abiutils.go

Documentation: cmd/compile/internal/abi

     1  // Copyright 2020 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package abi
     6  
     7  import (
     8  	"cmd/compile/internal/base"
     9  	"cmd/compile/internal/ir"
    10  	"cmd/compile/internal/types"
    11  	"cmd/internal/obj"
    12  	"cmd/internal/src"
    13  	"fmt"
    14  	"math"
    15  	"sync"
    16  )
    17  
    18  //......................................................................
    19  //
    20  // Public/exported bits of the ABI utilities.
    21  //
    22  
    23  // ABIParamResultInfo stores the results of processing a given
    24  // function type to compute stack layout and register assignments. For
    25  // each input and output parameter we capture whether the param was
    26  // register-assigned (and to which register(s)) or the stack offset
    27  // for the param if is not going to be passed in registers according
    28  // to the rules in the Go internal ABI specification (1.17).
    29  type ABIParamResultInfo struct {
    30  	inparams          []ABIParamAssignment // Includes receiver for method calls.  Does NOT include hidden closure pointer.
    31  	outparams         []ABIParamAssignment
    32  	offsetToSpillArea int64
    33  	spillAreaSize     int64
    34  	inRegistersUsed   int
    35  	outRegistersUsed  int
    36  	config            *ABIConfig // to enable String() method
    37  }
    38  
    39  func (a *ABIParamResultInfo) Config() *ABIConfig {
    40  	return a.config
    41  }
    42  
    43  func (a *ABIParamResultInfo) InParams() []ABIParamAssignment {
    44  	return a.inparams
    45  }
    46  
    47  func (a *ABIParamResultInfo) OutParams() []ABIParamAssignment {
    48  	return a.outparams
    49  }
    50  
    51  func (a *ABIParamResultInfo) InRegistersUsed() int {
    52  	return a.inRegistersUsed
    53  }
    54  
    55  func (a *ABIParamResultInfo) OutRegistersUsed() int {
    56  	return a.outRegistersUsed
    57  }
    58  
    59  func (a *ABIParamResultInfo) InParam(i int) *ABIParamAssignment {
    60  	return &a.inparams[i]
    61  }
    62  
    63  func (a *ABIParamResultInfo) OutParam(i int) *ABIParamAssignment {
    64  	return &a.outparams[i]
    65  }
    66  
    67  func (a *ABIParamResultInfo) SpillAreaOffset() int64 {
    68  	return a.offsetToSpillArea
    69  }
    70  
    71  func (a *ABIParamResultInfo) SpillAreaSize() int64 {
    72  	return a.spillAreaSize
    73  }
    74  
    75  // ArgWidth returns the amount of stack needed for all the inputs
    76  // and outputs of a function or method, including ABI-defined parameter
    77  // slots and ABI-defined spill slots for register-resident parameters.
    78  // The name is inherited from (*Type).ArgWidth(), which it replaces.
    79  func (a *ABIParamResultInfo) ArgWidth() int64 {
    80  	return a.spillAreaSize + a.offsetToSpillArea - a.config.LocalsOffset()
    81  }
    82  
    83  // RegIndex stores the index into the set of machine registers used by
    84  // the ABI on a specific architecture for parameter passing.  RegIndex
    85  // values 0 through N-1 (where N is the number of integer registers
    86  // used for param passing according to the ABI rules) describe integer
    87  // registers; values N through M (where M is the number of floating
    88  // point registers used).  Thus if the ABI says there are 5 integer
    89  // registers and 7 floating point registers, then RegIndex value of 4
    90  // indicates the 5th integer register, and a RegIndex value of 11
    91  // indicates the 7th floating point register.
    92  type RegIndex uint8
    93  
    94  // ABIParamAssignment holds information about how a specific param or
    95  // result will be passed: in registers (in which case 'Registers' is
    96  // populated) or on the stack (in which case 'Offset' is set to a
    97  // non-negative stack offset). The values in 'Registers' are indices
    98  // (as described above), not architected registers.
    99  type ABIParamAssignment struct {
   100  	Type      *types.Type
   101  	Name      *ir.Name
   102  	Registers []RegIndex
   103  	offset    int32
   104  }
   105  
   106  // Offset returns the stack offset for addressing the parameter that "a" describes.
   107  // This will panic if "a" describes a register-allocated parameter.
