[llvm] [IR] LangRef: state explicitly that floats generally behave according to IEEE-754 (PR #102140)

[Matt Arsenault via llvm-commits]

================
@@ -3657,6 +3661,40 @@ specification on some architectures:
   LLVM does not correctly represent this. See `issue #60796
   <https://github.com/llvm/llvm-project/issues/60796>`_.
 
+.. _floatsem:
+
+Floating-Point Semantics
+------------------------
+
+This section defines the semantics for core floating-point operations on types
+that use a format specified by IEEE-745. These types are: ``half``, ``float``,
+``double``, and ``fp128``, which correspond to the binary16, binary32, binary64,
+and binary128 formats, respectively. The "core" operations are those defined in
+section 5 of IEEE-745, which all have corresponding LLVM operations.
+
+The value returned by those operations matches that of the corresponding
+IEEE-754 operation executed in the :ref:`default LLVM floating-point environment
+<floatenv>`, except that the behavior of NaN results is instead :ref:`as
+specified here <floatnan>`. In particular, such a floating-point instruction
+returning a non-NaN value is guaranteed to always return the same bit-identical
+result on all machines and optimization levels.
+
+This means that optimizations and backends may not change the observed bitwise
+result of these operations in any way (unless NaNs are returned), and frontends
+can rely on these operations providing perfectly rounded results as described in
----------------
arsenm wrote:

Term is usually "correctly rounded" not "perfectly rounded"

https://github.com/llvm/llvm-project/pull/102140