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(unextend): Rewrite type conversions to avoid overflow.

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From-SVN: r13165
This commit is contained in:
Jim Wilson 1996-11-15 10:20:53 -08:00
parent f6beed2169
commit cf85c69b0e
1 changed files with 13 additions and 3 deletions
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16
gcc/fold-const.c
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@ -3127,18 +3127,28 @@ unextend (c, p, unsignedp, mask)
if (p == modesize || unsignedp)
return c;
if (TREE_UNSIGNED (type))
c = convert (signed_type (type), c);
/* We work by getting just the sign bit into the low-order bit, then
into the high-order bit, then sign-extend. We then XOR that value
with C. */
temp = const_binop (RSHIFT_EXPR, c, size_int (p - 1), 0);
temp = const_binop (BIT_AND_EXPR, temp, size_int (1), 0);
/* We must use a signed type in order to get an arithmetic right shift.
However, we must also avoid introducing accidental overflows, so that
a subsequent call to integer_zerop will work. Hence we must
do the type conversion here. At this point, the constant is either
zero or one, and the conversion to a signed type can never overflow.
We could get an overflow if this conversion is done anywhere else. */
if (TREE_UNSIGNED (type))
temp = convert (signed_type (type), temp);
temp = const_binop (LSHIFT_EXPR, temp, size_int (modesize - 1), 0);
temp = const_binop (RSHIFT_EXPR, temp, size_int (modesize - p - 1), 0);
if (mask != 0)
temp = const_binop (BIT_AND_EXPR, temp, convert (TREE_TYPE (c), mask), 0);
/* If necessary, convert the type back to match the type of C. */
if (TREE_UNSIGNED (type))
temp = convert (type, temp);
return convert (type, const_binop (BIT_XOR_EXPR, c, temp, 0));
}