   108  func (a *ABIParamAssignment) Offset() int32 {
   109  	if len(a.Registers) > 0 {
   110  		base.Fatalf("register allocated parameters have no offset")
   111  	}
   112  	return a.offset
   113  }
   114  
   115  // RegisterTypes returns a slice of the types of the registers
   116  // corresponding to a slice of parameters.  The returned slice
   117  // has capacity for one more, likely a memory type.
   118  func RegisterTypes(apa []ABIParamAssignment) []*types.Type {
   119  	rcount := 0
   120  	for _, pa := range apa {
   121  		rcount += len(pa.Registers)
   122  	}
   123  	if rcount == 0 {
   124  		// Note that this catches top-level struct{} and [0]Foo, which are stack allocated.
   125  		return make([]*types.Type, 0, 1)
   126  	}
   127  	rts := make([]*types.Type, 0, rcount+1)
   128  	for _, pa := range apa {
   129  		if len(pa.Registers) == 0 {
   130  			continue
   131  		}
   132  		rts = appendParamTypes(rts, pa.Type)
   133  	}
   134  	return rts
   135  }
   136  
   137  func (pa *ABIParamAssignment) RegisterTypesAndOffsets() ([]*types.Type, []int64) {
   138  	l := len(pa.Registers)
   139  	if l == 0 {
   140  		return nil, nil
   141  	}
   142  	typs := make([]*types.Type, 0, l)
   143  	offs := make([]int64, 0, l)
   144  	offs, _ = appendParamOffsets(offs, 0, pa.Type) // 0 is aligned for everything.
   145  	return appendParamTypes(typs, pa.Type), offs
   146  }
   147  
   148  func appendParamTypes(rts []*types.Type, t *types.Type) []*types.Type {
   149  	w := t.Size()
   150  	if w == 0 {
   151  		return rts
   152  	}
   153  	if t.IsScalar() || t.IsPtrShaped() {
   154  		if t.IsComplex() {
   155  			c := types.FloatForComplex(t)
   156  			return append(rts, c, c)
   157  		} else {
   158  			if int(t.Size()) <= types.RegSize {
   159  				return append(rts, t)
   160  			}
   161  			// assume 64bit int on 32-bit machine
   162  			// TODO endianness? Should high-order (sign bits) word come first?
   163  			if t.IsSigned() {
   164  				rts = append(rts, types.Types[types.TINT32])
   165  			} else {
   166  				rts = append(rts, types.Types[types.TUINT32])
   167  			}
   168  			return append(rts, types.Types[types.TUINT32])
   169  		}
   170  	} else {
   171  		typ := t.Kind()
   172  		switch typ {
   173  		case types.TARRAY:
   174  			for i := int64(0); i < t.NumElem(); i++ { // 0 gets no registers, plus future-proofing.
   175  				rts = appendParamTypes(rts, t.Elem())
   176  			}
   177  		case types.TSTRUCT:
   178  			for _, f := range t.Fields() {
   179  				if f.Type.Size() > 0 { // embedded zero-width types receive no registers
   180  					rts = appendParamTypes(rts, f.Type)
   181  				}
   182  			}
   183  		case types.TSLICE:
   184  			return appendParamTypes(rts, synthSlice)
   185  		case types.TSTRING:
   186  			return appendParamTypes(rts, synthString)
   187  		case types.TINTER:
   188  			return appendParamTypes(rts, synthIface)
   189  		}
   190  	}
   191  	return rts
   192  }
   193  
   194  // appendParamOffsets appends the offset(s) of type t, starting from "at",
   195  // to input offsets, and returns the longer slice and the next unused offset.
   196  // at should already be aligned for t.
   197  func appendParamOffsets(offsets []int64, at int64, t *types.Type) ([]int64, int64) {
   198  	w := t.Size()
   199  	if w == 0 {
   200  		return offsets, at
   201  	}
   202  	if t.IsScalar() || t.IsPtrShaped() {
   203  		if t.IsComplex() || int(t.Size()) > types.RegSize { // complex and *int64 on 32-bit
   204  			s := w / 2
   205  			return append(offsets, at, at+s), at + w
   206  		} else {
   207  			return append(offsets, at), at + w
   208  		}
   209  	} else {
   210  		typ := t.Kind()
   211  		switch typ {
   212  		case types.TARRAY:
   213  			te := t.Elem()
   214  			for i := int64(0); i < t.NumElem(); i++ {
   215  				at = align(at, te)
   216  				offsets, at = appendParamOffsets(offsets, at, te)
   217  			}
   218  		case types.TSTRUCT:
   219  			at0 := at
   220  			for i, f := range t.Fields() {
   221  				at = at0 + f.Offset // Fields may be over-aligned, see wasm32.
