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[LangRef] describe the default FP environment 

Follow-up for D44216: add a section and examples to describe the FP env.
Also, add pointers from the FP instructions to this new section to reduce
bloat.

Differential Revision: https://reviews.llvm.org/D44318

llvm-svn: 327998
This commit is contained in:
Sanjay Patel 2018-03-20 16:38:22 +00:00
parent 73be83dec5
commit 54b161e47f
1 changed files with 38 additions and 15 deletions
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53
llvm/docs/LangRef.rst
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@ -2295,6 +2295,32 @@ or ``syncscope("<target-scope>")`` *synchronizes with* and participates in the
seq\_cst total orderings of other operations that are not marked
``syncscope("singlethread")`` or ``syncscope("<target-scope>")``.
.. _floatenv:
Floating-Point Environment
--------------------------
The default LLVM floating-point environment assumes that floating-point
instructions do not have side effects. Results assume the round-to-nearest
rounding mode. No floating-point exception state is maintained in this
environment. Therefore, there is no attempt to create or preserve invalid
operation (SNaN) or division-by-zero exceptions in these examples:
.. code-block:: llvm
%A = fdiv 0x7ff0000000000001, %X ; 64-bit SNaN hex value
%B = fdiv %X, 0.0
Safe:
%A = NaN
%B = NaN
The benefit of this exception-free assumption is that floating-point
operations may be speculated freely without any other fast-math relaxations
to the floating-point model.
Code that requires different behavior than this should use the
:ref:`Constrained Floating-Point Intrinsics <_constrainedfp>`.
.. _fastmath:
Fast-Math Flags
@ -6415,9 +6441,8 @@ Semantics:
""""""""""
The value produced is the floating-point sum of the two operands.
This instruction is assumed to execute in the default floating-point
environment. It has no side effects. Users can not assume that any
floating-point exception state is updated by this instruction.
This instruction is assumed to execute in the default :ref`floating-point
environment <_floatenv>`.
This instruction can also take any number of :ref:`fast-math
flags <fastmath>`, which are optimization hints to enable otherwise
unsafe floating-point optimizations:
@ -6513,9 +6538,8 @@ Semantics:
""""""""""
The value produced is the floating-point difference of the two operands.
This instruction is assumed to execute in the default floating-point
environment. It has no side effects. Users can not assume that any
floating-point exception state is updated by this instruction.
This instruction is assumed to execute in the default :ref`floating-point
environment <_floatenv>`.
This instruction can also take any number of :ref:`fast-math
flags <fastmath>`, which are optimization hints to enable otherwise
unsafe floating-point optimizations:
@ -6609,9 +6633,8 @@ Semantics:
""""""""""
The value produced is the floating-point product of the two operands.
This instruction is assumed to execute in the default floating-point
environment. It has no side effects. Users can not assume that any
floating-point exception state is updated by this instruction.
This instruction is assumed to execute in the default :ref`floating-point
environment <_floatenv>`.
This instruction can also take any number of :ref:`fast-math
flags <fastmath>`, which are optimization hints to enable otherwise
unsafe floating-point optimizations:
@ -6744,9 +6767,8 @@ Semantics:
""""""""""
The value produced is the floating-point quotient of the two operands.
This instruction is assumed to execute in the default floating-point
environment. It has no side effects. Users can not assume that any
floating-point exception state is updated by this instruction.
This instruction is assumed to execute in the default :ref`floating-point
environment <_floatenv>`.
This instruction can also take any number of :ref:`fast-math
flags <fastmath>`, which are optimization hints to enable otherwise
unsafe floating-point optimizations:
@ -6891,9 +6913,8 @@ The value produced is the floating-point remainder of the two operands.
This is the same output as a libm '``fmod``' function, but without any
possibility of setting ``errno``. The remainder has the same sign as the
dividend.
This instruction is assumed to execute in the default floating-point
environment. It has no side effects. Users can not assume that any
floating-point exception state is updated by this instruction.
This instruction is assumed to execute in the default :ref`floating-point
environment <_floatenv>`.
This instruction can also take any number of :ref:`fast-math
flags <fastmath>`, which are optimization hints to enable otherwise
unsafe floating-point optimizations:
@ -12906,6 +12927,8 @@ Semantics:
Returns another pointer that aliases its argument but which is considered different
for the purposes of ``load``/``store`` ``invariant.group`` metadata.
.. _constrainedfp:
Constrained Floating Point Intrinsics
-------------------------------------