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Diffstat (limited to 'docs')
| -rw-r--r-- | docs/users_guide/9.8.1-notes.rst | 18 | ||||
| -rw-r--r-- | docs/users_guide/using.rst | 18 |
2 files changed, 36 insertions, 0 deletions
diff --git a/docs/users_guide/9.8.1-notes.rst b/docs/users_guide/9.8.1-notes.rst index e7e9acdf75..84d9105efd 100644 --- a/docs/users_guide/9.8.1-notes.rst +++ b/docs/users_guide/9.8.1-notes.rst @@ -142,6 +142,24 @@ Runtime system - ``sameMutVar#``, ``sameTVar#``, ``sameMVar#`` - ``sameIOPort#``, ``eqStableName#``. +- New primops for fused multiply-add operations. These primops combine a + multiplication and an addition, compiling to a single instruction when + the ``-mfma`` flag is enabled and the architecture supports it. + + The new primops are ``fmaddFloat#, fmsubFloat#, fnmaddFloat#, fnmsubFloat# :: Float# -> Float# -> Float# -> Float#`` + and ``fmaddDouble#, fmsubDouble#, fnmaddDouble#, fnmsubDouble# :: Double# -> Double# -> Double# -> Double#``. + + These implement the following operations, while performing one single + rounding at the end, leading to a more accurate result: + + - ``fmaddFloat# x y z``, ``fmaddDouble# x y z`` compute ``x * y + z``. + - ``fmsubFloat# x y z``, ``fmsubDouble# x y z`` compute ``x * y - z``. + - ``fnmaddFloat# x y z``, ``fnmaddDouble# x y z`` compute ``- x * y + z``. + - ``fnmsubFloat# x y z``, ``fnmsubDouble# x y z`` compute ``- x * y - z``. + + Warning: on unsupported architectures, the software emulation provided by + the fallback to the C standard library is not guaranteed to be IEEE-compliant. + ``ghc`` library ~~~~~~~~~~~~~~~ diff --git a/docs/users_guide/using.rst b/docs/users_guide/using.rst index 787b6a0503..8de7dd3533 100644 --- a/docs/users_guide/using.rst +++ b/docs/users_guide/using.rst @@ -1732,6 +1732,24 @@ Some flags only make sense for particular target platforms. :ref:`native code generator <native-code-gen>`. The resulting compiled code will only run on processors that support BMI2 (Intel Haswell and newer, AMD Excavator, Zen and newer). +.. ghc-flag:: -mfma + :shortdesc: Use native FMA instructions for fused multiply-add floating-point operations + :type: dynamic + :category: platform-options + + :since: 9.8.1 + + Use native FMA instructions to implement the fused multiply-add floating-point + operations of the form ``x * y + z``. + This allows computing a multiplication and addition in a single instruction, + without an intermediate rounding step. + Supported architectures: X86 with the FMA3 instruction set (this includes + most consumer processors since 2013), PowerPC and AArch64. + + When this flag is disabled, GHC falls back to the C implementation of fused + multiply-add, which might perform non-IEEE-compliant software emulation on + some platforms (depending on the implementation of the C standard library). + Haddock ------- |