[PATCH] D67360: [FPEnv] Document that constrained FP intrinsics cannot be mixed with non-constrained

[Kevin P. Neal via Phabricator via llvm-commits]

kpn updated this revision to Diff 220095.
kpn added a comment.

Incorporate changes and text requested by Andrew Kaylor. Remove mention of the lack of barriers and include text on how to accomplish mixing without barriers.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D67360/new/

https://reviews.llvm.org/D67360

Files:
  docs/LangRef.rst


Index: docs/LangRef.rst
===================================================================
--- docs/LangRef.rst
+++ docs/LangRef.rst
@@ -15051,12 +15051,21 @@
 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:
 
 ::
 
@@ -15086,9 +15095,9 @@
 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:
 
 ::
 


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D67360.220095.patch
Type: text/x-patch
Size: 1996 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20190913/1d72bbac/attachment.bin>