github.com/4ad/go@v0.0.0-20161219182952-69a12818b605/src/runtime/sys_x86.go (about)

     1  // Copyright 2013 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  // +build amd64 amd64p32 386
     6  
     7  package runtime
     8  
     9  import (
    10  	"runtime/internal/sys"
    11  	"unsafe"
    12  )
    13  
    14  // adjust Gobuf as it if executed a call to fn with context ctxt
    15  // and then did an immediate gosave.
    16  func gostartcall(buf *gobuf, fn, ctxt unsafe.Pointer) {
    17  	sp := buf.sp
    18  	if sys.RegSize > sys.PtrSize {
    19  		sp -= sys.PtrSize
    20  		*(*uintptr)(unsafe.Pointer(sp)) = 0
    21  	}
    22  	sp -= sys.PtrSize
    23  	*(*uintptr)(unsafe.Pointer(sp)) = buf.pc
    24  	buf.sp = sp
    25  	buf.pc = uintptr(fn)
    26  	buf.ctxt = ctxt
    27  }
    28  
    29  // Called to rewind context saved during morestack back to beginning of function.
    30  // To help us, the linker emits a jmp back to the beginning right after the
    31  // call to morestack. We just have to decode and apply that jump.
    32  func rewindmorestack(buf *gobuf) {
    33  	pc := (*[8]byte)(unsafe.Pointer(buf.pc))
    34  	if pc[0] == 0xe9 { // jmp 4-byte offset
    35  		buf.pc = buf.pc + 5 + uintptr(int64(*(*int32)(unsafe.Pointer(&pc[1]))))
    36  		return
    37  	}
    38  	if pc[0] == 0xeb { // jmp 1-byte offset
    39  		buf.pc = buf.pc + 2 + uintptr(int64(*(*int8)(unsafe.Pointer(&pc[1]))))
    40  		return
    41  	}
    42  	if pc[0] == 0xcc {
    43  		// This is a breakpoint inserted by gdb. We could use
    44  		// runtime·findfunc to find the function. But if we
    45  		// do that, then we will continue execution at the
    46  		// function entry point, and we will not hit the gdb
    47  		// breakpoint. So for this case we don't change
    48  		// buf.pc, so that when we return we will execute
    49  		// the jump instruction and carry on. This means that
    50  		// stack unwinding may not work entirely correctly
    51  		// (https://golang.org/issue/5723) but the user is
    52  		// running under gdb anyhow.
    53  		return
    54  	}
    55  	print("runtime: pc=", pc, " ", hex(pc[0]), " ", hex(pc[1]), " ", hex(pc[2]), " ", hex(pc[3]), " ", hex(pc[4]), "\n")
    56  	throw("runtime: misuse of rewindmorestack")
    57  }