<p>Here is a small example below showing the <em>eightbyte</em> rule in action:</p>
<p>short-short-int fits in a eightbyte, but short-int-short does not, it requires 2 eightbytes ($RAX, $RDX)<br>
float-int fits in a eightbyte, so is passed in $RAX (integer dominates)<br>
double-int does not, so is returned in $XMMO,$RAX</p>
<p>We cannot return 3 eightbytes registers (like 2 long, 1 double), so the struct is passed by pointer in $RDI int this case, and $RDI is copied in $RAX on return.</p>
<pre><code>/*
* test returning struct by value
* clang --shared -Os -o libalientest.dylib alientest.c
* objdump --disassemble libalientest.dylib
*/
typedef struct sd2 {double a,b;} sd2;
typedef struct sf2 {float a,b;} sf2;
typedef struct sl2 {long a,b;} sl2;
typedef struct si2 {int a,b;} si2;
typedef struct ss2 {short a,b;} ss2;
typedef struct ssi {short a; int b;} ssi;
typedef struct sfi {float a; int b;} sfi;
typedef struct sdi {double a; int b;} sdi;
typedef struct sf2d {float a,b; double c;} sf2d;
typedef struct sfdf {float a; double b; float c;} sfdf;
typedef struct ss2i {short a,b; int c;} ss2i;
typedef struct ssis {short a; int b; short c;} ssis;
typedef struct ssls {short a; long b; short c;} ssls;
typedef struct sslf {short a; long b; float c;} sslf;
typedef struct sf4 {float a,b,c,d;} sf4;
typedef struct ss4 {short a,b,c,d;} ss4;
sd2 id2 (double a,double b) { sd2 v = {a,b}; return v; }
sf2 if2 (float a,float b) { sf2 v = {a,b}; return v; }
sl2 il2 (long a,long b) { sl2 v = {a,b}; return v; }
si2 ii2 (int a,int b) { si2 v = {a,b}; return v; }
ss2 is2 (short a,short b) { ss2 v = {a,b}; return v; }
ssi isi (short a,int b) { ssi v = {a,b}; return v; }
sfi ifi (float a,int b) { sfi v = {a,b}; return v; }
sdi idi (double a,int b) { sdi v = {a,b}; return v; }
sf2d if2d(float a,float b, double c) { sf2d v = {a,b,c}; return v; }
sfdf ifdf(float a,double b, float c) { sfdf v = {a,b,c}; return v; }
ss2i is2i(short a,short b, int c) { ss2i v = {a,b,c}; return v; }
ssis isis(short a,int b, short c) { ssis v = {a,b,c}; return v; }
ssls isls(short a,long b, short c) { ssls v = {a,b,c}; return v; }
sslf islf(short a,long b, float c) { sslf v = {a,b,c}; return v; }
sf4 if4 (float a,float b,float c,float d) { sf4 v = {a,b,c,d}; return v; }
ss4 is4 (short a,short b,short c,short d) { ss4 v = {a,b,c,d}; return v; }
sd2 ed2 () { return id2(1.0 ,2.0 ); }
sf2 ef2 () { return if2(1.0f,2.0f); }
sl2 el2 () { return il2(1L,2L); }
si2 ei2 () { return ii2(1,2); }
ss2 es2 () { return is2(1,2); }
sfi efi () { return ifi(1.0f,2); }
sdi edi () { return idi(1.0,2); }
ssi esi () { return isi(1,2); }
sf2d ef2d() { return if2d(1.0f,2.0f,3.0); }
sfdf efdf() { return ifdf(1.0f,2.0,3.0f); }
ss2i es2i() { return is2i(1,2,3); }
ssis esis() { return isis(1,2,3); }
ssls esls() { return isls(1,2L,3); }
sslf eslf() { return islf(1,2L,3.0f); }
sf4 ef4 () { return if4(1.0f,2.0f,3.0f,4.0f); }
ss4 es4 () { return is4(1,2,3,4); }
</code></pre>
<p>which disassemble to:</p>
<pre><code>libalientest.dylib: file format Mach-O 64-bit x86-64