   222  				offsets, at = appendParamOffsets(offsets, at, f.Type)
   223  				if f.Type.Size() == 0 && i == t.NumFields()-1 {
   224  					at++ // last field has zero width
   225  				}
   226  			}
   227  			at = align(at, t) // type size is rounded up to its alignment
   228  		case types.TSLICE:
   229  			return appendParamOffsets(offsets, at, synthSlice)
   230  		case types.TSTRING:
   231  			return appendParamOffsets(offsets, at, synthString)
   232  		case types.TINTER:
   233  			return appendParamOffsets(offsets, at, synthIface)
   234  		}
   235  	}
   236  	return offsets, at
   237  }
   238  
   239  // FrameOffset returns the frame-pointer-relative location that a function
   240  // would spill its input or output parameter to, if such a spill slot exists.
   241  // If there is none defined (e.g., register-allocated outputs) it panics.
   242  // For register-allocated inputs that is their spill offset reserved for morestack;
   243  // for stack-allocated inputs and outputs, that is their location on the stack.
   244  // (In a future version of the ABI, register-resident inputs may lose their defined
   245  // spill area to help reduce stack sizes.)
   246  func (a *ABIParamAssignment) FrameOffset(i *ABIParamResultInfo) int64 {
   247  	if a.offset == -1 {
   248  		base.Fatalf("function parameter has no ABI-defined frame-pointer offset")
   249  	}
   250  	if len(a.Registers) == 0 { // passed on stack
   251  		return int64(a.offset) - i.config.LocalsOffset()
   252  	}
   253  	// spill area for registers
   254  	return int64(a.offset) + i.SpillAreaOffset() - i.config.LocalsOffset()
   255  }
   256  
   257  // RegAmounts holds a specified number of integer/float registers.
   258  type RegAmounts struct {
   259  	intRegs   int
   260  	floatRegs int
   261  }
   262  
   263  // ABIConfig captures the number of registers made available
   264  // by the ABI rules for parameter passing and result returning.
   265  type ABIConfig struct {
   266  	// Do we need anything more than this?
   267  	offsetForLocals int64 // e.g., obj.(*Link).Arch.FixedFrameSize -- extra linkage information on some architectures.
   268  	regAmounts      RegAmounts
   269  	which           obj.ABI
   270  }
   271  
   272  // NewABIConfig returns a new ABI configuration for an architecture with
   273  // iRegsCount integer/pointer registers and fRegsCount floating point registers.
   274  func NewABIConfig(iRegsCount, fRegsCount int, offsetForLocals int64, which uint8) *ABIConfig {
   275  	return &ABIConfig{offsetForLocals: offsetForLocals, regAmounts: RegAmounts{iRegsCount, fRegsCount}, which: obj.ABI(which)}
   276  }
   277  
   278  // Copy returns config.
   279  //
   280  // TODO(mdempsky): Remove.
   281  func (config *ABIConfig) Copy() *ABIConfig {
   282  	return config
   283  }
   284  
   285  // Which returns the ABI number
   286  func (config *ABIConfig) Which() obj.ABI {
   287  	return config.which
   288  }
   289  
   290  // LocalsOffset returns the architecture-dependent offset from SP for args and results.
   291  // In theory this is only used for debugging; it ought to already be incorporated into
   292  // results from the ABI-related methods
   293  func (config *ABIConfig) LocalsOffset() int64 {
   294  	return config.offsetForLocals
   295  }
   296  
   297  // FloatIndexFor translates r into an index in the floating point parameter
   298  // registers.  If the result is negative, the input index was actually for the
   299  // integer parameter registers.
   300  func (config *ABIConfig) FloatIndexFor(r RegIndex) int64 {
   301  	return int64(r) - int64(config.regAmounts.intRegs)
   302  }
   303  
   304  // NumParamRegs returns the total number of registers used to
   305  // represent a parameter of the given type, which must be register
   306  // assignable.
