[Docs] Documentation for the saturation addition and subtraction intrinsics

[leonardchan]

Summary

Patches for addition: https://reviews.llvm.org/D53053, https://reviews.llvm.org/D53340
Patches for subtraction: https://reviews.llvm.org/D53783

[bjope]

LGTM!

[bjope]

llvm/docs/LangRef.rst

12579 ↗

(On Diff #174690)

nit: As I also wrote in https://reviews.llvm.org/D54719 we will get a tiny problem when adding support for smul.fix.sat if we have one chapter called "Saturation Arithmetic Intrinsics" and one called "Fixed Point Arithmetic Intrinsics" in the LangRef. Maybe we can solve that by describing smul.fix.sat as a "Fixed Point Arithmetic Intrinsics" and maybe add a reference or something in this chapter. Or should all these saturation/fixedpoint intrinsics be in a common chapter?

[leonardchan]

llvm/docs/LangRef.rst

12579 ↗

(On Diff #174690)

I'd be up for either. My initial plan was what your first suggestion was of putting it under the Fixed Point section with a reference or some mention to the Saturation section.

[JosephTremoulet]

Thanks!

llvm/trunk/docs/LangRef.rst

 
 .. code-block:: llvm
 
       %res = call {i32, i1} @llvm.umul.with.overflow.i32(i32 %a, i32 %b)
       %sum = extractvalue {i32, i1} %res, 0
       %obit = extractvalue {i32, i1} %res, 1
       br i1 %obit, label %overflow, label %normal
 
