[llvm-bugs] [Bug 39251] New: Return of structs in C calling convention is done on stack, incompatible with avr-gcc usually using registers

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39251

            Bug ID: 39251
           Summary: Return of structs in C calling convention is done on
                    stack, incompatible with avr-gcc usually using
                    registers
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: AVR
          Assignee: unassignedbugs at nondot.org
          Reporter: me at dylanmckay.io
                CC: llvm-bugs at lists.llvm.org

Created attachment 20989
  --> https://bugs.llvm.org/attachment.cgi?id=20989&action=edit
the test C file I wrote containing structures of different sizes

The AVR-GCC documentation states that

   | Return values with a size of 1 byte up to and including a size of
   | 8 bytes will be returned in registers. Return values whose size
   | is outside that range will be returned in memory. 

     https://gcc.gnu.org/wiki/avr-gcc#Calling_Convention

However, this does not seem to be the case when the return value is a struct.
>From experimental tests, I've seen that avr-gcc will return the struct via
registers if and only if the struct is less than or equal to four bytes in
size. Any struct with sizeof(self) > 5 will be returned on the stack with
AVR-GCC.

Currently, AVR-LLVM does not place ANY struct return values in registers. They
all go to the stack.

We need to modify the AVR backend so that it matches AVR-GCC's behaviour -
structures of four bytes or less should be assigned to registers, whereas
structures larger than this should be relegated to stack.

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20181011/7ee5ef58/attachment.html>