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authorSzabolcs Nagy <nsz@port70.net>2018-10-16 22:20:39 +0000
committerRich Felker <dalias@aerifal.cx>2019-04-17 13:02:47 -0400
commite980ca7a571465e8a4c887a199491c2cd8d0c0ee (patch)
treed86a07a748a2c59bf1f12609f497cfd12f0dfc7a /src/string
parent65c8be380431eebe4d70d130bd38563f8df9a7d7 (diff)
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define FP_FAST_FMA* when fma* can be inlined
FP_FAST_FMA can be defined if "the fma function generally executes about
as fast as, or faster than, a multiply and an add of double operands",
which can only be true if the fma call is inlined as an instruction.

gcc sets __FP_FAST_FMA if __builtin_fma is inlined as an instruction,
but that does not mean an fma call will be inlined (e.g. it is defined
with -fno-builtin-fma), other compilers (clang) don't even have such
macro, but this is the closest we can get.

(even if the libc fma implementation is a single instruction, the extern
call overhead is already too big when the macro is used to decide between
x*y+z and fma(x,y,z) so it cannot be based on libc only, defining the
macro unconditionally on targets which have fma in the base isa is also
incorrect: the compiler might not inline fma anyway.)

this solution works with gcc unless fma inlining is explicitly turned off.
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