avcodec/jpeg2000: Simplify exp2fi for numbers used here

The call to ff_exp2fi() here always uses arguments in the normal
range, so that the branches in ff_exp2fi() are unnecessary.
This is so because JPEG2000 itself only supports up to
128 bits per component per pixel (we only support far less);
furthermore, expn is always 0..31 for the decoder and also
sane for the encoder, so that the difference between these
two values is always in the normal range of -126..128.

Reviewed-by: Tomas Härdin <git@haerdin.se>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/jpeg2000.c

@@ -32,7 +32,6 @@
 #include "libavutil/mem.h"
 #include "libavutil/thread.h"
 #include "avcodec.h"
-#include "internal.h"
 #include "jpeg2000.h"
 
 #define SHL(a, n) ((n) >= 0 ? (a) << (n) : (a) >> -(n))
@@ -201,6 +200,17 @@ void ff_jpeg2000_set_significance(Jpeg2000T1Context *t1, int x, int y,
 
 // static const uint8_t lut_gain[2][4] = { { 0, 0, 0, 0 }, { 0, 1, 1, 2 } }; (unused)
 
+/**
+ * 2^(x) for integer x in the range -126..128.
+ * @return correctly rounded float
+ */
+static av_always_inline float exp2fi(int x)
+{
+    av_assert2(-126 <= x && x <= 128);
+    /* Normal range */
+    return av_int2float((x+127) << 23);
+}
+
 static void init_band_stepsize(AVCodecContext *avctx,
                                Jpeg2000Band *band,
                                Jpeg2000CodingStyle *codsty,
@@ -230,7 +240,7 @@ static void init_band_stepsize(AVCodecContext *avctx,
          * R_b = R_I + log2 (gain_b )
          * see ISO/IEC 15444-1:2002 E.1.1 eqn. E-3 and E-4 */
         gain            = cbps;
-        band->f_stepsize  = ff_exp2fi(gain - qntsty->expn[gbandno]);
+        band->f_stepsize  = exp2fi(gain - qntsty->expn[gbandno]);
         band->f_stepsize *= qntsty->mant[gbandno] / 2048.0 + 1.0;
         break;
     default:
@@ -391,7 +401,7 @@ static int init_band(AVCodecContext *avctx,
                      Jpeg2000CodingStyle *codsty,
                      Jpeg2000QuantStyle *qntsty,
                      int bandno, int gbandno, int reslevelno,
-                     int cbps, int dx, int dy)
+                     const int cbps, int dx, int dy)
 {
     Jpeg2000Band *band = reslevel->band + bandno;
     uint8_t log2_band_prec_width, log2_band_prec_height;
@@ -466,7 +476,7 @@ static int init_band(AVCodecContext *avctx,
 int ff_jpeg2000_init_component(Jpeg2000Component *comp,
                                Jpeg2000CodingStyle *codsty,
                                Jpeg2000QuantStyle *qntsty,
-                               int cbps, int dx, int dy,
+                               const int cbps, int dx, int dy,
                                AVCodecContext *avctx)
 {
     int reslevelno, bandno, gbandno = 0, ret, i, j;