redundant minus signs.
This commit is contained in:
Sei Lisa 2018-03-23 18:15:28 +01:00
parent d0d6d6744b
commit 877d5fc10c
1 changed files with 28 additions and 1 deletions
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29
lslopt/lslfoldconst.py
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@ -708,7 +708,15 @@ class foldconst(object):
subexpr['ch'] = [subexpr['ch'][b], subexpr['ch'][a]]
parent[index] = subexpr
return
if snt == '!=' or snt == '^':
if snt == '!=' or snt == '^' or snt == '-' or snt == '+':
if snt == '+':
# Change !(x + y) -> -x == y, and make another pass
# to get rid of the signs where possible
subexpr['ch'][0] = {'nt':'NEG', 't':'integer',
'ch':[subexpr['ch'][0]]}
if 'SEF' in subexpr['ch'][0]['ch'][0]:
subexpr['ch'][0]['SEF'] = True
subexpr['nt'] = '=='
parent[index] = subexpr
self.FoldTree(parent, index)
@ -1171,6 +1179,9 @@ class foldconst(object):
if child[b]['nt'] != 'CONST':
a, b = 1, 0
# a == -1 (in any order) -> !~a,
# a == 0 -> !a
# a == 1 -> !~-a
if child[b]['nt'] == 'CONST':
if child[b]['value'] in (-1, 0, 1):
node = child[a]
@ -1189,6 +1200,22 @@ class foldconst(object):
del child
self.FoldTree(parent, index)
return
# -a == -b -> a == b with const variations.
# Note this changes the sign of two CONSTs but that case
# should not reach here, as those are resolved earlier.
if ((child[0]['nt'] == 'NEG' or child[0]['nt'] == 'CONST')
and
(child[1]['nt'] == 'NEG' or child[1]['nt'] == 'CONST')
):
for a in (0, 1):
if child[a]['nt'] == 'NEG':
child[a] = child[a]['ch'][0] # remove sign
else:
child[a]['value'] = lslfuncs.neg(
child[a]['value'])
if self.CompareTrees(child[0], child[1]):
# expr == expr -> 1
parent[index] = {'nt':'CONST', 't':'integer', 'value':1,