[FPEnv] Document that constrained FP intrinsics cannot be mixed with non-constrained

Reviewed by:	andrew.w.kaylor, cameron.mcinally, uweigand
Approved by:	andrew.w.kaylor
Differential Revision:	https://reviews.llvm.org/D67360

llvm-svn: 371888

Author: kpneal

llvm/docs/LangRef.rst

@@ -15064,12 +15064,21 @@ Constrained FP intrinsics are used to support non-default rounding modes and
 accurately preserve exception behavior without compromising LLVM's ability to
 optimize FP code when the default behavior is used.
 
-Each of these intrinsics corresponds to a normal floating-point operation.  The
-first two arguments and the return value are the same as the corresponding FP
+If any FP operation in a function is constrained then they all must be
+constrained. This is required for correct LLVM IR. Optimizations that
+move code around can create miscompiles if mixing of constrained and normal
+operations is done. The correct way to mix constrained and less constrained
+operations is to use the rounding mode and exception handling metadata to
+mark constrained intrinsics as having LLVM's default behavior.
+
+Each of these intrinsics corresponds to a normal floating-point operation. The
+data arguments and the return value are the same as the corresponding FP
 operation.
 
-The third argument is a metadata argument specifying the rounding mode to be
-assumed. This argument must be one of the following strings:
+The rounding mode argument is a metadata string specifying what 
+assumptions, if any, the optimizer can make when transforming constant 
+values. Some constrained FP intrinsics omit this argument. If required 
+by the intrinsic, this argument must be one of the following strings:
 
 ::
 
@@ -15099,9 +15108,9 @@ the specified rounding mode, but this is not guaranteed.  Using a specific
 non-dynamic rounding mode which does not match the actual rounding mode at
 runtime results in undefined behavior.
 
-The fourth argument to the constrained floating-point intrinsics specifies the
-required exception behavior.  This argument must be one of the following
-strings:
+The exception behavior argument is a metadata string describing the floating
+point exception semantics that required for the intrinsic. This argument
+must be one of the following strings:
 
 ::