[IR] redefine 'UnsafeAlgebra' / 'reassoc' fast-math-flags and add 'trans' fast-math-flag

As discussed on llvm-dev:
http://lists.llvm.org/pipermail/llvm-dev/2016-November/107104.html
and again more recently:
http://lists.llvm.org/pipermail/llvm-dev/2017-October/118118.html

...this is a step in cleaning up our fast-math-flags implementation in IR to better match
the capabilities of both clang's user-visible flags and the backend's flags for SDNode.

As proposed in the above threads, we're replacing the 'UnsafeAlgebra' bit (which had the 
'umbrella' meaning that all flags are set) with a new bit that only applies to algebraic 
reassociation - 'AllowReassoc'.

We're also adding a bit to allow approximations for library functions called 'ApproxFunc' 
(this was initially proposed as 'libm' or similar).

...and we're out of bits. 7 bits ought to be enough for anyone, right? :) FWIW, I did 
look at getting this out of SubclassOptionalData via SubclassData (spacious 16-bits), 
but that's apparently already used for other purposes. Also, I don't think we can just 
add a field to FPMathOperator because Operator is not intended to be instantiated. 
We'll defer movement of FMF to another day.

We keep the 'fast' keyword. I thought about removing that, but seeing IR like this:
%f.fast = fadd reassoc nnan ninf nsz arcp contract afn float %op1, %op2
...made me think we want to keep the shortcut synonym.

Finally, this change is binary incompatible with existing IR as seen in the 
compatibility tests. This statement:
"Newer releases can ignore features from older releases, but they cannot miscompile 
them. For example, if nsw is ever replaced with something else, dropping it would be 
a valid way to upgrade the IR." 
( http://llvm.org/docs/DeveloperPolicy.html#ir-backwards-compatibility )
...provides the flexibility we want to make this change without requiring a new IR 
version. Ie, we're not loosening the FP strictness of existing IR. At worst, we will 
fail to optimize some previously 'fast' code because it's no longer recognized as 
'fast'. This should get fixed as we audit/squash all of the uses of 'isFast()'.

Note: an inter-dependent clang commit to use the new API name should closely follow 
commit.

Differential Revision: https://reviews.llvm.org/D39304



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317488 91177308-0d34-0410-b5e6-96231b3b80d8

Author: rotateright

docs/LangRef.rst

@@ -2272,11 +2272,11 @@ seq\_cst total orderings of other operations that are not marked
 Fast-Math Flags
 ---------------
 
-LLVM IR floating-point binary ops (:ref:`fadd <i_fadd>`,
+LLVM IR floating-point operations (:ref:`fadd <i_fadd>`,
 :ref:`fsub <i_fsub>`, :ref:`fmul <i_fmul>`, :ref:`fdiv <i_fdiv>`,
 :ref:`frem <i_frem>`, :ref:`fcmp <i_fcmp>`) and :ref:`call <i_call>`
-instructions have the following flags that can be set to enable
-otherwise unsafe floating point transformations.
+may use the following flags to enable otherwise unsafe 
+floating-point transformations.
 
 ``nnan``
    No NaNs - Allow optimizations to assume the arguments and result are not
@@ -2300,10 +2300,17 @@ otherwise unsafe floating point transformations.
    Allow floating-point contraction (e.g. fusing a multiply followed by an
    addition into a fused multiply-and-add).
 
+``afn``
+   Approximate functions - Allow substitution of approximate calculations for
+   functions (sin, log, sqrt, etc). See floating-point intrinsic definitions 
+   for places where this can apply to LLVM's intrinsic math functions. 
+
+``reassoc``
+   Allow reassociation transformations for floating-point instructions. 
+   This may dramatically change results in floating point.
+
 ``fast``
-   Fast - Allow algebraically equivalent transformations that may
-   dramatically change results in floating point (e.g. reassociate). This
-   flag implies all the others.
+   This flag implies all of the others.
 
 .. _uselistorder:
 
@@ -10483,7 +10490,7 @@ Syntax:
 """""""
 
 This is an overloaded intrinsic. You can use ``llvm.sqrt`` on any
-floating point or vector of floating point type. Not all targets support
+floating-point or vector of floating-point type. Not all targets support
 all types however.
 
 ::
@@ -10497,20 +10504,22 @@ all types however.
 Overview:
 """""""""
 
-The '``llvm.sqrt``' intrinsics return the square root of the specified value,
-returning the same value as the libm '``sqrt``' functions would, but without
-trapping or setting ``errno``.
+The '``llvm.sqrt``' intrinsics return the square root of the specified value.
 
