[llvm-bugs] [Bug 42394] New: The preserve_all and preserve_most calling conventions on X86 trash the return value.

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42394

            Bug ID: 42394
           Summary: The preserve_all and preserve_most calling conventions
                    on X86 trash the return value.
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: sunil_srivastava at playstation.sony.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

The preserve_all and preserve_most calling convention attributes are meant to
save and restore more than just the normal callee-save set of registers. 

However, at least on x86,  they seem to save and restore the return register
also, thus trashing the return value.

//--------------------
#include <stdio.h>
int __attribute__((preserve_most)) foo(int x) { return x+1; }
int main() { printf("%d %d\n", foo(1), foo(2)); }
//--------------------

$ clang -o p.out p.cpp
$ .\p.out
4195616 4195616

Interestingly, with -O2, the result is correct because foo gets inlined. If the
caller and the callee are in two different files, -O2 shows the same problem.

The assembly code shows clearly what is happening:

_Z3fooi:                                # @_Z3fooi
        pushq   %rdi
        pushq   %rax
        leal    1(%rdi), %eax
        popq    %rax           <--- return value trashed
        popq    %rdi
        retq

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