nowb.go

Documentation: cmd/compile/internal/ssagen

     1  // Copyright 2009 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 ssagen
     6  
     7  import (
     8  	"fmt"
     9  	"strings"
    10  
    11  	"cmd/compile/internal/base"
    12  	"cmd/compile/internal/ir"
    13  	"cmd/compile/internal/typecheck"
    14  	"cmd/compile/internal/types"
    15  	"cmd/internal/obj"
    16  	"cmd/internal/src"
    17  )
    18  
    19  func EnableNoWriteBarrierRecCheck() {
    20  	nowritebarrierrecCheck = newNowritebarrierrecChecker()
    21  }
    22  
    23  func NoWriteBarrierRecCheck() {
    24  	// Write barriers are now known. Check the
    25  	// call graph.
    26  	nowritebarrierrecCheck.check()
    27  	nowritebarrierrecCheck = nil
    28  }
    29  
    30  var nowritebarrierrecCheck *nowritebarrierrecChecker
    31  
    32  type nowritebarrierrecChecker struct {
    33  	// extraCalls contains extra function calls that may not be
    34  	// visible during later analysis. It maps from the ODCLFUNC of
    35  	// the caller to a list of callees.
    36  	extraCalls map[*ir.Func][]nowritebarrierrecCall
    37  
    38  	// curfn is the current function during AST walks.
    39  	curfn *ir.Func
    40  }
    41  
    42  type nowritebarrierrecCall struct {
    43  	target *ir.Func // caller or callee
    44  	lineno src.XPos // line of call
    45  }
    46  
    47  // newNowritebarrierrecChecker creates a nowritebarrierrecChecker. It
    48  // must be called before walk.
    49  func newNowritebarrierrecChecker() *nowritebarrierrecChecker {
    50  	c := &nowritebarrierrecChecker{
    51  		extraCalls: make(map[*ir.Func][]nowritebarrierrecCall),
    52  	}
    53  
    54  	// Find all systemstack calls and record their targets. In
    55  	// general, flow analysis can't see into systemstack, but it's
    56  	// important to handle it for this check, so we model it
    57  	// directly. This has to happen before transforming closures in walk since
    58  	// it's a lot harder to work out the argument after.
    59  	for _, n := range typecheck.Target.Funcs {
    60  		c.curfn = n
    61  		if c.curfn.ABIWrapper() {
    62  			// We only want "real" calls to these
    63  			// functions, not the generated ones within
    64  			// their own ABI wrappers.
    65  			continue
    66  		}
    67  		ir.Visit(n, c.findExtraCalls)
    68  	}
    69  	c.curfn = nil
    70  	return c
    71  }
    72  
    73  func (c *nowritebarrierrecChecker) findExtraCalls(nn ir.Node) {
    74  	if nn.Op() != ir.OCALLFUNC {
    75  		return
    76  	}
    77  	n := nn.(*ir.CallExpr)
    78  	if n.Fun == nil || n.Fun.Op() != ir.ONAME {
    79  		return
    80  	}
    81  	fn := n.Fun.(*ir.Name)
    82  	if fn.Class != ir.PFUNC || fn.Defn == nil {
    83  		return
    84  	}
    85  	if types.RuntimeSymName(fn.Sym()) != "systemstack" {
    86  		return
    87  	}
    88  
    89  	var callee *ir.Func
    90  	arg := n.Args[0]
    91  	switch arg.Op() {
    92  	case ir.ONAME:
    93  		arg := arg.(*ir.Name)
    94  		callee = arg.Defn.(*ir.Func)
    95  	case ir.OCLOSURE:
    96  		arg := arg.(*ir.ClosureExpr)
    97  		callee = arg.Func
    98  	default:
    99  		base.Fatalf("expected ONAME or OCLOSURE node, got %+v", arg)
   100  	}
   101  	c.extraCalls[c.curfn] = append(c.extraCalls[c.curfn], nowritebarrierrecCall{callee, n.Pos()})
   102  }
   103  
   104  // recordCall records a call from ODCLFUNC node "from", to function
   105  // symbol "to" at position pos.
   106  //
   107  // This should be done as late as possible during compilation to
   108  // capture precise call graphs. The target of the call is an LSym
   109  // because that's all we know after we start SSA.
   110  //
   111  // This can be called concurrently for different from Nodes.