 Arguments:
 """"""""""
 
-The argument and return value are floating point numbers of the same type.
+The argument and return value are floating-point numbers of the same type.
 
 Semantics:
 """"""""""
 
-This function returns the square root of the operand if it is a nonnegative
-floating point number.
+Return the same value as a corresponding libm '``sqrt``' function but without
+trapping or setting ``errno``. For types specified by IEEE-754, the result 
+matches a conforming libm implementation.
+
+When specified with the fast-math-flag 'afn', the result may be approximated 
+using a less accurate calculation.
 
 '``llvm.powi.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -10557,7 +10566,7 @@ Syntax:
 """""""
 
 This is an overloaded intrinsic. You can use ``llvm.sin`` on any
-floating point or vector of floating point type. Not all targets support
+floating-point or vector of floating-point type. Not all targets support
 all types however.
 
 ::
@@ -10576,14 +10585,16 @@ The '``llvm.sin.*``' intrinsics return the sine of the operand.
 Arguments:
 """"""""""
 
-The argument and return value are floating point numbers of the same type.
+The argument and return value are floating-point numbers of the same type.
 
 Semantics:
 """"""""""
 
-This function returns the sine of the specified operand, returning the
-same values as the libm ``sin`` functions would, and handles error
-conditions in the same way.
+Return the same value as a corresponding libm '``sin``' function but without
+trapping or setting ``errno``.
+
+When specified with the fast-math-flag 'afn', the result may be approximated 
+using a less accurate calculation.
 
 '``llvm.cos.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -10592,7 +10603,7 @@ Syntax:
 """""""
 
 This is an overloaded intrinsic. You can use ``llvm.cos`` on any
-floating point or vector of floating point type. Not all targets support
+floating-point or vector of floating-point type. Not all targets support
 all types however.
 
 ::
@@ -10611,14 +10622,16 @@ The '``llvm.cos.*``' intrinsics return the cosine of the operand.
 Arguments:
 """"""""""
 
-The argument and return value are floating point numbers of the same type.
+The argument and return value are floating-point numbers of the same type.
 
 Semantics:
 """"""""""
 
-This function returns the cosine of the specified operand, returning the
-same values as the libm ``cos`` functions would, and handles error
-conditions in the same way.
+Return the same value as a corresponding libm '``cos``' function but without
+trapping or setting ``errno``.
+
+When specified with the fast-math-flag 'afn', the result may be approximated 
+using a less accurate calculation.
 
 '``llvm.pow.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -10627,7 +10640,7 @@ Syntax:
 """""""
 
 This is an overloaded intrinsic. You can use ``llvm.pow`` on any
-floating point or vector of floating point type. Not all targets support
+floating-point or vector of floating-point type. Not all targets support
 all types however.
 
 ::
@@ -10647,15 +10660,16 @@ specified (positive or negative) power.
 Arguments:
 """"""""""
 
-The second argument is a floating point power, and the first is a value
-to raise to that power.
+The arguments and return value are floating-point numbers of the same type.
 
 Semantics:
 """"""""""
 
-This function returns the first value raised to the second power,
-returning the same values as the libm ``pow`` functions would, and
-handles error conditions in the same way.
+Return the same value as a corresponding libm '``pow``' function but without
+trapping or setting ``errno``.
+
+When specified with the fast-math-flag 'afn', the result may be approximated 
+using a less accurate calculation.
 
 '``llvm.exp.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -10664,7 +10678,7 @@ Syntax:
 """""""
 
 This is an overloaded intrinsic. You can use ``llvm.exp`` on any
-floating point or vector of floating point type. Not all targets support
+floating-point or vector of floating-point type. Not all targets support
 all types however.
 
 ::
@@ -10684,13 +10698,16 @@ value.
 Arguments:
 """"""""""
 
-The argument and return value are floating point numbers of the same type.
+The argument and return value are floating-point numbers of the same type.
 
 Semantics:
 """"""""""
 
-This function returns the same values as the libm ``exp`` functions
-would, and handles error conditions in the same way.
+Return the same value as a corresponding libm '``exp``' function but without
+trapping or setting ``errno``.
+
+When specified with the fast-math-flag 'afn', the result may be approximated 
+using a less accurate calculation.
 
