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endianmacros: Replace borked PA_FLOAT32_SWAP() with PA_READ_FLOAT32RE() / PA_WRITE_FLOAT32RE()

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building PA with -O0 leads to test failure in mix-test on i386

issue reported by Felipe, see
http://lists.freedesktop.org/archives/pulseaudio-discuss/2014-August/021406.html

the problem is the value 0xbeffbd7f: when byte-swapped it becomes 0x7fbdffbe and according
to IEEE-754 represents a signalling NaN (starting with s111 1111 10, see http://en.wikipedia.org/wiki/NaN)

when this value is assigned to a floating point register, it becomes 0x7ffdffbe, representing
a quiet NaN (starting with s111 1111 11) -- a signalling NaN is turned into a quiet NaN!

so PA_FLOAT32_SWAP(PA_FLOAT32_SWAP(x)) != x for certain values, uhuh!

the following test code can be used; due to volatile, it will always demonstrate the issue;
without volatile, it depends on the optimization level (i386, 32-bit, gcc 4.9):

// snip

static inline float PA_FLOAT32_SWAP(float x) {
    union {
        float f;
        uint32_t u;
    } t;

    t.f = x;
    t.u = bswap_32(t.u);
    return t.f;
}

int main() {
  unsigned x = 0xbeffbd7f;
  volatile float f = PA_FLOAT32_SWAP(*(float *)&x);
  printf("%08x %08x %08x %f\n", 0xbeffbd7f, *(unsigned *)&f, bswap_32(*(unsigned *)&f), f);
}
// snip

the problem goes away with optimization when no temporary floating point registers are used

the proposed solution is to avoid passing swapped floating point data in a
float; this is done with new functions PA_READ_FLOAT32RE() and PA_WRITE_FLOAT32RE()
which use uint32_t to dereference a pointer and byte-swap the data, hence no temporary
float variable is used

also delete PA_FLOAT32_TO_LE()/_BE(), not used

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Reported-by: Felipe Sateler <fsateler@debian.org>
This commit is contained in:
Peter Meerwald 2014-09-02 23:53:09 +02:00
parent 5d7b5e509c
commit 293a1739e2
7 changed files with 32 additions and 40 deletions
Download patch file Download diff file Expand all files Collapse all files

4
src/pulsecore/svolume_c.c
View file

@ -125,9 +125,9 @@ static void pa_volume_float32re_c(float *samples, const float *volumes, unsigned
for (channel = 0; length; length--) {
float t;
t = PA_FLOAT32_SWAP(*samples);
t = PA_READ_FLOAT32RE(samples);
t *= volumes[channel];
*samples++ = PA_FLOAT32_SWAP(t);
PA_WRITE_FLOAT32RE(samples++, t);
if (PA_UNLIKELY(++channel >= channels))
channel = 0;