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audio/mixeng: fix clang 10+ warning

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The code in CONV_NATURAL_FLOAT() and CLIP_NATURAL_FLOAT()
seems to use the constant 2^31-0.5 to convert float to integer
and back. But the float type lacks the required precision and
the constant used for the conversion is 2^31. This is equiva-
lent to a [-1.f, 1.f] <-> [INT32_MIN, INT32_MAX + 1] mapping.

This patch explicitly writes down the used constant. The
compiler generated code doesn't change.

The constant 2^31 has an exact float representation and the
clang 10 compiler stops complaining about an implicit int to
float conversion with a changed value.

A few notes:
- The conversion of 1.f to INT32_MAX + 1 doesn't overflow. The
  type of the destination variable is int64_t.
- At a later stage one of the clip_* functions in
  audio/mixeng_template.h limits INT32_MAX + 1 to the integer
  range.
- The clip_natural_float_* functions in audio/mixeng.c convert
  INT32_MAX and INT32_MAX + 1 to 1.f.

Buglink: https://bugs.launchpad.net/bugs/1878627
Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-id: 20200523201712.23908-1-vr_qemu@t-online.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
This commit is contained in:
Volker Rümelin 2020-05-23 22:17:12 +02:00 committed by Gerd Hoffmann
parent 2e44570321
commit 9c61fcc89a
1 changed files with 3 additions and 2 deletions
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5
audio/mixeng.c
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@ -271,11 +271,12 @@ f_sample *mixeng_clip[2][2][2][3] = {
#define CONV_NATURAL_FLOAT(x) (x)
#define CLIP_NATURAL_FLOAT(x) (x)
#else
static const float float_scale = UINT_MAX / 2.f;
/* macros to map [-1.f, 1.f] <-> [INT32_MIN, INT32_MAX + 1] */
static const float float_scale = (int64_t)INT32_MAX + 1;
#define CONV_NATURAL_FLOAT(x) ((x) * float_scale)
#ifdef RECIPROCAL
static const float float_scale_reciprocal = 2.f / UINT_MAX;
static const float float_scale_reciprocal = 1.f / ((int64_t)INT32_MAX + 1);
#define CLIP_NATURAL_FLOAT(x) ((x) * float_scale_reciprocal)
#else
#define CLIP_NATURAL_FLOAT(x) ((x) / float_scale)