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New semantics for the integer 'for' loop

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The numerical 'for' loop over integers now uses a precomputed counter
to control its number of iteractions. This change eliminates several
weird cases caused by overflows (wrap-around) in the control variable.
(It also ensures that every integer loop halts.)

Also, the special opcodes for the usual case of step==1 were removed.
(The new code is already somewhat complex for the usual case,
but efficient.)
This commit is contained in:
Roberto Ierusalimschy
2019-03-19 10:53:18 -03:00
parent 1e0c73d5b6
commit 9b37a4695e
10 changed files with 213 additions and 185 deletions
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42
lparser.c
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@@ -1371,18 +1371,14 @@ static void repeatstat (LexState *ls, int line) {
/*
** Read an expression and generate code to put its results in next
** stack slot. Return true if expression is a constant integer and,
** if 'i' is not-zero, its value is equal to 'i'.
** stack slot.
**
*/
static int exp1 (LexState *ls, int i) {
static void exp1 (LexState *ls) {
expdesc e;
int res;
expr(ls, &e);
res = luaK_isKint(&e) && (i == 0 || i == e.u.ival);
luaK_exp2nextreg(ls->fs, &e);
lua_assert(e.k == VNONRELOC);
return res;
}
@@ -1403,31 +1399,29 @@ static void fixforjump (FuncState *fs, int pc, int dest, int back) {
/*
** Generate code for a 'for' loop. 'kind' can be zero (a common for
** loop), one (a basic for loop, with integer values and increment of
** 1), or two (a generic for loop).
** Generate code for a 'for' loop.
*/
static void forbody (LexState *ls, int base, int line, int nvars, int kind) {
static void forbody (LexState *ls, int base, int line, int nvars, int isgen) {
/* forbody -> DO block */
static OpCode forprep[3] = {OP_FORPREP, OP_FORPREP1, OP_TFORPREP};
static OpCode forloop[3] = {OP_FORLOOP, OP_FORLOOP1, OP_TFORLOOP};
static OpCode forprep[2] = {OP_FORPREP, OP_TFORPREP};
static OpCode forloop[2] = {OP_FORLOOP, OP_TFORLOOP};
BlockCnt bl;
FuncState *fs = ls->fs;
int prep, endfor;
checknext(ls, TK_DO);
prep = luaK_codeABx(fs, forprep[kind], base, 0);
prep = luaK_codeABx(fs, forprep[isgen], base, 0);
enterblock(fs, &bl, 0); /* scope for declared variables */
adjustlocalvars(ls, nvars);
luaK_reserveregs(fs, nvars);
block(ls);
leaveblock(fs); /* end of scope for declared variables */
fixforjump(fs, prep, luaK_getlabel(fs), 0);
if (kind == 2) { /* generic for? */
if (isgen) { /* generic for? */
luaK_codeABC(fs, OP_TFORCALL, base, 0, nvars);
luaK_fixline(fs, line);
base += 2; /* base for 'OP_TFORLOOP' (skips function and state) */
}
endfor = luaK_codeABx(fs, forloop[kind], base, 0);
endfor = luaK_codeABx(fs, forloop[isgen], base, 0);
fixforjump(fs, endfor, prep + 1, 1);
luaK_fixline(fs, line);
}
@@ -1437,26 +1431,22 @@ static void fornum (LexState *ls, TString *varname, int line) {
/* fornum -> NAME = exp,exp[,exp] forbody */
FuncState *fs = ls->fs;
int base = fs->freereg;
int basicfor = 1; /* true if it is a "basic" 'for' (integer + 1) */
new_localvarliteral(ls, "(for index)");
new_localvarliteral(ls, "(for limit)");
new_localvarliteral(ls, "(for step)");
new_localvar(ls, varname);
checknext(ls, '=');
if (!exp1(ls, 0)) /* initial value not an integer? */
basicfor = 0; /* not a basic 'for' */
exp1(ls); /* initial value */
checknext(ls, ',');
exp1(ls, 0); /* limit */
if (testnext(ls, ',')) {
if (!exp1(ls, 1)) /* optional step not 1? */
basicfor = 0; /* not a basic 'for' */
}
exp1(ls); /* limit */
if (testnext(ls, ','))
exp1(ls); /* optional step */
else { /* default step = 1 */
luaK_int(fs, fs->freereg, 1);
luaK_reserveregs(fs, 1);
}
adjustlocalvars(ls, 3); /* control variables */
forbody(ls, base, line, 1, basicfor);
forbody(ls, base, line, 1, 0);
}
@@ -1484,7 +1474,7 @@ static void forlist (LexState *ls, TString *indexname) {
adjust_assign(ls, 4, explist(ls, &e), &e);
adjustlocalvars(ls, 4); /* control variables */
luaK_checkstack(fs, 3); /* extra space to call generator */
forbody(ls, base, line, nvars - 4, 2);
forbody(ls, base, line, nvars - 4, 1);
}
@@ -1633,7 +1623,7 @@ static void tocloselocalstat (LexState *ls) {
luaO_pushfstring(ls->L, "unknown attribute '%s'", getstr(attr)));
new_localvar(ls, str_checkname(ls));
checknext(ls, '=');
exp1(ls, 0);
exp1(ls);
markupval(fs, fs->nactvar);
fs->bl->insidetbc = 1; /* in the scope of a to-be-closed variable */
adjustlocalvars(ls, 1);