   112  func (c *nowritebarrierrecChecker) recordCall(fn *ir.Func, to *obj.LSym, pos src.XPos) {
   113  	// We record this information on the *Func so this is concurrent-safe.
   114  	if fn.NWBRCalls == nil {
   115  		fn.NWBRCalls = new([]ir.SymAndPos)
   116  	}
   117  	*fn.NWBRCalls = append(*fn.NWBRCalls, ir.SymAndPos{Sym: to, Pos: pos})
   118  }
   119  
   120  func (c *nowritebarrierrecChecker) check() {
   121  	// We walk the call graph as late as possible so we can
   122  	// capture all calls created by lowering, but this means we
   123  	// only get to see the obj.LSyms of calls. symToFunc lets us
   124  	// get back to the ODCLFUNCs.
   125  	symToFunc := make(map[*obj.LSym]*ir.Func)
   126  	// funcs records the back-edges of the BFS call graph walk. It
   127  	// maps from the ODCLFUNC of each function that must not have
   128  	// write barriers to the call that inhibits them. Functions
   129  	// that are directly marked go:nowritebarrierrec are in this
   130  	// map with a zero-valued nowritebarrierrecCall. This also
   131  	// acts as the set of marks for the BFS of the call graph.
   132  	funcs := make(map[*ir.Func]nowritebarrierrecCall)
   133  	// q is the queue of ODCLFUNC Nodes to visit in BFS order.
   134  	var q ir.NameQueue
   135  
   136  	for _, fn := range typecheck.Target.Funcs {
   137  		symToFunc[fn.LSym] = fn
   138  
   139  		// Make nowritebarrierrec functions BFS roots.
   140  		if fn.Pragma&ir.Nowritebarrierrec != 0 {
   141  			funcs[fn] = nowritebarrierrecCall{}
   142  			q.PushRight(fn.Nname)
   143  		}
   144  		// Check go:nowritebarrier functions.
   145  		if fn.Pragma&ir.Nowritebarrier != 0 && fn.WBPos.IsKnown() {
   146  			base.ErrorfAt(fn.WBPos, 0, "write barrier prohibited")
   147  		}
   148  	}
   149  
   150  	// Perform a BFS of the call graph from all
   151  	// go:nowritebarrierrec functions.
   152  	enqueue := func(src, target *ir.Func, pos src.XPos) {
   153  		if target.Pragma&ir.Yeswritebarrierrec != 0 {
   154  			// Don't flow into this function.
   155  			return
   156  		}
   157  		if _, ok := funcs[target]; ok {
   158  			// Already found a path to target.
   159  			return
   160  		}
   161  
   162  		// Record the path.
   163  		funcs[target] = nowritebarrierrecCall{target: src, lineno: pos}
   164  		q.PushRight(target.Nname)
   165  	}
   166  	for !q.Empty() {
   167  		fn := q.PopLeft().Func
   168  
   169  		// Check fn.
   170  		if fn.WBPos.IsKnown() {
   171  			var err strings.Builder
   172  			call := funcs[fn]
   173  			for call.target != nil {
   174  				fmt.Fprintf(&err, "\n\t%v: called by %v", base.FmtPos(call.lineno), call.target.Nname)
   175  				call = funcs[call.target]
   176  			}
   177  			// Seeing this error in a failed CI run? It indicates that
   178  			// a function in the runtime package marked nowritebarrierrec
   179  			// (the outermost stack element) was found, by a static
   180  			// reachability analysis over the fully lowered optimized code,
   181  			// to call a function (fn) that involves a write barrier.
   182  			//
   183  			// Even if the call path is infeasable,
   184  			// you will need to reorganize the code to avoid it.
   185  			base.ErrorfAt(fn.WBPos, 0, "write barrier prohibited by caller; %v%s", fn.Nname, err.String())
   186  			continue
   187  		}
   188  
   189  		// Enqueue fn's calls.
   190  		for _, callee := range c.extraCalls[fn] {
   191  			enqueue(fn, callee.target, callee.lineno)
   192  		}
   193  		if fn.NWBRCalls == nil {
   194  			continue
   195  		}
   196  		for _, callee := range *fn.NWBRCalls {
   197  			target := symToFunc[callee.Sym]
   198  			if target != nil {
   199  				enqueue(fn, target, callee.Pos)
   200  			}
   201  		}
   202  	}
   203  }
   204
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