Disassembly of section __TEXT,__text:
_id2:
d1f: 55 pushq %rbp
d20: 48 89 e5 movq %rsp, %rbp
d23: 5d popq %rbp
d24: c3 retq
_if2:
d25: 55 pushq %rbp
d26: 48 89 e5 movq %rsp, %rbp
d29: 66 0f 3a 21 c1 10 insertps $16, %xmm1, %xmm0
d2f: 5d popq %rbp
d30: c3 retq
_il2:
d31: 55 pushq %rbp
d32: 48 89 e5 movq %rsp, %rbp
d35: 48 89 f8 movq %rdi, %rax
d38: 48 89 f2 movq %rsi, %rdx
d3b: 5d popq %rbp
d3c: c3 retq
_ii2:
d3d: 55 pushq %rbp
d3e: 48 89 e5 movq %rsp, %rbp
d41: 48 c1 e6 20 shlq $32, %rsi
d45: 89 f8 movl %edi, %eax
d47: 48 09 f0 orq %rsi, %rax
d4a: 5d popq %rbp
d4b: c3 retq
_is2:
d4c: 55 pushq %rbp
d4d: 48 89 e5 movq %rsp, %rbp
d50: c1 e6 10 shll $16, %esi
d53: 0f b7 c7 movzwl %di, %eax
d56: 09 f0 orl %esi, %eax
d58: 5d popq %rbp
d59: c3 retq
_isi:
d5a: 55 pushq %rbp
d5b: 48 89 e5 movq %rsp, %rbp
d5e: 48 c1 e6 20 shlq $32, %rsi
d62: 0f b7 c7 movzwl %di, %eax
d65: 48 09 f0 orq %rsi, %rax
d68: 5d popq %rbp
d69: c3 retq
_ifi:
d6a: 55 pushq %rbp
d6b: 48 89 e5 movq %rsp, %rbp
d6e: 66 0f 7e c0 movd %xmm0, %eax
d72: 48 c1 e7 20 shlq $32, %rdi
d76: 48 09 f8 orq %rdi, %rax
d79: 5d popq %rbp
d7a: c3 retq
_idi:
d7b: 55 pushq %rbp
d7c: 48 89 e5 movq %rsp, %rbp
d7f: 89 f8 movl %edi, %eax
d81: 5d popq %rbp
d82: c3 retq
_if2d:
d83: 55 pushq %rbp
d84: 48 89 e5 movq %rsp, %rbp
d87: 66 0f 3a 21 c1 10 insertps $16, %xmm1, %xmm0
d8d: 0f 28 ca movaps %xmm2, %xmm1
d90: 5d popq %rbp
d91: c3 retq
_ifdf:
d92: 55 pushq %rbp
d93: 48 89 e5 movq %rsp, %rbp
d96: f3 0f 11 07 movss %xmm0, (%rdi)
d9a: f2 0f 11 4f 08 movsd %xmm1, 8(%rdi)
d9f: f3 0f 11 57 10 movss %xmm2, 16(%rdi)
da4: 48 89 f8 movq %rdi, %rax
da7: 5d popq %rbp
da8: c3 retq
_is2i:
da9: 55 pushq %rbp
daa: 48 89 e5 movq %rsp, %rbp
dad: 48 c1 e2 20 shlq $32, %rdx
db1: 0f b7 ce movzwl %si, %ecx
db4: 48 c1 e1 10 shlq $16, %rcx
db8: 0f b7 c7 movzwl %di, %eax
dbb: 48 09 c8 orq %rcx, %rax
dbe: 48 09 d0 orq %rdx, %rax
dc1: 5d popq %rbp
dc2: c3 retq
_isis:
dc3: 55 pushq %rbp
dc4: 48 89 e5 movq %rsp, %rbp
dc7: 48 c1 e6 20 shlq $32, %rsi
dcb: 0f b7 c7 movzwl %di, %eax
dce: 48 09 f0 orq %rsi, %rax
dd1: 5d popq %rbp
dd2: c3 retq
_isls:
dd3: 55 pushq %rbp
dd4: 48 89 e5 movq %rsp, %rbp
dd7: 66 89 37 movw %si, (%rdi)
dda: 48 89 57 08 movq %rdx, 8(%rdi)
dde: 66 89 4f 10 movw %cx, 16(%rdi)
de2: 48 89 f8 movq %rdi, %rax
de5: 5d popq %rbp
de6: c3 retq
_islf:
de7: 55 pushq %rbp
de8: 48 89 e5 movq %rsp, %rbp
deb: 66 89 37 movw %si, (%rdi)
dee: 48 89 57 08 movq %rdx, 8(%rdi)
df2: f3 0f 11 47 10 movss %xmm0, 16(%rdi)
df7: 48 89 f8 movq %rdi, %rax
dfa: 5d popq %rbp
dfb: c3 retq
_if4:
dfc: 55 pushq %rbp
dfd: 48 89 e5 movq %rsp, %rbp
e00: 66 0f 3a 21 c1 10 insertps $16, %xmm1, %xmm0
e06: 66 0f 3a 21 d3 10 insertps $16, %xmm3, %xmm2
e0c: 0f 28 ca movaps %xmm2, %xmm1
e0f: 5d popq %rbp
e10: c3 retq
_is4:
e11: 55 pushq %rbp
e12: 48 89 e5 movq %rsp, %rbp
e15: 48 c1 e1 30 shlq $48, %rcx
e19: 0f b7 d2 movzwl %dx, %edx
e1c: 48 c1 e2 20 shlq $32, %rdx
e20: 0f b7 f6 movzwl %si, %esi
e23: 48 c1 e6 10 shlq $16, %rsi
e27: 0f b7 c7 movzwl %di, %eax
e2a: 48 09 f0 orq %rsi, %rax
