p.go

     1  // Copyright 2022 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 p
     6  
     7  import (
     8  	"fmt"
     9  	"runtime"
    10  )
    11  
    12  var y int
    13  
    14  //go:noinline
    15  func Square(x int) {
    16  	var pc0, pc1 [1]uintptr
    17  	runtime.Callers(1, pc0[:]) // get PC at entry
    18  
    19  	// a switch using jump table
    20  	switch x {
    21  	case 1:
    22  		y = 1
    23  	case 2:
    24  		y = 4
    25  	case 3:
    26  		y = 9
    27  	case 4:
    28  		y = 16
    29  	case 5:
    30  		y = 25
    31  	case 6:
    32  		y = 36
    33  	case 7:
    34  		y = 49
    35  	case 8:
    36  		y = 64
    37  	default:
    38  		panic("too large")
    39  	}
    40  
    41  	// check PC is in the same function
    42  	runtime.Callers(1, pc1[:])
    43  	if pc1[0] < pc0[0] || pc1[0] > pc0[0]+1000000 {
    44  		fmt.Printf("jump across DSO boundary. pc0=%x, pc1=%x\n", pc0[0], pc1[0])
    45  		panic("FAIL")
    46  	}
    47  
    48  	if y != x*x {
    49  		fmt.Printf("x=%d y=%d!=%d\n", x, y, x*x)
    50  		panic("FAIL")
    51  	}
    52  }
    53
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