[Driver, CodeGen] add options to enable/disable an FP cast optimization

As discussed in the post-commit thread for:
rL330437 ( http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20180423/545906.html )

We need a way to opt-out of a float-to-int-to-float cast optimization because too much 
existing code relies on the platform-specific undefined result of those casts when the 
float-to-int overflows.

The LLVM changes associated with adding this function attribute are here:
rL330947
rL330950
rL330951

Also as suggested, I changed the LLVM doc to mention the specific sanitizer flag that 
catches this problem:
rL330958

Differential Revision: https://reviews.llvm.org/D46135


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@331041 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 10 insertions, 0 deletions

diff --git a/docs/UsersManual.rst b/docs/UsersManual.rst
index 3653d0088e..7425d2ad97 100644
--- a/docs/UsersManual.rst
+++ b/docs/UsersManual.rst
@@ -1255,6 +1255,16 @@ are listed below.
    flushed-to-zero number is preserved in the sign of 0, denormals are
    flushed to positive zero, respectively.
 
+.. option:: -f[no-]fp-cast-overflow-workaround
+
+   Enable a workaround for code that casts floating-point values to 
+   integers and back to floating-point. If the floating-point value 
+   is not representable in the intermediate integer type, the code is
+   incorrect according to the language standard. This flag will attempt 
+   to generate code as if the result of an overflowing conversion matches
+   the overflowing behavior of a target's native float-to-int conversion
+   instructions.
+
 .. option:: -fwhole-program-vtables
 
    Enable whole-program vtable optimizations, such as single-implementation