compiler: implement syscall.Syscall* as builtins

Treating them as builtins is easier to implement and likely reduces code
size.

Author: aykevl

compiler/compiler.go

@@ -1686,6 +1686,11 @@ func (c *Compiler) parseCall(frame *Frame, instr *ssa.CallCommon) (llvm.Value, e
 			return c.builder.CreateCall(target, args, ""), nil
 		}
 
+		switch fn.RelString(nil) {
+		case "syscall.Syscall", "syscall.Syscall6":
+			return c.emitSyscall(frame, instr)
+		}
+
 		targetFunc := c.ir.GetFunction(fn)
 		if targetFunc.LLVMFn.IsNil() {
 			return llvm.Value{}, c.makeError(instr.Pos(), "undefined function: "+targetFunc.LinkName())

compiler/syscall.go

@@ -0,0 +1,67 @@
+package compiler
+
+// This file implements the syscall.Syscall and syscall.Syscall6 instructions as
+// compiler builtins.
+
+import (
+	"go/constant"
+
+	"golang.org/x/tools/go/ssa"
+	"tinygo.org/x/go-llvm"
+)
+
+// emitSyscall emits an inline system call instruction, depending on the target
+// OS/arch.
+func (c *Compiler) emitSyscall(frame *Frame, call *ssa.CallCommon) (llvm.Value, error) {
+	num, _ := constant.Uint64Val(call.Args[0].(*ssa.Const).Value)
+	switch {
+	case c.GOARCH == "amd64" && c.GOOS == "linux":
+		// Sources:
+		//   https://stackoverflow.com/a/2538212
+		//   https://en.wikibooks.org/wiki/X86_Assembly/Interfacing_with_Linux#syscall
+		args := []llvm.Value{llvm.ConstInt(c.uintptrType, num, false)}
+		argTypes := []llvm.Type{c.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}",
+			}[i]
+			llvmValue, err := c.parseExpr(frame, arg)
+			if err != nil {
+				return llvm.Value{}, err
+			}
+			args = append(args, llvmValue)
+			argTypes = append(argTypes, llvmValue.Type())
+		}
+		constraints += ",~{rcx},~{r11}"
+		fnType := llvm.FunctionType(c.uintptrType, argTypes, false)
+		target := llvm.InlineAsm(fnType, "syscall", constraints, true, false, llvm.InlineAsmDialectIntel)
+		syscallResult := c.builder.CreateCall(target, args, "")
+		// Return values: r1, r1, err uintptr
+		// Pseudocode:
+		//     var err uintptr
+		//     if syscallResult < 0 && syscallResult > -4096 {
+		//         err = -syscallResult
+		//     }
+		//     return syscallResult, 0, err
+		zero := llvm.ConstInt(c.uintptrType, 0, false)
+		inrange1 := c.builder.CreateICmp(llvm.IntSLT, syscallResult, llvm.ConstInt(c.uintptrType, 0, false), "")
+		inrange2 := c.builder.CreateICmp(llvm.IntSGT, syscallResult, llvm.ConstInt(c.uintptrType, 0xfffffffffffff000, true), "") // -4096
+		hasError := c.builder.CreateAnd(inrange1, inrange2, "")
+		errResult := c.builder.CreateSelect(hasError, c.builder.CreateNot(syscallResult, ""), zero, "syscallError")
+		retval := llvm.Undef(llvm.StructType([]llvm.Type{c.uintptrType, c.uintptrType, c.uintptrType}, false))
+		retval = c.builder.CreateInsertValue(retval, syscallResult, 0, "")
+		retval = c.builder.CreateInsertValue(retval, zero, 1, "")
+		retval = c.builder.CreateInsertValue(retval, errResult, 2, "")
+		return retval, nil
+	default:
+		return llvm.Value{}, c.makeError(call.Pos(), "unknown GOOS/GOARCH for syscall: "+c.GOOS+"/"+c.GOARCH)
+	}
+}