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diff --git a/src/pkg/runtime/stack.h b/src/pkg/runtime/stack.h
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+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Stack layout parameters.
+Included both by runtime (compiled via 6c) and linkers (compiled via gcc).
+
+The per-goroutine g->stackguard is set to point StackGuard bytes
+above the bottom of the stack.  Each function compares its stack
+pointer against g->stackguard to check for overflow.  To cut one
+instruction from the check sequence for functions with tiny frames,
+the stack is allowed to protrude StackSmall bytes below the stack
+guard.  Functions with large frames don't bother with the check and
+always call morestack.  The sequences are (for amd64, others are
+similar):
+ 
+	guard = g->stackguard
+	frame = function's stack frame size
+	argsize = size of function arguments (call + return)
+
+	stack frame size <= StackSmall:
+		CMPQ guard, SP
+		JHI 3(PC)
+		MOVQ m->morearg, $(argsize << 32)
+		CALL morestack(SB)
+
+	stack frame size > StackSmall but < StackBig
+		LEAQ (frame-StackSmall)(SP), R0
+		CMPQ guard, R0
+		JHI 3(PC)
+		MOVQ m->morearg, $(argsize << 32)
+		CALL morestack(SB)
+
+	stack frame size >= StackBig:
+		MOVQ m->morearg, $((argsize << 32) | frame)
+		CALL morestack(SB)
+
+The bottom StackGuard - StackSmall bytes are important: there has
+to be enough room to execute functions that refuse to check for
+stack overflow, either because they need to be adjacent to the
+actual caller's frame (deferproc) or because they handle the imminent
+stack overflow (morestack).
+
+For example, deferproc might call malloc, which does one of the
+above checks (without allocating a full frame), which might trigger
+a call to morestack.  This sequence needs to fit in the bottom
+section of the stack.  On amd64, morestack's frame is 40 bytes, and
+deferproc's frame is 56 bytes.  That fits well within the
+StackGuard - StackSmall = 128 bytes at the bottom.  
+The linkers explore all possible call traces involving non-splitting
+functions to make sure that this limit cannot be violated.
+ */
+
+enum {
+	// The amount of extra stack to allocate beyond the size
+	// needed for the single frame that triggered the split.
+	StackExtra = 1024,
+
+	// The minimum stack segment size to allocate.
+	// If the amount needed for the splitting frame + StackExtra
+	// is less than this number, the stack will have this size instead.
+	StackMin = 4096,
+
+	// Functions that need frames bigger than this call morestack
+	// unconditionally.  That is, on entry to a function it is assumed
+	// that the amount of space available in the current stack segment
+	// couldn't possibly be bigger than StackBig.  If stack segments
+	// do run with more space than StackBig, the space may not be
+	// used efficiently.  As a result, StackBig should not be significantly
+	// smaller than StackMin or StackExtra.
+	StackBig = 4096,
+
+	// The stack guard is a pointer this many bytes above the
+	// bottom of the stack.
+	StackGuard = 256,
+
+	// After a stack split check the SP is allowed to be this
+	// many bytes below the stack guard.  This saves an instruction
+	// in the checking sequence for tiny frames.
+	StackSmall = 128,
+
+	// The maximum number of bytes that a chain of NOSPLIT
+	// functions can use.
+	StackLimit = StackGuard - StackSmall,
+};