abi.go

     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  	"internal/goarch"
     9  	"unsafe"
    10  )
    11  
    12  // RegArgs is a struct that has space for each argument
    13  // and return value register on the current architecture.
    14  //
    15  // Assembly code knows the layout of the first two fields
    16  // of RegArgs.
    17  //
    18  // RegArgs also contains additional space to hold pointers
    19  // when it may not be safe to keep them only in the integer
    20  // register space otherwise.
    21  type RegArgs struct {
    22  	// Values in these slots should be precisely the bit-by-bit
    23  	// representation of how they would appear in a register.
    24  	//
    25  	// This means that on big endian arches, integer values should
    26  	// be in the top bits of the slot. Floats are usually just
    27  	// directly represented, but some architectures treat narrow
    28  	// width floating point values specially (e.g. they're promoted
    29  	// first, or they need to be NaN-boxed).
    30  	Ints   [IntArgRegs]uintptr  // untyped integer registers
    31  	Floats [FloatArgRegs]uint64 // untyped float registers
    32  
    33  	// Fields above this point are known to assembly.
    34  
    35  	// Ptrs is a space that duplicates Ints but with pointer type,
    36  	// used to make pointers passed or returned  in registers
    37  	// visible to the GC by making the type unsafe.Pointer.
    38  	Ptrs [IntArgRegs]unsafe.Pointer
    39  
    40  	// ReturnIsPtr is a bitmap that indicates which registers
    41  	// contain or will contain pointers on the return path from
    42  	// a reflectcall. The i'th bit indicates whether the i'th
    43  	// register contains or will contain a valid Go pointer.
    44  	ReturnIsPtr IntArgRegBitmap
    45  }
    46  
    47  func (r *RegArgs) Dump() {
    48  	print("Ints:")
    49  	for _, x := range r.Ints {
    50  		print(" ", x)
    51  	}
    52  	println()
    53  	print("Floats:")
    54  	for _, x := range r.Floats {
    55  		print(" ", x)
    56  	}
    57  	println()
    58  	print("Ptrs:")
    59  	for _, x := range r.Ptrs {
    60  		print(" ", x)
    61  	}
    62  	println()
    63  }
    64  
    65  // IntRegArgAddr returns a pointer inside of r.Ints[reg] that is appropriately
    66  // offset for an argument of size argSize.
    67  //
    68  // argSize must be non-zero, fit in a register, and a power-of-two.
    69  //
    70  // This method is a helper for dealing with the endianness of different CPU
    71  // architectures, since sub-word-sized arguments in big endian architectures
    72  // need to be "aligned" to the upper edge of the register to be interpreted
    73  // by the CPU correctly.
    74  func (r *RegArgs) IntRegArgAddr(reg int, argSize uintptr) unsafe.Pointer {
    75  	if argSize > goarch.PtrSize || argSize == 0 || argSize&(argSize-1) != 0 {
    76  		panic("invalid argSize")
    77  	}
    78  	offset := uintptr(0)
    79  	if goarch.BigEndian {
    80  		offset = goarch.PtrSize - argSize
    81  	}
    82  	return unsafe.Pointer(uintptr(unsafe.Pointer(&r.Ints[reg])) + offset)
    83  }
    84  
    85  // IntArgRegBitmap is a bitmap large enough to hold one bit per
    86  // integer argument/return register.
    87  type IntArgRegBitmap [(IntArgRegs + 7) / 8]uint8
    88  
    89  // Set sets the i'th bit of the bitmap to 1.
    90  func (b *IntArgRegBitmap) Set(i int) {
    91  	b[i/8] |= uint8(1) << (i % 8)
    92  }
    93  
    94  // Get returns whether the i'th bit of the bitmap is set.
    95  //
    96  // nosplit because it's called in extremely sensitive contexts, like
    97  // on the reflectcall return path.
    98  //
    99  //go:nosplit
   100  func (b *IntArgRegBitmap) Get(i int) bool {
   101  	return b[i/8]&(uint8(1)<<(i%8)) != 0
   102  }
   103
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