[llvm-bugs] [Bug 27022] New: Using -freg-struct-return and -fno-reg-struct-return compiles can fail

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27022

            Bug ID: 27022
           Summary: Using -freg-struct-return and -fno-reg-struct-return
                    compiles can fail
           Product: clang
           Version: 3.7
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: wink at saville.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

When I compile any program using clang 3.7.1 with -m64 and -freg-struct-return:

$ clang --version
clang version 3.7.1 (tags/RELEASE_371/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix

$ cat t.c
int main(void) {
  return 0;
}

I get the error unsupported option even though the compiler is generating the
expected code as defined by System V ABI by default, so this should be trivial
to support:

$ clang -m64 -freg-struct-return t.c
clang-3.7: error: unsupported option '-freg-struct-return' for target
'x86_64-unknown-linux-gnu'


It would also be nice if the -fno-reg-struct-return worked and didn't do the
optimization, but it also complains that the option isn't supported:

$ clang -m64 -fno-reg-struct-return t.c
clang-3.7: error: unsupported option '-fno-reg-struct-return' for target
'x86_64-unknown-linux-gnu'


When compiling 32bit application the -freg-struct-return works and generates
reasonable code:

$ clang -m32 -freg-struct-return t.c
$

But the -fno-reg-struct-return doesn't work, and it too should be trivial as it
is the default case for -m32:

$ clang -m32 -fno-reg-struct-return t.c
clang-3.7: error: unknown argument: '-fno-reg-struct-return'


Note, all combination are accepted with gcc:

$ gcc -m64 -freg-struct-return t.c
$ gcc -m64 -fno-reg-struct-return t.c
$ gcc -m32 -freg-struct-return t.c
$ gcc -m32 -fno-reg-struct-return t.c
$


It should also be noted that in a performance test I created comparing how to
return two values from a function I found that gcc was generating faster code.
This maybe because my test, but I thought I'd note it here.

Here is a link to the performance test:
https://github.com/winksaville/ret_two_values

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