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: [email protected]
ReportedBy: [email protected]
CC: [email protected]
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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