PR17164: Change clang's default behavior from -flax-vector-conversions=all to -flax-vector-conversions=integer.

Summary:
See proposal on cfe-dev:
http://lists.llvm.org/pipermail/cfe-dev/2019-April/062030.html

Reviewers: SjoerdMeijer, eli.friedman

Subscribers: kristof.beyls, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67678

(cherry picked from commit b72a8c65e4e34779b6bc9e466203f553f5294486)

2 files changed, 34 insertions, 1 deletions

diff --git a/clang/docs/CommandGuide/clang.rst b/clang/docs/CommandGuide/clang.rst
index 7b0873600fc..6947450beb4 100644
--- a/clang/docs/CommandGuide/clang.rst
+++ b/clang/docs/CommandGuide/clang.rst
@@ -278,9 +278,18 @@ Language Selection and Mode Options
  Make all string literals default to writable.  This disables uniquing of
  strings and other optimizations.
 
-.. option:: -flax-vector-conversions
+.. option:: -flax-vector-conversions, -flax-vector-conversions=<kind>, -fno-lax-vector-conversions
 
  Allow loose type checking rules for implicit vector conversions.
+ Possible values of <kind>:
+
+ - ``none``: allow no implicit conversions between vectors
+ - ``integer``: allow implicit bitcasts between integer vectors of the same
+   overall bit-width
+ - ``all``: allow implicit bitcasts between any vectors of the same
+   overall bit-width
+
+ <kind> defaults to ``integer`` if unspecified.
 
 .. option:: -fblocks
 
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 881dcb78b66..8c48724e7c6 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -102,6 +102,15 @@ Non-comprehensive list of changes in this release
   the found gcc installation is older than 4.7.0. Add ``-fno-use-init-array`` to
   get the old behavior (``.ctors``).
 
+* Lax vector conversions involving floating-point vectors have been disabled
+  by default, and can no longer be enabled with ``-flax-vector-conversions``.
+  This matches the behavior of these flags in GCC, but code relying on implicit
+  vector bitcasts between integer and floating-point types that used to compile
+  with older versions of Clang is no longer accepted by default in Clang 10.
+  The old behavior can be restored with ``-flax-vector-conversions=all``.
+  In a future release of Clang, we intend to change the default to
+  ``-fno-lax-vector-conversions``.
+
 New Compiler Flags
 ------------------
 
@@ -132,6 +141,21 @@ Modified Compiler Flags
 - RISC-V now sets the architecture (riscv32/riscv64) based on the value provided
   to the ``-march`` flag, overriding the target provided by ``-triple``.
 
+- ``-flax-vector-conversions`` has been split into three different levels of
+  laxness:
+
+  - ``-flax-vector-conversions=all``: This is Clang's historical default, and
+    permits implicit vector conversions (performed as bitcasts) between any
+    two vector types of the same overall bit-width.
+
+  - ``-flax-vector-conversions=integer``: This is Clang's current default,
+    and permits implicit vector conversions (performed as bitcasts) between
+    any two integer vector types of the same overall bit-width.
+    Synonym: ``-flax-vector-conversions``.
+
+  - ``-flax-vector-conversions=none``: Do not perform any implicit bitcasts
+    between vector types. Synonym: ``-fno-lax-vector-conversions``.
+
 New Pragmas in Clang
 --------------------