   307  func (config *ABIConfig) NumParamRegs(typ *types.Type) int {
   308  	intRegs, floatRegs := typ.Registers()
   309  	if intRegs == math.MaxUint8 && floatRegs == math.MaxUint8 {
   310  		base.Fatalf("cannot represent parameters of type %v in registers", typ)
   311  	}
   312  	return int(intRegs) + int(floatRegs)
   313  }
   314  
   315  // ABIAnalyzeTypes takes slices of parameter and result types, and returns an ABIParamResultInfo,
   316  // based on the given configuration.  This is the same result computed by config.ABIAnalyze applied to the
   317  // corresponding method/function type, except that all the embedded parameter names are nil.
   318  // This is intended for use by ssagen/ssa.go:(*state).rtcall, for runtime functions that lack a parsed function type.
   319  func (config *ABIConfig) ABIAnalyzeTypes(params, results []*types.Type) *ABIParamResultInfo {
   320  	setup()
   321  	s := assignState{
   322  		stackOffset: config.offsetForLocals,
   323  		rTotal:      config.regAmounts,
   324  	}
   325  
   326  	assignParams := func(params []*types.Type, isResult bool) []ABIParamAssignment {
   327  		res := make([]ABIParamAssignment, len(params))
   328  		for i, param := range params {
   329  			res[i] = s.assignParam(param, nil, isResult)
   330  		}
   331  		return res
   332  	}
   333  
   334  	info := &ABIParamResultInfo{config: config}
   335  
   336  	// Inputs
   337  	info.inparams = assignParams(params, false)
   338  	s.stackOffset = types.RoundUp(s.stackOffset, int64(types.RegSize))
   339  	info.inRegistersUsed = s.rUsed.intRegs + s.rUsed.floatRegs
   340  
   341  	// Outputs
   342  	s.rUsed = RegAmounts{}
   343  	info.outparams = assignParams(results, true)
   344  	// The spill area is at a register-aligned offset and its size is rounded up to a register alignment.
   345  	// TODO in theory could align offset only to minimum required by spilled data types.
   346  	info.offsetToSpillArea = alignTo(s.stackOffset, types.RegSize)
   347  	info.spillAreaSize = alignTo(s.spillOffset, types.RegSize)
   348  	info.outRegistersUsed = s.rUsed.intRegs + s.rUsed.floatRegs
   349  
   350  	return info
   351  }
   352  
   353  // ABIAnalyzeFuncType takes a function type 'ft' and an ABI rules description
   354  // 'config' and analyzes the function to determine how its parameters
   355  // and results will be passed (in registers or on the stack), returning
   356  // an ABIParamResultInfo object that holds the results of the analysis.
   357  func (config *ABIConfig) ABIAnalyzeFuncType(ft *types.Type) *ABIParamResultInfo {
   358  	setup()
   359  	s := assignState{
   360  		stackOffset: config.offsetForLocals,
   361  		rTotal:      config.regAmounts,
   362  	}
   363  
   364  	assignParams := func(params []*types.Field, isResult bool) []ABIParamAssignment {
   365  		res := make([]ABIParamAssignment, len(params))
   366  		for i, param := range params {
   367  			var name *ir.Name
   368  			if param.Nname != nil {
   369  				name = param.Nname.(*ir.Name)
   370  			}
   371  			res[i] = s.assignParam(param.Type, name, isResult)
   372  		}
   373  		return res
   374  	}
   375  
   376  	info := &ABIParamResultInfo{config: config}
   377  
   378  	// Inputs
   379  	info.inparams = assignParams(ft.RecvParams(), false)
   380  	s.stackOffset = types.RoundUp(s.stackOffset, int64(types.RegSize))
   381  	info.inRegistersUsed = s.rUsed.intRegs + s.rUsed.floatRegs
   382  
   383  	// Outputs
   384  	s.rUsed = RegAmounts{}
   385  	info.outparams = assignParams(ft.Results(), true)
   386  	// The spill area is at a register-aligned offset and its size is rounded up to a register alignment.
   387  	// TODO in theory could align offset only to minimum required by spilled data types.
   388  	info.offsetToSpillArea = alignTo(s.stackOffset, types.RegSize)
   389  	info.spillAreaSize = alignTo(s.spillOffset, types.RegSize)
   390  	info.outRegistersUsed = s.rUsed.intRegs + s.rUsed.floatRegs
   391  	return info
   392  }
   393  
   394  // ABIAnalyze returns the same result as ABIAnalyzeFuncType, but also
   395  // updates the offsets of all the receiver, input, and output fields.
