powerpc64 parameter passing of small structs does not match PPC64 ELF ABI

[llvmbot]

Bugzilla Link	13627
Resolution	FIXED
Resolved on	Sep 19, 2012 10:52
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor

Extended Description

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 G#4 , 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 G#4 :

.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

[llvmbot]

mentioned in issue llvm/llvm-bugzilla-archive#13949

[llvmbot]

Fixed under r164228.

[llvmbot]

Unions are likely padded on the right instead of the left when passed in registers. Example code that shows the problem:

union x { char c; short s; int j; };

union x test_union_1 (union x foo) { foo.c++; foo.s++; foo.j++; return foo; }