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authorSzabolcs Nagy <nsz@port70.net>2017-09-17 17:31:20 +0000
committerRich Felker <dalias@aerifal.cx>2017-10-13 22:30:45 -0400
commit907476925fca05f24ebca5fcdc21f1e58ba7b313 (patch)
tree79aa3b08e17ed7e04d0ac53d612a62d43dbac3cc /README
parentb3516058eb9ff9e52a8720f0e5c0f6936cce6047 (diff)
downloadmusl-907476925fca05f24ebca5fcdc21f1e58ba7b313.tar.gz
math: rewrite fma with mostly int arithmetics
the freebsd fma code failed to raise underflow exception in some cases in nearest rounding mode (affects fmal too) e.g. fma(-0x1p-1000, 0x1.000001p-74, 0x1p-1022) and the inexact exception may be raised spuriously since the fenv is not saved/restored around the exact multiplication algorithm (affects x86 fma too). another issue is that the underflow behaviour when the rounded result is the minimal normal number is target dependent, ieee754 allows two ways to raise underflow for inexact results: raise if the result before rounding is in the subnormal range (e.g. aarch64, arm, powerpc) or if the result after rounding with infinite exponent range is in the subnormal range (e.g. x86, mips, sh). to avoid all these issues the algorithm was rewritten with mostly int arithmetics and float arithmetics is only used to get correct rounding and raise exceptions according to the behaviour of the target without any fenv.h dependency. it also unifies x86 and non-x86 fma. fmaf is not affected, fmal need to be fixed too. this algorithm depends on a_clz_64 and it required a few spurious instructions to make sure underflow exception is raised in a particular corner case. (normally FORCE_EVAL(tiny*tiny) would be used for this, but on i386 gcc is broken if the expression is constant https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57245 and there is no easy portable fix for the macro.)
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