Native 64-Bit (N64) and N32 Subprogram Interface for MIPS Architectures

The principal interface for 64-bit native code is similar to the 32-bit ABI standard, with all 32-bit objects replaced by 64-bit objects. Note that square brackets [ ] indicate different n32-bit and o32-bit ABI conventions.

In particular, this implies:

• All integer parameters are promoted (that is, sign- or zero-extended to 64-bit integers and passed in a single register). Typically, no code is required for the promotion.

• All pointers and addresses are 64-bit objects. [Under n32, pointers and addresses are 32 bits.]

• Floating point parameters are passed as single- or double-precision according to the ANSI C rules. [This is the same under n32.]

• All stack parameter slots become 64-bit doublewords, even for parameters that are smaller (for example, floats and 32-bit integers). [This is also true for n32.]

• All stack regions are quadword aligned. [The 32-bit ABI specifies only doubleword alignment.]

• Up to eight integer registers ($4 .. $11) may be used to pass integer arguments. [The 32-bit ABI uses only the four registers $4 .. $7.]

• Up to eight floating point registers ($f12 .. $f19) may be used to pass floating point arguments. [The 32-bit ABI uses only the four registers $f12 .. $f15, with the odd registers used only for halves of double-precision arguments.]

• The argument registers may be viewed as an image of the initial eight doublewords of a structure containing all of the arguments, where each of the argument fields is a multiple of 64 bits in size with doubleword alignment. The integer and floating point registers are distinct images, that is, the first doubleword is passed in either $4 or $f1, depending on its type; the second in either $5 or $f1; and so on. [The 32-bit ABI associates each floating point argument with an even/odd pair of integer or floating point argument registers.]

• Within each of the 64-bit save area slots, smaller scalar parameters are right-justified, that is, they are placed at the highest possible address (for big-endian targets). This is relevant to float parameters and to integer parameters of 32 or fewer bits. Of these, only int parameters arise in C except for prototyped cases - floats are promoted to doubles, and smaller integers are promoted to int. [This is true for the 32-bit ABI, but is relevant only to prototyped small integers since all the other types were at least register-sized.]

• 32-bit integer (int) parameters are always sign-extended when passed in registers, whether of signed or unsigned type. [This issue does not arise in the 32-bit ABI.]

• Quad-precision floating point parameters (C long double or Fortran REAL*16) are always 16-byte aligned. This requires that they be passed in even-odd floating point register pairs, even if doing so requires skipping a register parameter and/or a 64-bit save area slot. (The 32-bit ABI does not consider long double parameters, since they were not supported.)

• Structs, unions, or other composite types are treated as a sequence of doublewords, and are passed in integer or floating point registers as though they were simple scalar parameters to the extent that they fit, with any excess on the stack packed according to the normal memory layout of the object. More specifically:

  • Regardless of the struct field structure, it is treated as a sequence of 64-bit chunks. If a chunk consists solely of a double float field (but not a double, which is part of a union), it is passed in a floating point register. Any other chunk is passed in an integer register.

  • A union, either as the parameter itself or as a struct parameter field, is treated as a sequence of integer doublewords for purposes of assignment to integer parameter registers. No attempt is made to identify floating point components for passing in floating point registers.

  • Array fields of structs are passed like unions. Array parameters are passed by reference (unless the relevant language standard requires otherwise).

  • Right-justifying small scalar parameters in their save area slots notwithstanding, struct parameters are always left-justified. This applies both to the case of a struct smaller than 64 bits, and to the final chunk of a struct which is not an integral multiple of 64 bits in size. The implication of this rule is that the address of the first chunk's save area slot is the address of the struct, and the struct is laid out in the save area memory exactly as if it were allocated normally (once any part in registers has been stored to the save area). [These rules are analogous to the 32-bit ABI treatment - only the chunk size and the ability to pass double fields in floating point registers are different.]

• Whenever possible, floating point arguments are passed in floating point registers regardless of whether they are preceded by integer parameters. [The 32-bit ABI allows only leading floating point (FP) arguments to be passed in FP registers; those coming after integer registers must be moved to integer registers.]

• Variable argument routines require an exception to the previous rule. Any floating point parameters in the variable part of the argument list (leading or otherwise) are passed in integer registers. Several important cases are involved:

  • If a varargs prototype (or the actual definition of the callee) is available to the caller, it places floating point parameters directly in the integer register required, and no problems occur.

  • If no prototype is available to the caller for a direct call, the caller's parameter profile is provided in the object file (as are all global subprogram formal parameter profiles), and the linker (ld/rld) generates an error message if the linked entry point turns out to be a varargs routine.

    Note: If you add -TENV:varargs_prototypes=off to the compilation command line, the floating point parameters appear in both floating point registers and integer registers. This decreases the performance of not only varargs routines with floating point parameters, but also of any unprototyped routines that pass floating point parameters. The program compiles and executes correctly; however, a warning message about unprototyped varargs routines still is present.

  • If no prototype is available to the caller for an indirect call (that is, via a function pointer), the caller assumes that the callee is not a varargs routine and places floating point parameters in floating point registers (if the callee is varargs, it is not ANSI-conformant).