   396  // If setNname is true, it also sets the FrameOffset of the Nname for
   397  // the field(s); this is for use when compiling a function and figuring out
   398  // spill locations.  Doing this for callers can cause races for register
   399  // outputs because their frame location transitions from BOGUS_FUNARG_OFFSET
   400  // to zero to an as-if-AUTO offset that has no use for callers.
   401  func (config *ABIConfig) ABIAnalyze(t *types.Type, setNname bool) *ABIParamResultInfo {
   402  	result := config.ABIAnalyzeFuncType(t)
   403  
   404  	// Fill in the frame offsets for receiver, inputs, results
   405  	for i, f := range t.RecvParams() {
   406  		config.updateOffset(result, f, result.inparams[i], false, setNname)
   407  	}
   408  	for i, f := range t.Results() {
   409  		config.updateOffset(result, f, result.outparams[i], true, setNname)
   410  	}
   411  	return result
   412  }
   413  
   414  func (config *ABIConfig) updateOffset(result *ABIParamResultInfo, f *types.Field, a ABIParamAssignment, isResult, setNname bool) {
   415  	if f.Offset != types.BADWIDTH {
   416  		base.Fatalf("field offset for %s at %s has been set to %d", f.Sym, base.FmtPos(f.Pos), f.Offset)
   417  	}
   418  
   419  	// Everything except return values in registers has either a frame home (if not in a register) or a frame spill location.
   420  	if !isResult || len(a.Registers) == 0 {
   421  		// The type frame offset DOES NOT show effects of minimum frame size.
   422  		// Getting this wrong breaks stackmaps, see liveness/plive.go:WriteFuncMap and typebits/typebits.go:Set
   423  		off := a.FrameOffset(result)
   424  		if setNname && f.Nname != nil {
   425  			f.Nname.(*ir.Name).SetFrameOffset(off)
   426  			f.Nname.(*ir.Name).SetIsOutputParamInRegisters(false)
   427  		}
   428  	} else {
   429  		if setNname && f.Nname != nil {
   430  			fname := f.Nname.(*ir.Name)
   431  			fname.SetIsOutputParamInRegisters(true)
   432  			fname.SetFrameOffset(0)
   433  		}
   434  	}
   435  }
   436  
   437  //......................................................................
   438  //
   439  // Non-public portions.
   440  
   441  // regString produces a human-readable version of a RegIndex.
   442  func (c *RegAmounts) regString(r RegIndex) string {
   443  	if int(r) < c.intRegs {
   444  		return fmt.Sprintf("I%d", int(r))
   445  	} else if int(r) < c.intRegs+c.floatRegs {
   446  		return fmt.Sprintf("F%d", int(r)-c.intRegs)
   447  	}
   448  	return fmt.Sprintf("<?>%d", r)
   449  }
   450  
   451  // ToString method renders an ABIParamAssignment in human-readable
   452  // form, suitable for debugging or unit testing.
   453  func (ri *ABIParamAssignment) ToString(config *ABIConfig, extra bool) string {
   454  	regs := "R{"
   455  	offname := "spilloffset" // offset is for spill for register(s)
   456  	if len(ri.Registers) == 0 {
   457  		offname = "offset" // offset is for memory arg
   458  	}
   459  	for _, r := range ri.Registers {
   460  		regs += " " + config.regAmounts.regString(r)
   461  		if extra {
   462  			regs += fmt.Sprintf("(%d)", r)
   463  		}
   464  	}
   465  	if extra {
   466  		regs += fmt.Sprintf(" | #I=%d, #F=%d", config.regAmounts.intRegs, config.regAmounts.floatRegs)
   467  	}
   468  	return fmt.Sprintf("%s } %s: %d typ: %v", regs, offname, ri.offset, ri.Type)
   469  }
   470  
   471  // String method renders an ABIParamResultInfo in human-readable
   472  // form, suitable for debugging or unit testing.
   473  func (ri *ABIParamResultInfo) String() string {
   474  	res := ""
   475  	for k, p := range ri.inparams {
   476  		res += fmt.Sprintf("IN %d: %s\n", k, p.ToString(ri.config, false))
   477  	}
   478  	for k, r := range ri.outparams {
   479  		res += fmt.Sprintf("OUT %d: %s\n", k, r.ToString(ri.config, false))
   480  	}
   481  	res += fmt.Sprintf("offsetToSpillArea: %d spillAreaSize: %d",
   482  		ri.offsetToSpillArea, ri.spillAreaSize)
   483  	return res
   484  }
   485  
   486  // assignState holds intermediate state during the register assigning process
   487  // for a given function signature.
