Should return structs of two capabilities in ca0/ca1

[arichardson]

EDIT: The description below applies to MIPS code generation, but the same issue also applies to RISC-V: https://cheri-compiler-explorer.cl.cam.ac.uk/z/c88q34 where we should return this pair in ca0/ca1.

---

The MIPS n64 ABI returns the following struct in $v0/$v1 whereas for CHERI we store to the stack instead.
This could be causing some performance differences.
However, it would be a flag day if we make this change (which is easier now that we have an ABI version that the kernel and RTLD check)

struct TwoPointers {
  void* first;
  void* second;
};

struct TwoPointers return_two_pointers() {
  struct TwoPointers result;
  result.first = (void*)(__UINTPTR_TYPE__)111;
  result.second = (void*)(__UINTPTR_TYPE__)222;
  return result;
}

IR generated for MIPS (-O2):

define inreg { i64, i64 } @return_two_pointers() local_unnamed_addr #0 {
entry:
  ret { i64, i64 } { i64 111, i64 222 }
}

IR for CHERI (-O2):

define void @return_two_pointers(%struct.TwoPointers addrspace(200)* noalias nocapture sret %agg.result) local_unnamed_addr addrspace(200) #0 {
entry:
  %0 = tail call i8 addrspace(200)* @llvm.cheri.cap.address.set.i64(i8 addrspace(200)* null, i64 111)
  %first = getelementptr inbounds %struct.TwoPointers, %struct.TwoPointers addrspace(200)* %agg.result, i64 0, i32 0
  store i8 addrspace(200)* %0, i8 addrspace(200)* addrspace(200)* %first, align 16, !tbaa !2
  %1 = tail call i8 addrspace(200)* @llvm.cheri.cap.address.set.i64(i8 addrspace(200)* null, i64 222)
  %second = getelementptr inbounds %struct.TwoPointers, %struct.TwoPointers addrspace(200)* %agg.result, i64 0, i32 1
  store i8 addrspace(200)* %1, i8 addrspace(200)* addrspace(200)* %second, align 16, !tbaa !7
  ret void
}

MIPS assembly code:

	.ent	return_two_pointers
return_two_pointers:
	.frame	$sp,0,$ra
	.mask 	0x00000000,0
	.fmask	0x00000000,0
	.set	noreorder
	.set	nomacro
	.set	noat
	daddiu	$2, $zero, 111
	jr	$ra
	daddiu	$3, $zero, 222
	.set	at
	.set	macro
	.set	reorder
	.end	return_two_pointers

CHERI code:

	.ent	return_two_pointers
return_two_pointers:
	.frame	$c11,0,$c17
	.mask 	0x00000000,0
	.fmask	0x00000000,0
	.set	noreorder
	.set	nomacro
	.set	noat
	cincoffset	$c1, $cnull, 111
	csc	$c1, $zero, 0($c3)
	cincoffset	$c1, $cnull, 222
	cjr	$c17
	csc	$c1, $zero, 16($c3)
	.set	at
	.set	macro
	.set	reorder
	.end	return_two_pointers

[arichardson]

This also affects RISC-V, updating description to remove old MIPS references.

https://cheri-compiler-explorer.cl.cam.ac.uk/z/c88q34

[jrtc27]

This also affects RISC-V, updating description to remove old MIPS references.

https://cheri-compiler-explorer.cl.cam.ac.uk/

Wrong link

[arichardson]

This also affects RISC-V, updating description to remove old MIPS references.
https://cheri-compiler-explorer.cl.cam.ac.uk/

Wrong link

Should be fixed now.