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fix of undefined multiply (int32 overflow)

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the problem was the incorporation of the extra constant 1<<16 in the kC1
constant, to emulate the addition. It's now removed and the addition is
performed explicitly.

No real speed difference observed.

cf. issue #278

Change-Id: I2c6499031571d98afff392fb5ebe21a5fa60722d
This commit is contained in:
Pascal Massimino 2015-11-24 23:06:50 -08:00
parent 912c9fdf0c
commit f5ca40e05f
1 changed files with 13 additions and 13 deletions
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26
src/dsp/dec.c
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@ -34,9 +34,8 @@ static WEBP_INLINE uint8_t clip_8b(int v) {
STORE(3, y, DC - (d)); \
} while (0)
static const int kC1 = 20091 + (1 << 16);
static const int kC2 = 35468;
#define MUL(a, b) (((a) * (b)) >> 16)
#define MUL1(a) ((((a) * 20091) >> 16) + (a))
#define MUL2(a) (((a) * 35468) >> 16)
static void TransformOne(const int16_t* in, uint8_t* dst) {
int C[4 * 4], *tmp;
@ -45,8 +44,8 @@ static void TransformOne(const int16_t* in, uint8_t* dst) {
for (i = 0; i < 4; ++i) { // vertical pass
const int a = in[0] + in[8]; // [-4096, 4094]
const int b = in[0] - in[8]; // [-4095, 4095]
const int c = MUL(in[4], kC2) - MUL(in[12], kC1); // [-3783, 3783]
const int d = MUL(in[4], kC1) + MUL(in[12], kC2); // [-3785, 3781]
const int c = MUL2(in[4]) - MUL1(in[12]); // [-3783, 3783]
const int d = MUL1(in[4]) + MUL2(in[12]); // [-3785, 3781]
tmp[0] = a + d; // [-7881, 7875]
tmp[1] = b + c; // [-7878, 7878]
tmp[2] = b - c; // [-7878, 7878]
@ -55,7 +54,7 @@ static void TransformOne(const int16_t* in, uint8_t* dst) {
in++;
}
// Each pass is expanding the dynamic range by ~3.85 (upper bound).
// The exact value is (2. + (kC1 + kC2) / 65536).
// The exact value is (2. + (20091 + 35468) / 65536).
// After the second pass, maximum interval is [-3794, 3794], assuming
// an input in [-2048, 2047] interval. We then need to add a dst value
// in the [0, 255] range.
@ -66,8 +65,8 @@ static void TransformOne(const int16_t* in, uint8_t* dst) {
const int dc = tmp[0] + 4;
const int a = dc + tmp[8];
const int b = dc - tmp[8];
const int c = MUL(tmp[4], kC2) - MUL(tmp[12], kC1);
const int d = MUL(tmp[4], kC1) + MUL(tmp[12], kC2);
const int c = MUL2(tmp[4]) - MUL1(tmp[12]);
const int d = MUL1(tmp[4]) + MUL2(tmp[12]);
STORE(0, 0, a + d);
STORE(1, 0, b + c);
STORE(2, 0, b - c);
@ -80,16 +79,17 @@ static void TransformOne(const int16_t* in, uint8_t* dst) {
// Simplified transform when only in[0], in[1] and in[4] are non-zero
static void TransformAC3(const int16_t* in, uint8_t* dst) {
const int a = in[0] + 4;
const int c4 = MUL(in[4], kC2);
const int d4 = MUL(in[4], kC1);
const int c1 = MUL(in[1], kC2);
const int d1 = MUL(in[1], kC1);
const int c4 = MUL2(in[4]);
const int d4 = MUL1(in[4]);
const int c1 = MUL2(in[1]);
const int d1 = MUL1(in[1]);
STORE2(0, a + d4, d1, c1);
STORE2(1, a + c4, d1, c1);
STORE2(2, a - c4, d1, c1);
STORE2(3, a - d4, d1, c1);
}
#undef MUL
#undef MUL1
#undef MUL2
#undef STORE2
static void TransformTwo(const int16_t* in, uint8_t* dst, int do_two) {