   488  type assignState struct {
   489  	rTotal      RegAmounts // total reg amounts from ABI rules
   490  	rUsed       RegAmounts // regs used by params completely assigned so far
   491  	stackOffset int64      // current stack offset
   492  	spillOffset int64      // current spill offset
   493  }
   494  
   495  // align returns a rounded up to t's alignment.
   496  func align(a int64, t *types.Type) int64 {
   497  	return alignTo(a, int(uint8(t.Alignment())))
   498  }
   499  
   500  // alignTo returns a rounded up to t, where t must be 0 or a power of 2.
   501  func alignTo(a int64, t int) int64 {
   502  	if t == 0 {
   503  		return a
   504  	}
   505  	return types.RoundUp(a, int64(t))
   506  }
   507  
   508  // nextSlot allocates the next available slot for typ.
   509  func nextSlot(offsetp *int64, typ *types.Type) int64 {
   510  	offset := align(*offsetp, typ)
   511  	*offsetp = offset + typ.Size()
   512  	return offset
   513  }
   514  
   515  // allocateRegs returns an ordered list of register indices for a parameter or result
   516  // that we've just determined to be register-assignable. The number of registers
   517  // needed is assumed to be stored in state.pUsed.
   518  func (state *assignState) allocateRegs(regs []RegIndex, t *types.Type) []RegIndex {
   519  	if t.Size() == 0 {
   520  		return regs
   521  	}
   522  	ri := state.rUsed.intRegs
   523  	rf := state.rUsed.floatRegs
   524  	if t.IsScalar() || t.IsPtrShaped() {
   525  		if t.IsComplex() {
   526  			regs = append(regs, RegIndex(rf+state.rTotal.intRegs), RegIndex(rf+1+state.rTotal.intRegs))
   527  			rf += 2
   528  		} else if t.IsFloat() {
   529  			regs = append(regs, RegIndex(rf+state.rTotal.intRegs))
   530  			rf += 1
   531  		} else {
   532  			n := (int(t.Size()) + types.RegSize - 1) / types.RegSize
   533  			for i := 0; i < n; i++ { // looking ahead to really big integers
   534  				regs = append(regs, RegIndex(ri))
   535  				ri += 1
   536  			}
   537  		}
   538  		state.rUsed.intRegs = ri
   539  		state.rUsed.floatRegs = rf
   540  		return regs
   541  	} else {
   542  		typ := t.Kind()
   543  		switch typ {
   544  		case types.TARRAY:
   545  			for i := int64(0); i < t.NumElem(); i++ {
   546  				regs = state.allocateRegs(regs, t.Elem())
   547  			}
   548  			return regs
   549  		case types.TSTRUCT:
   550  			for _, f := range t.Fields() {
   551  				regs = state.allocateRegs(regs, f.Type)
   552  			}
   553  			return regs
   554  		case types.TSLICE:
   555  			return state.allocateRegs(regs, synthSlice)
   556  		case types.TSTRING:
   557  			return state.allocateRegs(regs, synthString)
   558  		case types.TINTER:
   559  			return state.allocateRegs(regs, synthIface)
   560  		}
   561  	}
   562  	base.Fatalf("was not expecting type %s", t)
   563  	panic("unreachable")
   564  }
   565  
   566  // synthOnce ensures that we only create the synth* fake types once.
   567  var synthOnce sync.Once
   568  
   569  // synthSlice, synthString, and syncIface are synthesized struct types
   570  // meant to capture the underlying implementations of string/slice/interface.
   571  var synthSlice *types.Type
   572  var synthString *types.Type
   573  var synthIface *types.Type
   574  
   575  // setup performs setup for the register assignment utilities, manufacturing
   576  // a small set of synthesized types that we'll need along the way.