 '``llvm.exp2.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -10699,7 +10716,7 @@ Syntax:
 """""""
 
 This is an overloaded intrinsic. You can use ``llvm.exp2`` on any
-floating point or vector of floating point type. Not all targets support
+floating-point or vector of floating-point type. Not all targets support
 all types however.
 
 ::
@@ -10719,13 +10736,16 @@ specified value.
 Arguments:
 """"""""""
 
-The argument and return value are floating point numbers of the same type.
+The argument and return value are floating-point numbers of the same type.
 
 Semantics:
 """"""""""
 
-This function returns the same values as the libm ``exp2`` functions
-would, and handles error conditions in the same way.
+Return the same value as a corresponding libm '``exp2``' function but without
+trapping or setting ``errno``.
+
+When specified with the fast-math-flag 'afn', the result may be approximated 
+using a less accurate calculation.
 
 '``llvm.log.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -10734,7 +10754,7 @@ Syntax:
 """""""
 
 This is an overloaded intrinsic. You can use ``llvm.log`` on any
-floating point or vector of floating point type. Not all targets support
+floating-point or vector of floating-point type. Not all targets support
 all types however.
 
 ::
@@ -10754,13 +10774,16 @@ value.
 Arguments:
 """"""""""
 
-The argument and return value are floating point numbers of the same type.
+The argument and return value are floating-point numbers of the same type.
 
 Semantics:
 """"""""""
 
-This function returns the same values as the libm ``log`` functions
-would, and handles error conditions in the same way.
+Return the same value as a corresponding libm '``log``' function but without
+trapping or setting ``errno``.
+
+When specified with the fast-math-flag 'afn', the result may be approximated 
+using a less accurate calculation.
 
 '``llvm.log10.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -10769,7 +10792,7 @@ Syntax:
 """""""
 
 This is an overloaded intrinsic. You can use ``llvm.log10`` on any
-floating point or vector of floating point type. Not all targets support
+floating-point or vector of floating-point type. Not all targets support
 all types however.
 
 ::
@@ -10789,13 +10812,16 @@ specified value.
 Arguments:
 """"""""""
 
-The argument and return value are floating point numbers of the same type.
+The argument and return value are floating-point numbers of the same type.
 
 Semantics:
 """"""""""
 
-This function returns the same values as the libm ``log10`` functions
-would, and handles error conditions in the same way.
+Return the same value as a corresponding libm '``log10``' function but without
+trapping or setting ``errno``.
+
+When specified with the fast-math-flag 'afn', the result may be approximated 
+using a less accurate calculation.
 
 '``llvm.log2.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -10804,7 +10830,7 @@ Syntax:
 """""""
 
 This is an overloaded intrinsic. You can use ``llvm.log2`` on any
-floating point or vector of floating point type. Not all targets support
+floating-point or vector of floating-point type. Not all targets support
 all types however.
 
 ::
@@ -10824,13 +10850,16 @@ value.
 Arguments:
 """"""""""
 
-The argument and return value are floating point numbers of the same type.
+The argument and return value are floating-point numbers of the same type.
 
 Semantics:
 """"""""""
 
-This function returns the same values as the libm ``log2`` functions
-would, and handles error conditions in the same way.
+Return the same value as a corresponding libm '``log2``' function but without
+trapping or setting ``errno``.
+
+When specified with the fast-math-flag 'afn', the result may be approximated 
+using a less accurate calculation.
 
 '``llvm.fma.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -10839,7 +10868,7 @@ Syntax:
 """""""
 
 This is an overloaded intrinsic. You can use ``llvm.fma`` on any
-floating point or vector of floating point type. Not all targets support
+floating-point or vector of floating-point type. Not all targets support
 all types however.
 
 ::
@@ -10853,20 +10882,21 @@ all types however.
 Overview:
 """""""""
 
-The '``llvm.fma.*``' intrinsics perform the fused multiply-add
-operation.
+The '``llvm.fma.*``' intrinsics perform the fused multiply-add operation.
 
 Arguments:
 """"""""""
 
-The argument and return value are floating point numbers of the same
-type.
+The arguments and return value are floating-point numbers of the same type.
 
 Semantics:
 """"""""""
 
-This function returns the same values as the libm ``fma`` functions
-would, and does not set errno.
+Return the same value as a corresponding libm '``fma``' function but without
+trapping or setting ``errno``.
+
+When specified with the fast-math-flag 'afn', the result may be approximated 
+using a less accurate calculation.
 
 '``llvm.fabs.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^