[LLVMbugs] [Bug 13627] New: powerpc64 parameter passing of small structs does not match PPC64 ELF ABI

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=13627

             Bug #: 13627
           Summary: powerpc64 parameter passing of small structs does not
                    match PPC64 ELF ABI
           Product: new-bugs
           Version: trunk
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: wschmidt at linux.vnet.ibm.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


On powerpc64-unknown-linux-gnu, passing of structs smaller than 8 bytes is not
always compatible with the 64-bit PowerPC ELF Application Binary Interface
Supplement (see
http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html).  From
section 3.2.3: "An aggregate or union smaller than one doubleword in size is
padded so that it appears in the least significant bits of the doubleword.  All
others are padded, if necessary, at their tail."  So structs smaller than 8
bytes will be passed "right-adjusted" in a GPR.

Consider:

struct s {
  char c;
  short d;
};

struct s
test_struct_2 (struct s foo)
{
  foo.c++;
  foo.d++;
  return foo;
}

LLVM generates:

.L.test_struct_2:
    std 4, 56(1)
    lbz 5, 56(1)    <-- c is at leftmost position in GPR4, should be 60(1)
    addi 5, 5, 1
    stb 5, 56(1)
    addi 4, 1, 56   <-- d is four bytes left of correct, should be 60 here
    lhz 5, 2(4)
    addi 5, 5, 1
    sth 5, 2(4)
    lwz 5, 56(1)
    sth 5, 2(3)
    srwi 5, 5, 16
    sth 5, 0(3)
    std 3, -8(1)
    blr 

Note that LLVM gets it right if "d" is omitted from the struct.  "c" is then
passed rightmost in GPR4:

.L.test_struct_1:
    stb 4, 59(1)
    lbz 5, 59(1)
    addi 5, 5, 1
    stb 5, 59(1)
    lbz 5, 59(1)
    stb 5, 0(3)
    addi 4, 1, 59
    std 4, -8(1)
    std 3, -16(1)
    blr

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