[llvm-bugs] [Bug 43666] New: -mlong-double-64/80/128 documentation incomplete

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https://bugs.llvm.org/show_bug.cgi?id=43666

            Bug ID: 43666
           Summary: -mlong-double-64/80/128 documentation incomplete
           Product: Documentation
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: General docs
          Assignee: unassignedbugs at nondot.org
          Reporter: tydeman at tybor.com
                CC: llvm-bugs at lists.llvm.org

Assuming that clang is implementing the same as gcc, the documentation of
-mlong-double-64/80/128 in
https://clang.llvm.org/docs/ClangCommandLineReference.html fails to mention
that changing the size of long double does NOT change the ABI.  Therefore,
calls to functions that use long doubles, eg, <math.h>, scanf(), printf(), ...,
will produce strange results.

gcc documentation has: 
Warning: if you override the default value for your target ABI,
           this changes the size of structures and arrays containing "long
           double" variables, as well as modifying the function calling
           convention for functions taking "long double".  Hence they are
           not binary-compatible with code compiled without that switch.

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