Hi everybody!
Recently, I noticed that there are some restrictions on the arguments
passed to functions in registers.
For example, if `a` is a struct, it must have fewer than 5 fields, and its
size must be less than `5 * ptrsz`. You can find these restrictions in
`cmd/compile/internal/ssa/value.go` at line 590 in the `CanSSA` function:
```
// CanSSA reports whether values of type t can be represented as a Value.
func CanSSA(t *types.Type) bool {
types.CalcSize(t)
if t.Size() > int64(4*types.PtrSize) {
// 4*Widthptr is an arbitrary constant. We want it
// to be at least 3*Widthptr so slices can be registerized.
// Too big and we'll introduce too much register pressure.
return false
}
switch t.Kind() {
...
case types.TSTRUCT:
if t.NumFields() > MaxStruct { // MaxStruct = 4
return false
}
}
}
```
Consider the following example:
```
type A struct {
s1, s2 string
i1 int64
}
func (a A) GetInt() int64 {
return a.i1
}
```
This compiles to:
```
f90007e0 MOVD R0, 8(RSP)
f9000be1 MOVD R1, 16(RSP)
f9000fe2 MOVD R2, 24(RSP)
f90013e3 MOVD R3, 32(RSP)
f90017e4 MOVD R4, 40(RSP)
aa0403e0 MOVD R4, R0
d65f03c0 RET
```
In the recent merged changes
(CL#611075)[https://go-review.googlesource.com/c/go/+/611075/4] and
(CL#611076)[https://go-review.googlesource.com/c/go/+/611076/6], support
was added for making structs with any number of fields SSA-able. With these
changes, I was able to remove the size restriction for structs that can be
SSA-ized.
Without these restrictions, the above example compiles to:
```
f90007e0 MOVD R0, 8(RSP)
f9000fe2 MOVD R2, 24(RSP)
aa0403e0 MOVD R4, R0
d65f03c0 RET
```
So, I am wondering: why does the restriction on size exist in the first
place? It seems unreasonable to place the argument in registers only to
later push it to the stack. The comment mentions that it helps reduce
register pressure, but can't the register allocator decide to spill the
argument if necessary? Also, if we’re preemptively pushing the structure to
the stack, why not just pass it on the stack from the beginning?
Thank you for your time and attention,
Arseny.
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