+Saturation Arithmetic Intrinsics
+---------------------------------
+
+Saturation arithmetic is a version of arithmetic in which operations are
+limited to a fixed range between a minimum and maximum value. If the result of
+an operation is greater than the maximum value, the result is set (or
+"clamped") to this maximum. If it is below the minimum, it is clamped to this
+minimum.
+
+
+'``llvm.sadd.sat.*``' Intrinsics
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax
+"""""""
+
+This is an overloaded intrinsic. You can use ``llvm.sadd.sat``
+on any integer bit width or vectors of integers.
+
+::
+
+      declare i16 @llvm.sadd.sat.i16(i16 %a, i16 %b)
+      declare i32 @llvm.sadd.sat.i32(i32 %a, i32 %b)
+      declare i64 @llvm.sadd.sat.i64(i64 %a, i64 %b)
+      declare <4 x i32> @llvm.sadd.sat.v4i32(<4 x i32> %a, <4 x i32> %b)
+
+Overview
+"""""""""
+
+The '``llvm.sadd.sat``' family of intrinsic functions perform signed
+saturation addition on the 2 arguments.
+
+Arguments
+""""""""""
+
+The arguments (%a and %b) and the result may be of integer types of any bit
+width, but they must have the same bit width. ``%a`` and ``%b`` are the two
+values that will undergo signed addition.
+
+Semantics:
+""""""""""
+
+The maximum value this operation can clamp to is the largest signed value
+representable by the bit width of the arguments. The minimum value is the
+smallest signed value representable by this bit width.
+
+
+Examples
+"""""""""
+
+.. code-block:: llvm
+
+      %res = call i4 @llvm.sadd.sat.i4(i4 1, i4 2)  ; %res = 3
+      %res = call i4 @llvm.sadd.sat.i4(i4 5, i4 6)  ; %res = 7
+      %res = call i4 @llvm.sadd.sat.i4(i4 -4, i4 2)  ; %res = -2
+      %res = call i4 @llvm.sadd.sat.i4(i4 -4, i4 -5)  ; %res = -8
+
+
+'``llvm.uadd.sat.*``' Intrinsics
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax
+"""""""
+
+This is an overloaded intrinsic. You can use ``llvm.uadd.sat``
+on any integer bit width or vectors of integers.
+
+::
+
+      declare i16 @llvm.uadd.sat.i16(i16 %a, i16 %b)
+      declare i32 @llvm.uadd.sat.i32(i32 %a, i32 %b)
+      declare i64 @llvm.uadd.sat.i64(i64 %a, i64 %b)
+      declare <4 x i32> @llvm.uadd.sat.v4i32(<4 x i32> %a, <4 x i32> %b)
+
+Overview
+"""""""""
+
+The '``llvm.uadd.sat``' family of intrinsic functions perform unsigned
+saturation addition on the 2 arguments.
+
+Arguments
+""""""""""
+
+The arguments (%a and %b) and the result may be of integer types of any bit
+width, but they must have the same bit width. ``%a`` and ``%b`` are the two
+values that will undergo unsigned addition.
+
+Semantics:
+""""""""""
+
+The maximum value this operation can clamp to is the largest unsigned value
+representable by the bit width of the arguments. Because this is an unsigned
+operation, the result will never saturate towards zero.
+
+
+Examples
+"""""""""
+
+.. code-block:: llvm
+
+      %res = call i4 @llvm.uadd.sat.i4(i4 1, i4 2)  ; %res = 3
+      %res = call i4 @llvm.uadd.sat.i4(i4 5, i4 6)  ; %res = 11
+      %res = call i4 @llvm.uadd.sat.i4(i4 8, i4 8)  ; %res = 15
+
+
+'``llvm.ssub.sat.*``' Intrinsics
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax
+"""""""
+
+This is an overloaded intrinsic. You can use ``llvm.ssub.sat``
+on any integer bit width or vectors of integers.
+
+::
+
+      declare i16 @llvm.ssub.sat.i16(i16 %a, i16 %b)
+      declare i32 @llvm.ssub.sat.i32(i32 %a, i32 %b)
+      declare i64 @llvm.ssub.sat.i64(i64 %a, i64 %b)
+      declare <4 x i32> @llvm.ssub.sat.v4i32(<4 x i32> %a, <4 x i32> %b)
+
+Overview
+"""""""""
+
+The '``llvm.ssub.sat``' family of intrinsic functions perform signed
+saturation subtraction on the 2 arguments.
+
+Arguments
+""""""""""
+
+The arguments (%a and %b) and the result may be of integer types of any bit
+width, but they must have the same bit width. ``%a`` and ``%b`` are the two
+values that will undergo signed subtraction.
+
+Semantics:
+""""""""""
+
+The maximum value this operation can clamp to is the largest signed value
+representable by the bit width of the arguments. The minimum value is the
+smallest signed value representable by this bit width.
+
+
+Examples
+"""""""""
+
+.. code-block:: llvm
+
+      %res = call i4 @llvm.ssub.sat.i4(i4 2, i4 1)  ; %res = 1
+      %res = call i4 @llvm.ssub.sat.i4(i4 2, i4 6)  ; %res = -4
+      %res = call i4 @llvm.ssub.sat.i4(i4 -4, i4 5)  ; %res = -8
+      %res = call i4 @llvm.ssub.sat.i4(i4 4, i4 -5)  ; %res = 7
+
+
+'``llvm.usub.sat.*``' Intrinsics
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax
+"""""""
+
+This is an overloaded intrinsic. You can use ``llvm.usub.sat``
+on any integer bit width or vectors of integers.
+
+::
+
+      declare i16 @llvm.usub.sat.i16(i16 %a, i16 %b)
+      declare i32 @llvm.usub.sat.i32(i32 %a, i32 %b)
+      declare i64 @llvm.usub.sat.i64(i64 %a, i64 %b)
+      declare <4 x i32> @llvm.usub.sat.v4i32(<4 x i32> %a, <4 x i32> %b)
+
+Overview
+"""""""""
+
+The '``llvm.usub.sat``' family of intrinsic functions perform unsigned
+saturation subtraction on the 2 arguments.
+
+Arguments
+""""""""""
+
+The arguments (%a and %b) and the result may be of integer types of any bit
+width, but they must have the same bit width. ``%a`` and ``%b`` are the two
+values that will undergo unsigned subtraction.
+
+Semantics:
+""""""""""
+
+The minimum value this operation can clamp to is 0, which is the smallest
+unsigned value representable by the bit width of the unsigned arguments.
+Because this is an unsigned operation, the result will never saturate towards
+the largest possible value representable by this bit width.
+
+
+Examples
+"""""""""
+
+.. code-block:: llvm
+
+      %res = call i4 @llvm.usub.sat.i4(i4 2, i4 1)  ; %res = 1
+      %res = call i4 @llvm.usub.sat.i4(i4 2, i4 6)  ; %res = 0
+
+
 Specialised Arithmetic Intrinsics
 ---------------------------------
 
 '``llvm.canonicalize.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Syntax:
 """""""