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syscall.go
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package compiler
// This file implements the syscall.Syscall and syscall.Syscall6 instructions as
// compiler builtins.
import (
"strconv"
"golang.org/x/tools/go/ssa"
"tinygo.org/x/go-llvm"
)
// createSyscall emits an inline system call instruction, depending on the
// target OS/arch.
func (b *builder) createSyscall(call *ssa.CallCommon) (llvm.Value, error) {
num := b.getValue(call.Args[0])
var syscallResult llvm.Value
switch {
case b.GOARCH() == "amd64":
if b.GOOS() == "darwin" {
// Darwin adds this magic number to system call numbers:
//
// > Syscall classes for 64-bit system call entry.
// > For 64-bit users, the 32-bit syscall number is partitioned
// > with the high-order bits representing the class and low-order
// > bits being the syscall number within that class.
// > The high-order 32-bits of the 64-bit syscall number are unused.
// > All system classes enter the kernel via the syscall instruction.
//
// Source: https://opensource.apple.com/source/xnu/xnu-792.13.8/osfmk/mach/i386/syscall_sw.h
num = b.CreateOr(num, llvm.ConstInt(b.uintptrType, 0x2000000, false), "")
}
// Sources:
// https://stackoverflow.com/a/2538212
// https://en.wikibooks.org/wiki/X86_Assembly/Interfacing_with_Linux#syscall
args := []llvm.Value{num}
argTypes := []llvm.Type{b.uintptrType}
// Constraints will look something like:
// "={rax},0,{rdi},{rsi},{rdx},{r10},{r8},{r9},~{rcx},~{r11}"
constraints := "={rax},0"
for i, arg := range call.Args[1:] {
constraints += "," + [...]string{
"{rdi}",
"{rsi}",
"{rdx}",
"{r10}",
"{r8}",
"{r9}",
"{r11}",
"{r12}",
"{r13}",
}[i]
llvmValue := b.getValue(arg)
args = append(args, llvmValue)
argTypes = append(argTypes, llvmValue.Type())
}
constraints += ",~{rcx},~{r11}"
fnType := llvm.FunctionType(b.uintptrType, argTypes, false)
target := llvm.InlineAsm(fnType, "syscall", constraints, true, false, llvm.InlineAsmDialectIntel)
syscallResult = b.CreateCall(target, args, "")
case b.GOARCH() == "386" && b.GOOS() == "linux":
// Sources:
// syscall(2) man page
// https://stackoverflow.com/a/2538212
// https://en.wikibooks.org/wiki/X86_Assembly/Interfacing_with_Linux#int_0x80
args := []llvm.Value{num}
argTypes := []llvm.Type{b.uintptrType}
// Constraints will look something like:
// "={eax},0,{ebx},{ecx},{edx},{esi},{edi},{ebp}"
constraints := "={eax},0"
for i, arg := range call.Args[1:] {
constraints += "," + [...]string{
"{ebx}",
"{ecx}",
"{edx}",
"{esi}",
"{edi}",
"{ebp}",
}[i]
llvmValue := b.getValue(arg)
args = append(args, llvmValue)
argTypes = append(argTypes, llvmValue.Type())
}
fnType := llvm.FunctionType(b.uintptrType, argTypes, false)
target := llvm.InlineAsm(fnType, "int 0x80", constraints, true, false, llvm.InlineAsmDialectIntel)
syscallResult = b.CreateCall(target, args, "")
case b.GOARCH() == "arm" && b.GOOS() == "linux":
// Implement the EABI system call convention for Linux.
// Source: syscall(2) man page.
args := []llvm.Value{}
argTypes := []llvm.Type{}
// Constraints will look something like:
// ={r0},0,{r1},{r2},{r7},~{r3}
constraints := "={r0}"
for i, arg := range call.Args[1:] {
constraints += "," + [...]string{
"0", // tie to output
"{r1}",
"{r2}",
"{r3}",
"{r4}",
"{r5}",
"{r6}",
}[i]
llvmValue := b.getValue(arg)
args = append(args, llvmValue)
argTypes = append(argTypes, llvmValue.Type())
}
args = append(args, num)
argTypes = append(argTypes, b.uintptrType)
constraints += ",{r7}" // syscall number
for i := len(call.Args) - 1; i < 4; i++ {
// r0-r3 get clobbered after the syscall returns
constraints += ",~{r" + strconv.Itoa(i) + "}"
}
fnType := llvm.FunctionType(b.uintptrType, argTypes, false)
target := llvm.InlineAsm(fnType, "svc #0", constraints, true, false, 0)
syscallResult = b.CreateCall(target, args, "")
case b.GOARCH() == "arm64" && b.GOOS() == "linux":
// Source: syscall(2) man page.
