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musl: Fix limiting the float precision in intermediates.

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musl itself expects to be configured to compile with either
-ffloat-store or -fexcess-precision=standard - but when imported
into Wine, those flags aren't used.

This seems to be essential for getting reasonable precision
from some math functions such as exp2() - without the expected
precision truncation, the output value of exp2() can be off by
as much as 0.2% in some cases.

As Wine doesn't build the musl sources with those flags, use
volatile to force storing/reloading floats in order to limit
their intermediate precision, as musl expects.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=56372
This commit is contained in:
Martin Storsjö 2024-03-11 14:38:27 +02:00 committed by Alexandre Julliard
parent e2e1ccb58a
commit 3607eae620
Notes: Alexandre Julliard 2024-10-15 00:04:51 +02:00 
Approved-by: Piotr Caban (@piotr)
Approved-by: Alexandre Julliard (@julliard)
Merge-Request: https://gitlab.winehq.org/wine/wine/merge_requests/6668
1 changed files with 6 additions and 3 deletions
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9
libs/musl/src/internal/libm.h
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@ -104,17 +104,20 @@ static int32_t converttoint(double_t);
/* Evaluate an expression as the specified type. With standard excess
precision handling a type cast or assignment is enough (with
-ffloat-store an assignment is required, in old compilers argument
passing and return statement may not drop excess precision). */
passing and return statement may not drop excess precision).
If compiled without -ffloat-store or -fexcess-precision=standard,
an extra volatile qualifier here will force limiting the precision. */
static inline float eval_as_float(float x)
{
float y = x;
volatile float y = x;
return y;
}
static inline double eval_as_double(double x)
{
double y = x;
volatile double y = x;
return y;
}