cmd/compile: adjust interface conversion function selection with 0-sized fields

0-sized fields do not affect how arguments are passed under the
register ABI. But it does affect the size and alignment of the
type, and may affect the choice of interface conversion function.
Specifically, struct { a [0]int32; b [4]byte } should be passed in
memory, therefore should not use convT32.

Change-Id: Idfa21af79b81c196b50253b0be1fa4edecd12b45
Reviewed-on: https://go-review.googlesource.com/c/go/+/308651
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

diff --git a/src/cmd/compile/internal/walk/convert.go b/src/cmd/compile/internal/walk/convert.go
index 547e08dcabbaaffe474f1955ab25173c7e454d1c..dfdd9540e764fbcc77f080141154d1ab5476a97c 100644 (file)
--- a/src/cmd/compile/internal/walk/convert.go
+++ b/src/cmd/compile/internal/walk/convert.go
@@ -312,11 +312,6 @@ func convFuncName(from, to *types.Type) (fnname string, needsaddr bool) {
                case types.TARRAY:
                        return t.NumElem() == 1 && isFloatLike(t.Elem())
                case types.TSTRUCT:
-                       // allow for the possibility that we have a series of
-                       // leading fields that are zero size before a float field.
-                       // in addition, if we find a float field, it needs to be
-                       // the last item in the struct (a trailing zero length
-                       // field would introduce padding).
                        fsl := t.FieldSlice()
                        for idx, f := range fsl {
                                if f.Type.Width == 0 {
@@ -332,14 +327,6 @@ func convFuncName(from, to *types.Type) (fnname string, needsaddr bool) {
                return false
        }
 
-       // Helper to determine whether a given type (when passed to a
-       // function) will fit into a single integer register, assuming
-       // that the reg abi is in effect. This is somewhat ad-hoc, there
-       // may be a cleaner way to do this.
-       fitsInSingleIntReg := func(t *types.Type) bool {
-               return from.IsScalar() || types.IsDirectIface(from)
-       }
-
        tkind := to.Tie()
        switch from.Tie() {
        case 'I':
@@ -352,11 +339,11 @@ func convFuncName(from, to *types.Type) (fnname string, needsaddr bool) {
                        return "convT16", false
                case from.Size() == 4 && isFloatLike(from):
                        return "convT32F", false
-               case from.Size() == 4 && from.Align == 4 && !from.HasPointers():
+               case from.Size() == 4 && from.Align == 4 && !from.HasPointers() && (!objabi.Experiment.RegabiArgs || from.NumComponents(types.CountBlankFields) == 1):
                        return "convT32", false
                case from.Size() == 8 && isFloatLike(from):
                        return "convT64F", false
-               case from.Size() == 8 && from.Align == types.Types[types.TUINT64].Align && !from.HasPointers() && (!objabi.Experiment.RegabiArgs || fitsInSingleIntReg(from)):
+               case from.Size() == 8 && from.Align == types.Types[types.TUINT64].Align && !from.HasPointers() && (!objabi.Experiment.RegabiArgs || from.NumComponents(types.CountBlankFields) == 1):
                        return "convT64", false
                }
                if sc := from.SoleComponent(); sc != nil {