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

[Joshua Cranmer via llvm-commits]

================
@@ -3572,6 +3572,39 @@ 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>")``.
 
+.. _floatsem:
+
+Floating-Point Semantics
+------------------------
+
+LLVM floating-point types fall into two categories:
+
+- half, float, double, and fp128, which correspond to the binary16, binary32,
+  binary64, and binary128 formats described in the IEEE-754 specification.
+- The remaining types, which do not directly correspond to a standard IEEE
+  format.
+
+For floating-point operations acting on types with a corresponding IEEE format,
+unless otherwise specified the value returned by that operation matches that of
+the corresponding IEEE-754 operation executed in the :ref:`default
+floating-point environment <floatenv>`, except that the behavior of NaN results
+is instead :ref:`as specified here <floatnan>`. (This statement concerns only
+the returned *value*; we make no statement about status flags or
+traps/exceptions.) In particular, a floating-point instruction returning a
+non-NaN value is guaranteed to always return the same bit-identical result on
----------------
jcranmer-intel wrote:

This is something that is hard to come up with a good term for. IEEE 754 has a core list of operations in section 5 which is a good starting point, but these omit the minimum/maximum operations (which are section 9.6). Section 9 is "recommended operations", and 9.2 is the main list of transcendental functions you're thinking of; IEEE 754 requires that they be correctly rounded, but C explicitly disclaims that requirement in Annex F. There's also a few functions in C that aren't in IEEE 754, notably ldexp and frexp.

(Note too that it was recently brought up in the Discourse forums that the libm intrinsics are meant to correspond to libm semantics, not IEEE 754 semantics.)

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