issue19078.go

     1  // run
     2  
     3  // Copyright 2017 The Go Authors. All rights reserved.
     4  // Use of this source code is governed by a BSD-style
     5  // license that can be found in the LICENSE file.
     6  
     7  // Issue 19078: liveness & zero-initialization of results
     8  // when there is a defer.
     9  package main
    10  
    11  import "unsafe"
    12  
    13  func main() {
    14  	// Construct an invalid pointer.  We do this by
    15  	// making a pointer which points to the unused space
    16  	// between the last 48-byte object in a span and the
    17  	// end of the span (there are 32 unused bytes there).
    18  	p := new([48]byte)              // make a 48-byte object
    19  	sink = &p                       // escape it, so it allocates for real
    20  	u := uintptr(unsafe.Pointer(p)) // get its address
    21  	u = u >> 13 << 13               // round down to page size
    22  	u += 1<<13 - 1                  // add almost a page
    23  
    24  	for i := 0; i < 1000000; i++ {
    25  		_ = identity(u)         // installs u at return slot
    26  		_ = liveReturnSlot(nil) // incorrectly marks return slot as live
    27  	}
    28  }
    29  
    30  //go:noinline
    31  func liveReturnSlot(x *int) *int {
    32  	defer func() {}() // causes return slot to be marked live
    33  	sink = &x         // causes x to be moved to the heap, triggering allocation
    34  	return x
    35  }
    36  
    37  //go:noinline
    38  func identity(x uintptr) uintptr {
    39  	return x
    40  }
    41  
    42  var sink interface{}
    43
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