   577  func setup() {
   578  	synthOnce.Do(func() {
   579  		fname := types.BuiltinPkg.Lookup
   580  		nxp := src.NoXPos
   581  		bp := types.NewPtr(types.Types[types.TUINT8])
   582  		it := types.Types[types.TINT]
   583  		synthSlice = types.NewStruct([]*types.Field{
   584  			types.NewField(nxp, fname("ptr"), bp),
   585  			types.NewField(nxp, fname("len"), it),
   586  			types.NewField(nxp, fname("cap"), it),
   587  		})
   588  		types.CalcStructSize(synthSlice)
   589  		synthString = types.NewStruct([]*types.Field{
   590  			types.NewField(nxp, fname("data"), bp),
   591  			types.NewField(nxp, fname("len"), it),
   592  		})
   593  		types.CalcStructSize(synthString)
   594  		unsp := types.Types[types.TUNSAFEPTR]
   595  		synthIface = types.NewStruct([]*types.Field{
   596  			types.NewField(nxp, fname("f1"), unsp),
   597  			types.NewField(nxp, fname("f2"), unsp),
   598  		})
   599  		types.CalcStructSize(synthIface)
   600  	})
   601  }
   602  
   603  // assignParam processes a given receiver, param, or result
   604  // of field f to determine whether it can be register assigned.
   605  // The result of the analysis is recorded in the result
   606  // ABIParamResultInfo held in 'state'.
   607  func (state *assignState) assignParam(typ *types.Type, name *ir.Name, isResult bool) ABIParamAssignment {
   608  	registers := state.tryAllocRegs(typ)
   609  
   610  	var offset int64 = -1
   611  	if registers == nil { // stack allocated; needs stack slot
   612  		offset = nextSlot(&state.stackOffset, typ)
   613  	} else if !isResult { // register-allocated param; needs spill slot
   614  		offset = nextSlot(&state.spillOffset, typ)
   615  	}
   616  
   617  	return ABIParamAssignment{
   618  		Type:      typ,
   619  		Name:      name,
   620  		Registers: registers,
   621  		offset:    int32(offset),
   622  	}
   623  }
   624  
   625  // tryAllocRegs attempts to allocate registers to represent a
   626  // parameter of the given type. If unsuccessful, it returns nil.
   627  func (state *assignState) tryAllocRegs(typ *types.Type) []RegIndex {
   628  	if typ.Size() == 0 {
   629  		return nil // zero-size parameters are defined as being stack allocated
   630  	}
   631  
   632  	intRegs, floatRegs := typ.Registers()
   633  	if int(intRegs) > state.rTotal.intRegs-state.rUsed.intRegs || int(floatRegs) > state.rTotal.floatRegs-state.rUsed.floatRegs {
   634  		return nil // too few available registers
   635  	}
   636  
   637  	regs := make([]RegIndex, 0, int(intRegs)+int(floatRegs))
   638  	return state.allocateRegs(regs, typ)
   639  }
   640  
   641  // ComputePadding returns a list of "post element" padding values in
   642  // the case where we have a structure being passed in registers. Given
   643  // a param assignment corresponding to a struct, it returns a list
   644  // containing padding values for each field, e.g. the Kth element in
   645  // the list is the amount of padding between field K and the following
   646  // field. For things that are not structs (or structs without padding)
   647  // it returns a list of zeros. Example:
   648  //
   649  //	type small struct {
   650  //		x uint16
   651  //		y uint8
   652  //		z int32
   653  //		w int32
   654  //	}
   655  //
   656  // For this struct we would return a list [0, 1, 0, 0], meaning that
   657  // we have one byte of padding after the second field, and no bytes of
   658  // padding after any of the other fields. Input parameter "storage" is
   659  // a slice with enough capacity to accommodate padding elements for
   660  // the architected register set in question.
   661  func (pa *ABIParamAssignment) ComputePadding(storage []uint64) []uint64 {
   662  	nr := len(pa.Registers)
   663  	padding := storage[:nr]
   664  	for i := 0; i < nr; i++ {
   665  		padding[i] = 0
   666  	}
   667  	if pa.Type.Kind() != types.TSTRUCT || nr == 0 {
   668  		return padding
   669  	}
   670  	types := make([]*types.Type, 0, nr)
   671  	types = appendParamTypes(types, pa.Type)
   672  	if len(types) != nr {
   673  		panic("internal error")
   674  	}
   675  	offsets, _ := appendParamOffsets([]int64{}, 0, pa.Type)
   676  	off := int64(0)
   677  	for idx, t := range types {
   678  		ts := t.Size()
   679  		off += int64(ts)
   680  		if idx < len(types)-1 {
   681  			noff := offsets[idx+1]
   682  			if noff != off {
   683  				padding[idx] = uint64(noff - off)
   684  			}
   685  		}
   686  	}
   687  	return padding
   688  }
   689
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