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tccgen.c: Optimise 0<<x, 0>>x, -1>>x, x&0, x*0, x|-1, x%1.

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More precisely, treat (0 << x) and so on as constant expressions, but
not if const_wanted as we do not want to allow "case (x*0):", ...

Do not optimise (0 / x) and (0 % x) here as x might be zero, though
for an architecture that does not generate an exception for division
by zero the back end might choose to optimise those.
This commit is contained in:
Edmund Grimley Evans 2015-03-07 11:25:27 +00:00
parent 9163393476
commit ac70e6b840
1 changed files with 25 additions and 10 deletions
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tccgen.c
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@ -1476,16 +1476,31 @@ static void gen_opic(int op)
c2 = c1; //c = c1, c1 = c2, c2 = c;
l2 = l1; //l = l1, l1 = l2, l2 = l;
}
/* Filter out NOP operations like x*1, x-0, x&-1... */
if (c2 && (((op == '*' || op == '/' || op == TOK_UDIV ||
op == TOK_PDIV) &&
l2 == 1) ||
((op == '+' || op == '-' || op == '|' || op == '^' ||
op == TOK_SHL || op == TOK_SHR || op == TOK_SAR) &&
l2 == 0) ||
(op == '&' &&
l2 == -1))) {
/* nothing to do */
if (!const_wanted &&
c1 && ((l1 == 0 &&
(op == TOK_SHL || op == TOK_SHR || op == TOK_SAR)) ||
(l1 == -1 && op == TOK_SAR))) {
/* treat (0 << x), (0 >> x) and (-1 >> x) as constant */
vtop--;
} else if (!const_wanted &&
c2 && ((l2 == 0 && (op == '&' || op == '*')) ||
(l2 == -1 && op == '|') ||
(l2 == 0xffffffff && t2 != VT_LLONG && op == '|') ||
(l2 == 1 && (op == '%' || op == TOK_UMOD)))) {
/* treat (x & 0), (x * 0), (x | -1) and (x % 1) as constant */
if (l2 == 1)
vtop->c.ll = 0;
vswap();
vtop--;
} else if (c2 && (((op == '*' || op == '/' || op == TOK_UDIV ||
op == TOK_PDIV) &&
l2 == 1) ||
((op == '+' || op == '-' || op == '|' || op == '^' ||
op == TOK_SHL || op == TOK_SHR || op == TOK_SAR) &&
l2 == 0) ||
(op == '&' &&
l2 == -1))) {
/* filter out NOP operations like x*1, x-0, x&-1... */
vtop--;
} else if (c2 && (op == '*' || op == TOK_PDIV || op == TOK_UDIV)) {
/* try to use shifts instead of muls or divs */