[HardwareLoops] LangRef Intrinsic descriptions

The HardwareLoop intrinsics were missing and not described in LangRef. This
adds these descriptions/definitions.

Differential Revision: https://reviews.llvm.org/D80316

docs/LangRef.rst

@@ -14871,6 +14871,169 @@ Examples:
       %r2 = call float @llvm.fmuladd.f32(float %a, float %b, float %c) ; yields float:r2 = (a * b) + c
 
 
+Hardware-Loop Intrinsics
+------------------------
+
+LLVM support several intrinsics to mark a loop as a hardware-loop. They are
+hints to the backend which are required to lower these intrinsics further to target
+specific instructions, or revert the hardware-loop to a normal loop if target
+specific restriction are not met and a hardware-loop can't be generated.
+
+These intrinsics may be modified in the future and are not intended to be used
+outside the backend. Thus, front-end and mid-level optimizations should not be
+generating these intrinsics.
+
+
+'``llvm.set.loop.iterations.*``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+This is an overloaded intrinsic.
+
+::
+
+      declare void @llvm.set.loop.iterations.i32(i32)
+      declare void @llvm.set.loop.iterations.i64(i64)
+
+Overview:
+"""""""""
+
+The '``llvm.set.loop.iterations.*``' intrinsics are used to specify the
+hardware-loop trip count. They are placed in the loop preheader basic block and
+are marked as ``IntrNoDuplicate`` to avoid optimizers duplicating these
+instructions.
+
+Arguments:
+""""""""""
+
+The integer operand is the loop trip count of the hardware-loop, and thus
+not e.g. the loop back-edge taken count.
+
+Semantics:
+""""""""""
+
+The '``llvm.set.loop.iterations.*``' intrinsics do not perform any arithmetic
+on their operand. It's a hint to the backend that can use this to set up the
+hardware-loop count with a target specific instruction, usually a move of this
+value to a special register or a hardware-loop instruction.
+
+'``llvm.test.set.loop.iterations.*``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+This is an overloaded intrinsic.
+
+::
+
+      declare void @llvm.test.set.loop.iterations.i32(i32)
+      declare void @llvm.test.set.loop.iterations.i64(i64)
+
+Overview:
+"""""""""
+
+The '``llvm.test.set.loop.iterations.*``' intrinsics are used to specify the
+the loop trip count, and also test that the given count is not zero, allowing
+it to control entry to a while-loop.  They are placed in the loop preheader's
+predecessor basic block, and are marked as ``IntrNoDuplicate`` to avoid
+optimizers duplicating these instructions.
+
+Arguments:
+""""""""""
+
+The integer operand is the loop trip count of the hardware-loop, and thus
+not e.g. the loop back-edge taken count.
+
+Semantics:
+""""""""""
+
+The '``llvm.test.set.loop.iterations.*``' intrinsics do not perform any
+arithmetic on their operand. It's a hint to the backend that can use this to
+set up the hardware-loop count with a target specific instruction, usually a
+move of this value to a special register or a hardware-loop instruction.
+
+'``llvm.loop.decrement.reg.*``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+This is an overloaded intrinsic.
+
+::
+
+      declare i32 @llvm.loop.decrement.reg.i32(i32, i32)
+      declare i64 @llvm.loop.decrement.reg.i64(i64, i64)
+
+Overview:
+"""""""""
+
+The '``llvm.loop.decrement.reg.*``' intrinsics are used to lower the loop
+iteration counter and return an updated value that will be used in the next
+loop test check.
+
+Arguments:
+""""""""""
+
+Both arguments must have identical integer types. The first operand is the
+loop iteration counter. The second operand is the maximum number of elements
+processed in an iteration.
+
+Semantics:
+""""""""""
+
+The '``llvm.loop.decrement.reg.*``' intrinsics do an integer ``SUB`` of its
+two operands, which is not allowed to wrap. They return the remaining number of
+iterations still to be executed, and can be used together with a ``PHI``,
+``ICMP`` and ``BR`` to control the number of loop iterations executed. Any
+optimisations are allowed to treat it is a ``SUB``, and it is supported by
+SCEV, so it's the backends responsibility to handle cases where it may be
+optimised. These intrinsics are marked as ``IntrNoDuplicate`` to avoid
+optimizers duplicating these instructions.
+
+
+'``llvm.loop.decrement.*``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+This is an overloaded intrinsic.
+
+::
+
+      declare i1 @llvm.loop.decrement.i32(i32)
+      declare i1 @llvm.loop.decrement.i64(i64)
+
+Overview:
+"""""""""
+
+The HardwareLoops pass allows the loop decrement value to be specified with an
+option. It defaults to a loop decrement value of 1, but it can be an unsigned
+integer value provided by this option.  The '``llvm.loop.decrement.*``'
+intrinsics decrement the loop iteration counter with this value, and return a
+false predicate if the loop should exit, and true otherwise.
+This is emitted if the loop counter is not updated via a ``PHI`` node, which
+can also be controlled with an option.
+
+Arguments:
+""""""""""
+
+The integer argument is the loop decrement value used to decrement the loop
+iteration counter.
+
+Semantics:
+""""""""""
+
+The '``llvm.loop.decrement.*``' intrinsics do a ``SUB`` of the loop iteration
+counter with the given loop decrement value, and return false if the loop
+should exit, this ``SUB`` is not allowed to wrap. The result is a condition
+that is used by the conditional branch controlling the loop.
+
+
 Experimental Vector Reduction Intrinsics
 ----------------------------------------