args := []llvm.Value{}
argTypes := []llvm.Type{}
// Constraints will look something like:
// ={x0},0,{x1},{x2},{x8},~{x3},~{x4},~{x5},~{x6},~{x7},~{x16},~{x17}
constraints := "={x0}"
for i, arg := range call.Args[1:] {
constraints += "," + [...]string{
"0", // tie to output
"{x1}",
"{x2}",
"{x3}",
"{x4}",
"{x5}",
}[i]
llvmValue := b.getValue(arg)
args = append(args, llvmValue)
argTypes = append(argTypes, llvmValue.Type())
}
args = append(args, num)
argTypes = append(argTypes, b.uintptrType)
constraints += ",{x8}" // syscall number
for i := len(call.Args) - 1; i < 8; i++ {
// x0-x7 may get clobbered during the syscall following the aarch64
// calling convention.
constraints += ",~{x" + strconv.Itoa(i) + "}"
}
constraints += ",~{x16},~{x17}" // scratch registers
fnType := llvm.FunctionType(b.uintptrType, argTypes, false)
target := llvm.InlineAsm(fnType, "svc #0", constraints, true, false, 0)
syscallResult = b.CreateCall(target, args, "")
default:
return llvm.Value{}, b.makeError(call.Pos(), "unknown GOOS/GOARCH for syscall: "+b.GOOS()+"/"+b.GOARCH())
}
switch b.GOOS() {
case "linux", "freebsd":
// Return values: r0, r1 uintptr, err Errno
// Pseudocode:
// var err uintptr
// if syscallResult < 0 && syscallResult > -4096 {
// err = -syscallResult
// }
// return syscallResult, 0, err
zero := llvm.ConstInt(b.uintptrType, 0, false)
inrange1 := b.CreateICmp(llvm.IntSLT, syscallResult, llvm.ConstInt(b.uintptrType, 0, false), "")
inrange2 := b.CreateICmp(llvm.IntSGT, syscallResult, llvm.ConstInt(b.uintptrType, 0xfffffffffffff000, true), "") // -4096
hasError := b.CreateAnd(inrange1, inrange2, "")
errResult := b.CreateSelect(hasError, b.CreateSub(zero, syscallResult, ""), zero, "syscallError")
retval := llvm.Undef(b.ctx.StructType([]llvm.Type{b.uintptrType, b.uintptrType, b.uintptrType}, false))
retval = b.CreateInsertValue(retval, syscallResult, 0, "")
retval = b.CreateInsertValue(retval, zero, 1, "")
retval = b.CreateInsertValue(retval, errResult, 2, "")
return retval, nil
case "darwin":
// Return values: r0, r1 uintptr, err Errno
// Pseudocode:
// var err uintptr
// if syscallResult != 0 {
// err = syscallResult
// }
// return syscallResult, 0, err
zero := llvm.ConstInt(b.uintptrType, 0, false)
hasError := b.CreateICmp(llvm.IntNE, syscallResult, llvm.ConstInt(b.uintptrType, 0, false), "")
errResult := b.CreateSelect(hasError, syscallResult, zero, "syscallError")
retval := llvm.Undef(b.ctx.StructType([]llvm.Type{b.uintptrType, b.uintptrType, b.uintptrType}, false))
retval = b.CreateInsertValue(retval, syscallResult, 0, "")
retval = b.CreateInsertValue(retval, zero, 1, "")
retval = b.CreateInsertValue(retval, errResult, 2, "")
return retval, nil
default:
return llvm.Value{}, b.makeError(call.Pos(), "unknown GOOS/GOARCH for syscall: "+b.GOOS()+"/"+b.GOARCH())
}
}