[LLVMbugs] [Bug 11688] New: i386-pc-win32 struct by value return does not match MSVC calling convention

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=11688

             Bug #: 11688
           Summary: i386-pc-win32 struct by value return does not match
                    MSVC calling convention
           Product: clang
           Version: 3.0
          Platform: PC
        OS/Version: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: arcata at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


When targeting i?86-pc-win32, Clang returns all structs by hidden pointer,
although MSVC returns structs sized {8,4,3,2,1} bytes in registers, as
documented at http://msdn.microsoft.com/en-us/library/984x0h58.aspx .
A simple test case:

---
#include <stdint.h>

struct Struct1 {
   uint64_t a;
};

struct Struct2 {
   uint32_t a;
};

struct Struct3 {
   char a, b, c;
};

struct Struct1 c_return_1(void) {
   struct Struct1 r = { 0xC1A4C1A4C1A4C1A4ULL };
   return r;
}

struct Struct2 c_return_2(void) {
   struct Struct2 r = { 0xC1A4C1A4U };
   return r;
}

struct Struct3 c_return_3(void) {
   struct Struct3 r = { 0xAA, 0xBB, 0xCC };
   return r;
}
---

cl.exe generates the following asm for c_return_1:

---
_c_return_1 PROC
; File
c:\users\joe\documents\code\clay\test\externals\abi\common\external_test1.c
; Line 15
       push    ebp
       mov     ebp, esp
       sub     esp, 8
; Line 16
       mov     DWORD PTR _r$[ebp], -1046167132         ; c1a4c1a4H
       mov     DWORD PTR _r$[ebp+4], -1046167132       ; c1a4c1a4H
; Line 17
       mov     eax, DWORD PTR _r$[ebp]
       mov     edx, DWORD PTR _r$[ebp+4]
; Line 18
       mov     esp, ebp
       pop     ebp
       ret     0
_c_return_1 ENDP
---

You can see under "Line 17" that it moves the return value into
edx:eax. By contrast, clang expects a hidden pointer argument:

---
_c_return_1:                            # @c_return_1
# BB#0:
       subl    $12, %esp
       movl    16(%esp), %eax
       movsd   L_c_return_1.r, %xmm0
       movsd   %xmm0, (%esp)
       movsd   %xmm0, (%eax)
       addl    $12, %esp
       ret     $4
---

Similar discrepancies occur for c_return_2 and c_return_3.

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