runtime: change minimum stack size to 2K.

It will be 8K on windows because it needs 4K for the OS.
Similarly, plan9 will be 4K.

On linux/amd64, reduces size of 100,000 goroutines
from ~819MB to ~245MB.

Update #7514

LGTM=dvyukov
R=golang-codereviews, dvyukov, khr, aram
CC=golang-codereviews
https://golang.org/cl/145790043

src/runtime/proc.c

@@ -2162,11 +2162,11 @@ runtime·newproc1(FuncVal *fn, byte *argp, int32 narg, int32 nret, void *callerp
 	siz = narg + nret;
 	siz = (siz+7) & ~7;
 
-	// We could instead create a secondary stack frame
-	// and make it look like goexit was on the original but
-	// the call to the actual goroutine function was split.
+	// We could allocate a larger initial stack if necessary.
 	// Not worth it: this is almost always an error.
-	if(siz > StackMin - 1024)
+	// 4*sizeof(uintreg): extra space added below
+	// sizeof(uintreg): caller's LR (arm) or return address (x86, in gostartcall).
+	if(siz >= StackMin - 4*sizeof(uintreg) - sizeof(uintreg))
 		runtime·throw("runtime.newproc: function arguments too large for new goroutine");
 
 	p = g->m->p;

src/runtime/stack.h

@@ -69,16 +69,19 @@ enum {
 #endif	// Plan 9
 #endif	// Windows
 
-	// The amount of extra stack to allocate beyond the size
-	// needed for the single frame that triggered the split.
-	StackExtra = 2048,
+	// The minimum size of stack used by Go code
+	StackMin = 2048,
 
-	// 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 = 8192,
-	StackSystemRounded = StackSystem + (-StackSystem & (StackMin-1)),
-	FixedStack = StackMin + StackSystemRounded,
+	// The minimum stack size to allocate.
+	// The hackery here rounds FixedStack0 up to a power of 2.
+	FixedStack0 = StackMin + StackSystem,
+	FixedStack1 = FixedStack0 - 1,
+	FixedStack2 = FixedStack1 | (FixedStack1 >> 1),
+	FixedStack3 = FixedStack2 | (FixedStack2 >> 2),
+	FixedStack4 = FixedStack3 | (FixedStack3 >> 4),
+	FixedStack5 = FixedStack4 | (FixedStack4 >> 8),
+	FixedStack6 = FixedStack5 | (FixedStack5 >> 16),
+	FixedStack = FixedStack6 + 1,
 
 	// Functions that need frames bigger than this use an extra
 	// instruction to do the stack split check, to avoid overflow