• The portion of the argument structure beyond the initial eight doublewords is passed in memory on the stack and pointed to by the stack pointer at the time of call. The caller does not reserve space for the register arguments; the callee is responsible for reserving it if required (either adjacent to any caller-saved stack arguments if required, or elsewhere as appropriate.) No requirement is placed on the callee either to allocate space and save the register parameters, or to save them in any particular place. [The 32-bit ABI requires the caller to reserve space for the register arguments as well.]

• Function results are returned in $2 (and $3 if needed), or $f0 (and $f2 if needed), as appropriate for the type. Composite results (struct, union, or array) are returned in $2/$f0 and $3/$f2 according to the following rules:

  • A struct with only one or two floating point fields is returned in $f0 (and $f2 if necessary). This is a generalization of the Fortran COMPLEX case.

  • Any other struct or union results of at most 128 bits are returned in $2 (first 64 bits) and $3 (remainder, if necessary).

  • Larger composite results are handled by converting the function to a procedure with an implicit first parameter, which is a pointer to an area reserved by the caller to receive the result. [The 32-bit ABI requires that all composite results be handled by conversion to implicit first parameters. The MIPS/SGI Fortran implementation has always made a specific exception to return COMPLEX results in the floating point registers.]

• There are eight callee-saved floating point registers, $f24..$f31 for the 64-bit interface. There are six for the n32 ABI, the six even registers in $f20..$f30. [The 32-bit ABI specifies the six even registers, or even/odd pairs, $f20..$f31.]

• Routines are not be restricted to a single exit block. [The 32-bit ABI makes this restriction, though it is not observed under all optimization options.]

PIC (position-independent code, for DSO support) is generated from the compiler directly, rather than converting it later with a separate tool. This allows better compiler control for instruction scheduling and other optimizations, and provides greater robustness.

In the 64-bit interface, gp becomes a callee-saved register. [The 32-bit ABI makes gp a caller-saved register.]

Table 2-1 specifies the use of registers in native 64-bit mode. Note that "Caller-saved" means only that the caller may not assume that the value in the register is preserved across the call.

Native 64-Bit and N32 Interface Register Conventions

Register Name	Software Name	Use	Saver
$0	zero	Hardware zero
$1 or $at	at	Assembler temporary	Caller-saved
$2..$3	v0..v1	Function results	Caller-saved
$4..$11	a0..a7	Subprogram arguments	Caller-saved
$12..$15	t4..t7	Temporaries	Caller-saved
$16..$23	s0..s7	Saved	Callee-saved
$24	t8	Temporary	Caller-saved
$25	t9	Temporary	Caller-saved
$26..$27	kt0..kt1	Reserved for kernel
$28 or $gp	gp	Global pointer	Callee-saved
$29 or $sp	sp	Stack pointer	Callee-saved
$30	s8	Frame pointer (if needed)	Callee-saved
$31	ra	Return address	Caller-saved
hi, lo		Multiply/divide special registers	Caller-saved
$f0, $f2		Floating point function results	Caller-saved
$f1, $f3		Floating point temporaries	Caller-saved
$f4..$f11		Floating point temporaries	Caller-saved
$f12..$f19		Floating point arguments	Caller-saved
$f20..$f23 (32-bit)		Floating point temporaries	Caller-saved
$f24..$f31 (64-bit)		Floating point	Callee-saved
$f20..$f31 even (n32)		Floating point temporaries	Callee-saved
$f20..$f31 odd (n32)		Floating point	Caller-saved

Table 2-2 shows several examples of parameter passing. It illustrates that at most eight values can be passed through registers. In the table note that:

• d1..d5 are double precision floating point arguments

• s1..s4 are single precision floating point arguments

• n1..n3 are integer arguments

  Native 64-Bit and N32 C Parameter Passing

  Argument List	Register and Stack Assignments
  d1,d2	$f12, $f13
  s1,s2	$f12, $f13
  s1,d1	$f12, $f13
  d1,s1	$f12, $f13
  n1,d1	$4,$f13
  d1,n1,d1	$f12, $5,$f14
  n1,n2,d1	$4, $5,$f14
  d1,n1,n2	$f12, $5,$6
  s1,n1,n2	$f12, $5,$6
  d1,s1,s2	$f12, $f13, $f14
  s1,s2,d1	$f12, $f13, $f14
  n1,n2,n3,n4	$4,$5,$6,$7
  n1,n2,n3,d1	$4,$5,$6,$f15
  n1,n2,n3,s1	$4,$5,$6, $f15
  s1,s2,s3,s4	$f12, $f13,$f14,$f15
  s1,n1,s2,n2	$f12, $5,$f14,$7
  n1,s1,n2,s2	$4,$f13,$6,$f15
  n1,s1,n2,n3	$4,$f13,$6,$7
  d1,d2,d3,d4,d5	$f12, $f13, $f14, $f15, $f16
  d1,d2,d3,d4,d5,s1,s2,s3,s4	$f12, $f13, $f14, $f15, $f16, $f17, $f18,$f19,stack
  d1,d2,d3,s1,s2,s3,n1,n2,n3	$f12, $f13, $f14, $f15, $f16, $f17, $10,$11, stack