system-clang (C) handles returning example struct based on it
being on the stack (-maix-struct-return style); gcc9 via
registers r3 and r4 (-msvr4-struct-return style). So this
somewhat tracks what was observed for the C++ ABI.
The evidence from on a old G4 PowerMac3,6 . . .
The source code:
# more just_struct.c
struct two {
int a,b;
};
struct two f(void) {
struct two r= { 0, 1};
return r;
}
# cc -std=c99 -pedantic -g -O2 -c just_struct.c
# objdump -d --prefix-addresses just_struct.o | more
just_struct.o: file format elf32-powerpc-freebsd
Disassembly of section .text:
00000000 <f> li r4,1
00000004 <f+0x4> stw r4,4(r3)
00000008 <f+0x8> li r4,0
0000000c <f+0xc> stw r4,0(r3)
00000010 <f+0x10> blr
So it expect r3 to point to the space the caller
provided, probably via stack space.
This appears to be -maix-struct-return style.
# /usr/local/bin/powerpc-unknown-freebsd13.0-gcc9 -std=c99 -pedantic -g -O2 -c
just_struct.c
# objdump -d --prefix-addresses just_struct.o | more
just_struct.o: file format elf32-powerpc-freebsd
Disassembly of section .text:
00000000 <f> li r3,0
00000004 <f+0x4> li r4,1
00000008 <f+0x8> blr
So it returned via register r3 and r4.
This appears to be -msvr4-struct-return style.
# gcc9 -std=c99 -pedantic -g -O2 -c just_struct.c
# objdump -d --prefix-addresses just_struct.o | more
just_struct.o: file format elf32-powerpc-freebsd
Disassembly of section .text:
00000000 <f> li r3,0
00000004 <f+0x4> li r4,1
00000008 <f+0x8> blr
So it returned via register r3 and r4.
This appears to be -msvr4-struct-return style.
So is clang using the aix ABI the right ABI?
Or does GCC need to change?
===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)
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