forked from OSchip/llvm-project
Revert "Create UsersManual section entitled 'Controlling Floating Point"
This reverts commit a08d5a4b0e.
llvm-svn: 372185
This commit is contained in:
Erich Keane
parent
5c62152275
commit
f478c8bf92
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@ -1128,165 +1128,6 @@ number of cases where the compilation environment is tightly controlled
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and the precompiled header cannot be generated after headers have been
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installed.
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.. _controlling-fp-behavior:
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Controlling Floating Point Behavior
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-----------------------------------
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Clang provides a number of ways to control floating point behavior. The options
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are listed below.
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.. option:: -ffast-math
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Enable fast-math mode. This option lets the
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compiler make aggressive, potentially-lossy assumptions about
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floating-point math. These include:
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* Floating-point math obeys regular algebraic rules for real numbers (e.g.
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``+`` and ``*`` are associative, ``x/y == x * (1/y)``, and
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``(a + b) * c == a * c + b * c``),
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* Operands to floating-point operations are not equal to ``NaN`` and
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``Inf``, and
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* ``+0`` and ``-0`` are interchangeable.
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``-ffast-math`` also defines the ``__FAST_MATH__`` preprocessor
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macro. Some math libraries recognize this macro and change their behavior.
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With the exception of ``-ffp-contract=fast``, using any of the options
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below to disable any of the individual optimizations in ``-ffast-math``
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will cause ``__FAST_MATH__`` to no longer be set.
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This option implies:
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* ``-fno-honor-infinities``
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* ``-fno-honor-nans``
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* ``-fno-math-errno``
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* ``-ffinite-math``
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* ``-fassociative-math``
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* ``-freciprocal-math``
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* ``-fno-signed-zeros``
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* ``-fno-trapping-math``
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* ``-ffp-contract=fast``
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.. option:: -fdenormal-fp-math=<value>
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Select which denormal numbers the code is permitted to require.
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Valid values are:
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* ``ieee`` - IEEE 754 denormal numbers
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* ``preserve-sign`` - the sign of a flushed-to-zero number is preserved in the sign of 0
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* ``positive-zero`` - denormals are flushed to positive zero
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Defaults to ``ieee``.
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.. option:: -f[no-]strict-float-cast-overflow
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When a floating-point value is not representable in a destination integer
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type, the code has undefined behavior according to the language standard.
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By default, Clang will not guarantee any particular result in that case.
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With the 'no-strict' option, Clang attempts to match the overflowing behavior
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of the target's native float-to-int conversion instructions.
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.. option:: -f[no-]math-errno
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Require math functions to indicate errors by setting errno.
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The default varies by ToolChain. ``-fno-math-errno`` allows optimizations
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that might cause standard C math functions to not set ``errno``.
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For example, on some systems, the math function ``sqrt`` is specified
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as setting ``errno`` to ``EDOM`` when the input is negative. On these
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systems, the compiler cannot normally optimize a call to ``sqrt`` to use
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inline code (e.g. the x86 ``sqrtsd`` instruction) without additional
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checking to ensure that ``errno`` is set appropriately.
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``-fno-math-errno`` permits these transformations.
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On some targets, math library functions never set ``errno``, and so
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``-fno-math-errno`` is the default. This includes most BSD-derived
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systems, including Darwin.
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.. option:: -f[no-]trapping-math
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``-fno-trapping-math`` allows optimizations that assume that
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floating point operations cannot generate traps such as divide-by-zero,
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overflow and underflow. Defaults to ``-ftrapping-math``.
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Currently this option has no effect.
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.. option:: -ffp-contract=<value>
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Specify when the compiler is permitted to form fused floating-point
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operations, such as fused multiply-add (FMA). Fused operations are
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permitted to produce more precise results than performing the same
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operations separately.
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The C standard permits intermediate floating-point results within an
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expression to be computed with more precision than their type would
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normally allow. This permits operation fusing, and Clang takes advantage
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of this by default. This behavior can be controlled with the
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``FP_CONTRACT`` pragma. Please refer to the pragma documentation for a
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description of how the pragma interacts with this option.
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Valid values are:
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* ``fast`` (everywhere)
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* ``on`` (according to FP_CONTRACT pragma, default)
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* ``off`` (never fuse)
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.. option:: -f[no-]honor-infinities
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If both ``-fno-honor-infinities`` and ``-fno-honor-nans`` are used,
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has the same effect as specifying ``-ffinite-math``.
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.. option:: -f[no-]honor-nans
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If both ``-fno-honor-infinities`` and ``-fno-honor-nans`` are used,
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has the same effect as specifying ``-ffinite-math``.
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.. option:: -f[no-]signed-zeros
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Allow optimizations that ignore the sign of floating point zeros.
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Defaults to ``-fno-signed-zeros``.
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.. option:: -f[no-]associative-math
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Allow floating point operations to be reassociated.
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Defaults to ``-fno-associative-math``.
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.. option:: -f[no-]reciprocal-math
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Allow division operations to be transformed into multiplication by a
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reciprocal. This can be significantly faster than an ordinary division
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but can also have significantly less precision. Defaults to
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``-fno-reciprocal-math``.
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.. option:: -f[no-]unsafe-math-optimizations
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Allow unsafe floating-point optimizations. Also implies:
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* ``-fassociative-math``
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* ``-freciprocal-math``
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* ``-fno-signed-zeroes``
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* ``-fno-trapping-math``.
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Defaults to ``-fno-unsafe-math-optimizations``.
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.. option:: -f[no-]finite-math
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Allow floating-point optimizations that assume arguments and results are
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not NaNs or +-Inf. This defines the ``__FINITE_MATH_ONLY__`` preprocessor macro.
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Also implies:
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* ``-fno-honor-infinities``
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* ``-fno-honor-nans``
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Defaults to ``-fno-finite-math``.
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.. _controlling-code-generation:
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Controlling Code Generation
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@ -1425,6 +1266,36 @@ are listed below.
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This enables better devirtualization. Turned off by default, because it is
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still experimental.
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.. option:: -ffast-math
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Enable fast-math mode. This defines the ``__FAST_MATH__`` preprocessor
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macro, and lets the compiler make aggressive, potentially-lossy assumptions
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about floating-point math. These include:
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* Floating-point math obeys regular algebraic rules for real numbers (e.g.
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``+`` and ``*`` are associative, ``x/y == x * (1/y)``, and
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``(a + b) * c == a * c + b * c``),
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* operands to floating-point operations are not equal to ``NaN`` and
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``Inf``, and
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* ``+0`` and ``-0`` are interchangeable.
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.. option:: -fdenormal-fp-math=[values]
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Select which denormal numbers the code is permitted to require.
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Valid values are: ``ieee``, ``preserve-sign``, and ``positive-zero``,
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which correspond to IEEE 754 denormal numbers, the sign of a
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flushed-to-zero number is preserved in the sign of 0, denormals are
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flushed to positive zero, respectively.
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.. option:: -f[no-]strict-float-cast-overflow
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When a floating-point value is not representable in a destination integer
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type, the code has undefined behavior according to the language standard.
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By default, Clang will not guarantee any particular result in that case.
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With the 'no-strict' option, Clang attempts to match the overflowing behavior
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of the target's native float-to-int conversion instructions.
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.. option:: -fwhole-program-vtables
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Enable whole-program vtable optimizations, such as single-implementation
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