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