e2d: 48 09 d0 orq %rdx, %rax
e30: 48 09 c8 orq %rcx, %rax
e33: 5d popq %rbp
e34: c3 retq
_ed2:
e35: 55 pushq %rbp
e36: 48 89 e5 movq %rsp, %rbp
e39: f2 0f 10 05 2f 01 00 00 movsd 303(%rip), %xmm0
e41: f2 0f 10 0d 2f 01 00 00 movsd 303(%rip), %xmm1
e49: 5d popq %rbp
e4a: c3 retq
_ef2:
e4b: 55 pushq %rbp
e4c: 48 89 e5 movq %rsp, %rbp
e4f: 0f 28 05 3a 01 00 00 movaps 314(%rip), %xmm0
e56: 5d popq %rbp
e57: c3 retq
_el2:
e58: 55 pushq %rbp
e59: 48 89 e5 movq %rsp, %rbp
e5c: b8 01 00 00 00 movl $1, %eax
e61: ba 02 00 00 00 movl $2, %edx
e66: 5d popq %rbp
e67: c3 retq
_ei2:
e68: 55 pushq %rbp
e69: 48 89 e5 movq %rsp, %rbp
e6c: 48 b8 01 00 00 00 02 00 00 00 movabsq $8589934593, %rax
e76: 5d popq %rbp
e77: c3 retq
_es2:
e78: 55 pushq %rbp
e79: 48 89 e5 movq %rsp, %rbp
e7c: b8 01 00 02 00 movl $131073, %eax
e81: 5d popq %rbp
e82: c3 retq
_efi:
e83: 55 pushq %rbp
e84: 48 89 e5 movq %rsp, %rbp
e87: 48 b8 00 00 80 3f 02 00 00 00 movabsq $9655287808, %rax
e91: 5d popq %rbp
e92: c3 retq
_edi:
e93: 55 pushq %rbp
e94: 48 89 e5 movq %rsp, %rbp
e97: f2 0f 10 05 d1 00 00 00 movsd 209(%rip), %xmm0
e9f: b8 02 00 00 00 movl $2, %eax
ea4: 5d popq %rbp
ea5: c3 retq
_esi:
ea6: 55 pushq %rbp
ea7: 48 89 e5 movq %rsp, %rbp
eaa: 48 b8 01 00 00 00 02 00 00 00 movabsq $8589934593, %rax
eb4: 5d popq %rbp
eb5: c3 retq
_ef2d:
eb6: 55 pushq %rbp
eb7: 48 89 e5 movq %rsp, %rbp
eba: 0f 28 05 cf 00 00 00 movaps 207(%rip), %xmm0
ec1: f2 0f 10 0d b7 00 00 00 movsd 183(%rip), %xmm1
ec9: 5d popq %rbp
eca: c3 retq
_efdf:
ecb: 55 pushq %rbp
ecc: 48 89 e5 movq %rsp, %rbp
ecf: c7 07 00 00 80 3f movl $1065353216, (%rdi)
ed5: 48 b8 00 00 00 00 00 00 00 40 movabsq $4611686018427387904, %rax
edf: 48 89 47 08 movq %rax, 8(%rdi)
ee3: c7 47 10 00 00 40 40 movl $1077936128, 16(%rdi)
eea: 48 89 f8 movq %rdi, %rax
eed: 5d popq %rbp
eee: c3 retq
_es2i:
eef: 55 pushq %rbp
ef0: 48 89 e5 movq %rsp, %rbp
ef3: 48 b8 01 00 02 00 03 00 00 00 movabsq $12885032961, %rax
efd: 5d popq %rbp
efe: c3 retq
_esis:
eff: 55 pushq %rbp
f00: 48 89 e5 movq %rsp, %rbp
f03: 48 b8 01 00 00 00 02 00 00 00 movabsq $8589934593, %rax
f0d: 66 ba 03 00 movw $3, %dx
f11: 5d popq %rbp
f12: c3 retq
_esls:
f13: 55 pushq %rbp
f14: 48 89 e5 movq %rsp, %rbp
f17: 66 c7 07 01 00 movw $1, (%rdi)
f1c: 48 c7 47 08 02 00 00 00 movq $2, 8(%rdi)
f24: 66 c7 47 10 03 00 movw $3, 16(%rdi)
f2a: 48 89 f8 movq %rdi, %rax
f2d: 5d popq %rbp
f2e: c3 retq
_eslf:
f2f: 55 pushq %rbp
f30: 48 89 e5 movq %rsp, %rbp
f33: 66 c7 07 01 00 movw $1, (%rdi)
f38: 48 c7 47 08 02 00 00 00 movq $2, 8(%rdi)
f40: c7 47 10 00 00 40 40 movl $1077936128, 16(%rdi)
f47: 48 89 f8 movq %rdi, %rax
f4a: 5d popq %rbp
f4b: c3 retq
_ef4:
f4c: 55 pushq %rbp
f4d: 48 89 e5 movq %rsp, %rbp
f50: 0f 28 05 39 00 00 00 movaps 57(%rip), %xmm0
f57: 0f 28 0d 42 00 00 00 movaps 66(%rip), %xmm1
f5e: 5d popq %rbp
f5f: c3 retq
_es4:
f60: 55 pushq %rbp
f61: 48 89 e5 movq %rsp, %rbp
f64: 48 b8 01 00 02 00 03 00 04 00 movabsq $1125912791875585, %rax
f6e: 5d popq %rbp
f6f: c3 retq
</code></pre>
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